ee 6351 electrical drives and control 3 0 0 100 …

EE 6351 ELECTRICAL DRIVES AND CONTROL 3 0 0 100

OBJECTIVE

1.

To understand the basic concepts of different types of electrical machines and their Performance. To study the different methods of starting D.C motors and induction motors. To study the conventional and solid-state drives.

8 INTRODUCTION

Basic Elements – Types of Electric Drives – factors influencing the choice of electrical drives – heating and cooling curves – Loading conditions and classes of duty – Selection of power rating for drive motors with regard to thermal overloading and Load variation factors

4. CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES

Types of D.C Motor starters – Typical control circuits for shunt and series motors – Three phase squirrel cage and slip ring induction motors.

10

Speed control of three phase induction motor – Voltage control, voltage / frequency control, slip power recovery scheme – Using inverters and AC voltage regulators – applications.

TOTAL : 45

McGraw-

TEXT BOOKS

1.

2.

REFERENCES

1.PILLAI.S.K “A first course on Electric drives”, Wiley Eastern Limited, 1998 2.M.D.SINGH, K.B.KHANCHANDANI, “Power Electronics”, Tata McGraw-Hill, 1998 H.Partab, “Art and Science and Utilisation of electrical energy”, Dhanpat Rai and Sons, 1994

VEDAM SUBRAHMANIAM, “Electric Drives (concepts and applications)”, Tata Hill, 2001 NAGRATH.I.J. & KOTHARI.D.P, “Electrical Machines”, Tata McGraw-Hill, 1998

5. CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C. DRIVES

Speed control of DC series and shunt motors – Armature and field control, Ward-Leonard control system - Using controlled rectifiers and DC choppers –applications.

10

3. STARTING METHODS

Mechanical characteristics – Speed-Torque characteristics of various types of load and drive motors – Braking of Electrical motors – DC motors: Shunt, series and compound - single phase and three phase induction motors.

8

2. DRIVE MOTOR CHARACTERISTICS 9

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UNIT-I

INTRODUCTION TO ELECTRICAL DRIVES

Drives are employed for systems that require motion control – e.g. transportation

system, fans, robots, pumps, machine tools, etc. Prime movers are required in drive systems to

provide the sources: diesel engines, petrol engines, hydraulic motors, electric motors etc.

movement or motion and energy that is used to provide the motion can come from various

Drives that use electric motors as the prime movers are known as electrical drives.

There are several advantages of electrical drives:

a. Flexible control characteristic – This is particularly true when power electronic

Converters are employed where the dynamic and steady state characteristics of the motor

can be controlled by controlling the applied voltage or current.

b. Available in wide range of speed, torque and power

c. High efficiency, lower noise, low maintenance requirements and cleaner operation

d. Electric energy is easy to be transported.

With the advancement of power electronics, microprocessors and digital electronics,

typical Electric drive systems nowadays are becoming more compact, efficient, cheaper and

versatile this is shown in Figure 2. The voltage and current applied to the motor can be

changed at will by employing power electronic converters. AC motor is no longer limited to

application where only AC source is available, however, it can also be used when the power

source available is DC or vice versa

A typical conventional electric drive system for variable speed application employing

multi-machine system is shown in Figure 1. The system is obviously bulky, expensive,

inflexible and require regular maintenance. In the past, induction and synchronous machines

were used for constant speed applications – this was mainly because of the unavailability of

variable frequency supply.

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COM PONENTS OF ELECTRICAL DRIVES

The main components of a modern electrical drive are the motors, power processor, control

unit and electrical source. These are briefly discussed below

a) Motors : Motors obtain power from electrical sources. They convert energy from

electrical to mechanical - therefore can be regarded as energy converters. In braking mode, the

flow of power is reversed. Depending upon the type of power converters

Electric drives is multi-disciplinary field. Various research areas can be sub-divided from

electric drives as shown in Figure 3.


used, it is also possible for the power to be fed back to the sources rather than dissipated as

heat

There are several types of motors used in electric drives – choice of type used depends on

applications, cost, environmental factors and also the type of sources available.. Broadly, they

can be classified as either DC or AC motors they can be classified as either DC or AC motors:

DC motors (wound or permanent magnet)

AC motors

Induction motors – squirrel cage, wound rotor

Synchronous motors – wound field, permanent magnet

Brushless DC motor – require power electronic converters

Stepper motors – require power electronic converters

Synchronous reluctance motors or switched reluctance motor – require power electronic

converters

b) Power processor or power modulator

Since the electrical sources are normally uncontrollable, it is therefore necessary to be able to

control the flow of power to the motor – this is achieved using power processor or power modulator.

With controllable sources, the motor can be reversed, brake or can be operated with variable speed.

Conventional methods used, for example, variable impedance or relays, to shape the voltage or

current that is supplied to the motor – these methods however are inflexible and inefficient. Modern

electric drives normally used power electronic converters to shape the desired voltage or current

supplied to the motor. In other words, the characteristic of the motors can be changed at will. Power

electronic converters have several advantages over classical methods of power conversion, such as

1) More efficient – since ideally no losses occur in power electronic converters

2) Flexible – voltage and current can be shaped by simply controlling switching functions of the

power converter.

3) Compact – smaller, compact and higher ratings solid–state power electronic devices are

continuously being developed – the prices are getting cheaper. Converters are used to convert and

possibly regulate (i.e. using closed-loop control) the available sources to suit the load i.e. motors.

These converters are efficient because the switches operate in either cut-off or saturation modes.

Several conversions are possible.


b) Control Unit

The complexity of the control unit depends on the desired drive performance and the type of

motors used. A controller can be as simple as few op-amps and/or a few digital ICs, or it can

be as complex as the combinations of several ASICs and digital signal processors (DSPs). The

types of the main controllers can be

• analog - which is noisy, inflexible. However analog circuit ideally has infinite bandwidth.

• digital – immune to noise, configurable. The bandwidth is obviously smaller than the analog

controllers – depends on sampling frequency.


• DSP/microprocessor – flexible, lower bandwidth compared to above. DSPs perform faster

operation than microprocessors (multiplication in single cycle). With DSP/micro., complex

estimations and observers can be easily implemented.

d) Source

Electrical sources or power supplies provide the energy to the electrical motors. For

high efficiency operation, the power obtained from the electrical sources need to be regulated

using power electronic converters Power sources can be of AC or D C in nature and normally

are uncontrollable, i.e. their magnitudes or frequencies are fixed or depend on the sources of

energy such as solar or wind. AC source can be either three-phase or single-phase; 3-phase

sources are normally for high power applications

There can be several factors that affect the selection of different configuration of electrical

drive system such as

a) Torque and speed profile - determine the ratings of converters and the quadrant of operation

required.

b) Capital and running cost – Drive systems will vary in terms of start-up cost and running

cost, e.g. maintenance

c) Space and weight restrictions

d) Environment and location

3.1 Overall Considerations.

• Check the Current rating of the inverter and the motor. Power rating is only a rough guide

• Check that you have selected the correct operating voltage. 230V three phase input

MICROMASTERs will operate with single or three phase inputs; MIDIMASTERs will

operate with three phase only. Single phase input units can be more cost effective in some

cases, but note that 230V units will be damaged if operated at 400V.

• Check the speed range you require. Operation above normal supply frequency (50 or 60Hz)

is usually only possible at reduced power. Operation at low frequency and high torque can

cause the motor to overheat due to lack of cooling

• Synchronous motors require de-rating, typically by 2 -3 times. This is because the power

factor, and hence the current, can be very high at low frequency.

• Check overload performance. The inverter will limit current to 150 or 200 % of full current

very quickly - a standard, fixed speed motor will tolerate these overloads.

• Do you need to stop quickly? If so, consider using a braking resistor (braking unit on

MIDIMASTERs) to absorb the energy.

3. Selecting a Drive

Often drive selection is straight forward, as a motor is already installed and the speed

range requirement is not excessive. However, when a drive system is selected from first

principles, careful consideration may avoid problems in installation and operation, and may

also save significant cost.


• Do you need to operate with cables longer than 50m, or screened or armoured cables

longer than 25m? If so, it may be necessary to de-rate, or fit a choke to compensate for the

cable capacitance.

3.2 Motor limitations

For more information concerning calculation of Power requirements, Torque, and

Moment of Inertia, see later. The motor speed is determined mainly by the applied frequency.

The motor slows down a little as the load increases and the slip increases. If the load is too

great the motor will exceed the maximum torque and stall or „pull out‟. Most motors and

inverters will operate at 150% load

The constructions of the machines are very complex; normally built from various types

of materials (heterogeneous) with complex geometrical shapes. To exactly predict the heat

flow and hence the temperature distribution is extremely difficult. Based on the assumptions

that the temperature limits of all parts does not exceed the temperature limits under certain

operating conditions, the motors can therefore adequately modeled as homogeneous bodies.

Obviously, this assumption cannot determine the specific internal thermal conditions for the

motors.

Friction and wind age losses:

Mainly due to ball bearings, brushes, ventilation losses

Core losses:

Mainly due to eddy current and hysteresis losses

Conductor losses:

Exist in the windings, cables, brushes, slip rings, commutator, etc. .

The losses in the machines contribute to the temperature increase in the machine. The

various parts of the machine use different type of insulation materials which have different

temperature limits. Particularly important is the insulation used for the windings which give

rise to the different classes of machines. Allowable power losses are higher for materials

which can withstand higher temperature which translates to higher costs. Three main cause of

power losses are:

Thermal considerations


Where is the thermal time constant? With power input p1

from 0 to ph at t=0, the solution for is

where is the coefficient of heat transfer. If we let equation

(12) can be written as

The heat dissipated by convection is given by

and a step change in the

At steady state,\

During cooling, i.e. when heat is removed at t=0, the temperature of the body decays to

the ambient temperature.

Let us assume that a homogeneous body shown in Figure 12 represents a motor which

has a thermal capacity C. The input power, which is the losses incurred in the motor, is

represented by p1 whereas the output power, which is the power released as heat by

convection, is represented by p2. The output power due to radiation is assumed negligible

because of the low operating temperature and back radiation. Under a steady state condition,

the input power equals the output power; this is when the steady state temperature is reached.

The equation describing the power balance is given by


Ratings of converters and motors

In order to accelerate to a given reference value, the motor torque has to be larger than the

load torque. According to (1), the difference between T1 and Te determines how fast the

angular acceleration is. For example, the speed and torque responses for a closed-loop speed

control DC drive with two different torque limit setting (10 Nm and 15 Nm) is shown in

Figure 7. The higher the torque during the speed transient, the faster is the speed gets to its

reference

If the thermal time constant is large, a temporary overload is therefore possible without

exceeding the temperature limits. Three typical modes of operation are:

- Continuous duty

- Short time intermittent duty

- Periodic intermittent duty


In most cases, the torque during this transient condition can be up to 3 times the rated

torque of the motor and for servo motor, it can be as high as 8 to 10 times the rated value. This

momentary high torque is possible due to the large thermal capacity of the motor with suitable

insulators used for the winding. The converter, which conducts the motor current, must be able

to sustain this condition. However since the thermal capacity of the converter is small, the

current cannot be higher than its rated value. Consequently, the current rating of the converter

is normally set to equal the maximum allowable motor current and this can be as high as the 3

times the motor rated current. The maximum allowable torque during transient of a drive

system is determined by the current rating of the converter used whereas the continuous torque

limit depends on the current rating of the motor. The operating area of a 4-quadrant motor

drive is shown in Figure 8. The converter is normally protected from the over-current

condition by the current limiter mechanism within the converter system, which means that

sustained overloads on the motor has to be protected by an additional thermal protection the

torque is limited by the maximum mechanism. Above the base speed, allowable power, which

depends on whether the transient or continuous torque limit is considered? The speed limit

basically depends on the mechanical limitation of the motor.


Fig. Limits for torque, speed and power for drive system

The motor will operate at the steady-state speed (point where T1 = Te) provided that

the speed is of stable equilibrium. The stable equilibrium speed is investigated using steady-

state torque- speed characteristics of the load and motor. A disturbance in any part of the drive

will result in a speed to depart from the steady state speed. However, if the steady-state speed

is of stable equilibrium, the speed will return to the stable equilibrium speed. On the other

hand, if the speed is not of the stable equilibrium, the disturbance will results in the speed to

drift away from the equilibrium speed. It can be shown that the condition for stable

equilibrium is:

Steady-state stability


A constant applied voltage V is assumed across the armature. As the armature current

Ia, varies the armature drop varies proportionally and one can plot the variation of the induced

emf E. The mmf of the field is assumed to be constant. The flux inside the machine however

slightly falls due to the effect of saturation and due to armature reaction.

The variation of these parameters is shown in Fig. Knowing the value of E and flux

one can determine the value of the speed. Also knowing the armature current and the flux, the

value of the torque is found out. This procedure is repeated for different values of the assumed

armature currents and the values are plotted as in Fig. (a). From these graphs, a graph

indicating speed as a function of torque or the torque-speed characteristics is plotted Fig.

(b)(i).

As seen from the figure the fall in the flux due to load increases the speed due to the

fact that the induced emf depends on the product of speed and flux. Thus the speed of the

machine remains more or less constant with load. With highly saturated machines

Torque speed characteristics of a shunt motor:

UNIT-II

DRIVE MOTOR CHARACTERISTICS


the on-load speed may even slightly increase at over load conditions. This effect gets more

pronounced if the machine is designed to have its normal field ampere turns much less than

the armature ampere turns. This type of external characteristics introduces instability during

operation Fig. (b)(ii) and hence must be avoided. This may be simply achieved by providing a

series stability winding which aids the shunt field mmf.


Load characteristics of a series motor

Following the procedure described earlier under shunt motor, the torque speed

Characteristics of a series motor can also be determined. The armature current also happens to

be the excitation current of the series field and hence the flux variation resembles the

magnetization curve of the machine. At large value of the armature currents the useful flux

would be less than the no-load magnetization curve for the machine. Similarly for small values

of the load currents the torque varies as a square of the armature currents as the flux is

proportional to armature current in this region. As the magnetic circuit becomes more and

more saturated the torque becomes proportional to Ia as flux variation becomes small. Fig. (a)

shows the variation of E1, flux, torque and speed following the above procedure from which

the torque-speed characteristics of the series motor for a given applied voltage V can be

plotted as shown in Fig.(b) The initial portion of this torque-speed curve is seen to be a

rectangular hyperbola and the final portion is nearly a straight line. The speed under light load

conditions is many times more than the rated speed of the motor. Such high speeds are unsafe,

as the centrifugal forces acting on the armature and


commutator can destroy them giving rise to a catastrophic break down. Hence series motors

are not recommended for use where there is a possibility of the load becoming zero. In order

to safeguard the motor and personnel, in the modern machines, a 'weak' shunt field is provided


Two situations arise in the case of compound motors. The mmf of the shunt field and

series field may oppose each other or they may aid each other. The first configuration is called

differential compounding and is rarely used. They lead to unstable operation of the machine

unless the armature mmf is small and there is no magnetic saturation. This mode may

sometimes result due to the motoring operation of a level-compounded generator, say by the

failure of the prime mover. Also, differential compounding may result in large negative mmf

under overload/starting condition and the machine may start in the reverse direction. In motors

intended for constant speed operation the level of compounding is very low as not to cause

any problem.

Cumulatively compounded motors are very widely used for industrial drives. High

degree of compounding will make the machine approach a series machine like characteristics

but with a safe no-load speed. The major benefit of the compounding is that the field is

strengthened on load. Thus the torque per ampere of the armature current is made high. This

feature makes a cumulatively compounded machine well suited for

Load characteristics of a compound motor

on series motors to ensure a definite, though small, value of flux even when the

armature current is nearly zero. This way the no-load speed is limited to a safe maximum

speed. It is needless to say, this field should be connected so as to aid the series field.


intermittent peak loads. Due to the large speed variation between light load and peak load

conditions, a y wheel can be used with such motors with advantage. Due to the reasons

provided under shunt and series motors for the provision of an additional series/shunt

winding, it can be seen that all modern machines are compound machines. The difference

between them is only in the level of compounding.

Braking of D.C. Motors

When a motor is switched off it `coasts' to rest under the action of frictional

forces. Braking is employed when rapid stopping is required. In many cases

mechanical braking is adopted. The electric braking may be done for various reasons

such as those mentioned below:

1. To augment the brake power of the mechanical brakes.

2. To save the life of the mechanical brakes.

3. To regenerate the electrical power and improve the energy efficiency.

4. In the case of emergencies to step the machine instantly.

5. To improve the throughput in many production processes by reducing the stopping time.

In many cases electric braking makes more brake power available to the braking process

where mechanical brakes are applied. This reduces the wear and tear of the mechanical brakes

and reduces the frequency of the replacement of these parts. By recovering the mechanical

energy stored in the rotating parts and pumping it into the supply lines the overall energy

efficiency is improved. This is called regeneration. Where the safety of the personnel or the

equipment is at stake the machine may be required to stop instantly.

Extremely large brake power is needed under those conditions. Electric braking can help

in these situations also. In processes where frequent starting and stopping is involved the

process time requirement can be reduced if braking time is reduced. The reduction of the

1. Dynamic

2. Regenerative

3. Reverse voltage braking or plugging

These are now explained briefly with reference to shunt, series and compound motors.

Dynamic braking

Shunt machine

In dynamic braking the motor is disconnected from the supply and connected to a

dynamic braking resistance RDB. In and Fig. 49 this is done by changing the switch from

position 1 to 2. The supply to the field should not be removed. Due to the rotation of the

armature during motoring mode and due to the inertia, the armature continues to rotate. An

emf is induced due to the presence of the field and the rotation. This voltage drives a


current through the braking resistance. The direction of this current is opposite to the one

which was owing before change in the connection. Therefore, torque developed also gets

reversed. The machine acts like a brake. The torque speed characteristics separate by excited

shunt of the machine under dynamic braking mode is as shown in Fig. (b). For a particular

value of RDB. The positive torque corresponds to the motoring operation. Fig. shows the

dynamic braking of a shunt excited motor and the corresponding torque-speed curve. Here the

machine behaves as a self-excited generator. Below a certain speed the self-excitation

collapses and the braking action becomes Zero. Process time improves the throughput.

Basically the electric braking involved is fairly simple. The electric motor can be made to

work as a generator by suitable terminal conditions and absorb mechanical energy.

This converted mechanical power is dissipated or used on the electrical network suitably.

Braking can be broadly classified into:


Figure: Dynamic braking of shunt excited shunt machine

Compound generators

In the case of compound machine, the situation is like in a shunt machine. A separately

excited shunt field and the armature connected across the braking resistance are used.

A cumulatively connected motor becomes differentially compounded generator and the

braking torque generated comes down. It is therefore necessary to reverse the series field if

large braking torques are desired.

In the case of a series machine the excitation current becomes zero as soon as the

armature is disconnected from the mains and hence the induced emf also vanishes. In order to

achieve dynamic braking the series field must be isolated and connected to a low voltage high

current source to provide the field. Rather, the motor is made to work like a separately excited

machine. When several machines are available at any spot, as in railway locomotives,

dynamic braking is feasible. Series connection of all the series fields with parallel connection

of all the armatures connected across a single dynamic braking resistor is used in that case.

Series machine


Regenerative braking

In regenerative braking as the name suggests the energy recovered from the rotating

masses is fed back into the d.c. power source. Thus this type of braking improves the energy

efficiency of the machine. The armature current can be made to reverse for a constant voltage

operation by increase in speed/excitation only. Increase in speed does not result in braking and

the increase in excitation is feasible only over a small range, which may be of the order of 10

to 15%. Hence the best method for obtaining the regenerative braking is to operate, the

machine on a variable voltage supply. As the voltage is continuously pulled below the value

of the induced emf the speed steadily comes down. The field current is held constant by means

of separate excitation. The variable d.c. supply voltage can be obtained by Ward-Leonard

arrangement, shown schematically in Fig. .

Braking torque can be obtained right up to zero speed. In modern times static Ward-

Leonard scheme is used for getting the variable d.c. voltage. This has many advantages over

its rotating machine counter part. Static set is compact, has higher efficiency, and requires

lesser space and silent in operation; however it suffers from drawbacks like large ripple at low

voltage levels, unidirectional power flow and low over load capacity. Bidirectional power

flow capacity is a must if regenerative braking is required. Series motors cannot be

regenerative braked as the characteristics do not extend to the second quadrant.

Plugging

The third method for braking is by plugging. Fig. shows the method of connection for the

plugging of a shunt motor. Initially the machine is connected to the supply with the switch S

in position number 1. If now the switch is moved to position 2, then a reverse voltage is

applied across the armature. The induced armature voltage E and supply voltage V aid each

other and a large reverse current flows through the armature. This produces a large negative

torque or braking torque. Hence plugging is also termed as reverse voltage braking. The

machine instantly comes to rest. If the motor is not switched off at this instant the direction of

rotation reverses and the motor starts rotating the reverse direction. This type of braking

therefore has two modes viz. 1) plug to reverse and 2) plug to stop. If we need the plugging

only for bringing the speed to zero, then we have to open the switch S at zero speed. If nothing

is done it is plug to reverse mode. Plugging is a convenient mode for quick reversal of

direction of rotation in reversible drives. Just as in starting, during


Figure: Plugging or reverse voltage braking of a shunt motor


Plugging also it is necessary to limit the current and thus the torque, to reduce the

stress on the mechanical system and the commutator. This is done by adding additional

resistance in series with the armature during plugging.

Series motors

In the case of series motors plugging cannot be employed as the field current too gets

reversed when reverse voltage is applied across the machine. This keeps the direction of the

torque produced unchanged. This fact is used with advantage, in operating a D.C. series motor

on D.C. or A.C. supply. Series motors thus qualify to be called as `Universal motors'.

From the equivalent circuit, many aspects of the steady state behavior of the machine can

be deduced. We will begin by looking at the speed-torque characteristic of the machine. We

will consider the approximate equivalent circuit of the machine. We have reasoned earlier that

the power consumed by the 'rotor-portion' of the equivalent circuit is the power transferred

across the air-gap. Out of that quantity the amount dissipated in R0 r is the rotor copper loss

and the quantity consumed by R0r(1 + s)=s is the mechanical power developed. Neglecting

mechanical losses, this is the power available at the shaft. The torque available can be

obtained by dividing this number by the shaft speed.

The complete torque-speed characteristic of Induction motor

In order to estimate the speed torque characteristic let us suppose that a sinusoidal voltage

is impressed on the machine. Recalling that the equivalent circuit is the per-phase

representation of the machine, the current drawn by the circuit is given by

Deducing the machine performance (Single phase Induction motor)

Plugging of compound motors proceeds on similar lines as the shunt motors. However

some precautions have to be observed due to the presence of series field winding. A

cumulatively compounded motor becomes differentially compounded on plugging. The MMF

due to the series field can 'over power' the shunt field forcing the flux to low values or even

reverse the net field. This decreases the braking torque, and increases the duration of the large

braking current. To avoid this it may be advisable to deactivate the series field at the time of

braking by short-circuiting the same. In such cases the braking proceeds just as in a shunt

motor. If plugging is done to operate the motor in the negative direction of rotation as well,

then the series field has to be reversed and connected for getting the proper mmf. Unlike

dynamic braking and regenerative braking where the motor is made to work as a generator

during braking period, plugging makes the motor work on reverse motoring mode.

Compound motors


Where Vs is the phase voltage phases and Is is the current phases. The magnetizing current is

neglected. Since this current is owing through, the air-gap power is given by

The mechanical power output was shown to be (1_s) Pg (power dissipated in R0r=s). The

torque is obtained by dividing this by the shaft speed .Thus we have,

The torque may be plotted as a function of `s' and is called the torque-slip (or torque-

speed, since slip indicates speed) characteristic a very important characteristic of the induction

machine. Equation 16 is valid for a two-pole (one pole pair) machine. In general, this

expression should be multiplied by p, the number of pole-pairs. A typical torque-speed

characteristic is shown in fig. 22. This plot corresponds to a 3 kW, 4 pole and 60 Hz machine.

The rated operating speed is 1780 rpm.

We must note that the approximate equivalent circuit was used in deriving this

relation. Readers with access to MATLAB or suitable equivalents (octave, scalar available

free under GNU at the time of this writing) may find out the difference caused by using the

`exact' equivalent circuit by using the script found here. A comparison between the two is

found in the plot of fig. The plots correspond to a 3 kW, 4 poles, 50 machines, with a rated

speed of 1440 rpm. It can be seen that the approximate equivalent circuit is a good

approximation in the operating speed range of the machine. Comparing the two figures. We

can see that the slope and shape of the characteristics are dependent intimately on the machine

parameters.

Further, this curve is obtained by varying slip with the applied voltage being held

constant. Coupled with the fact that this is an equivalent circuit valid under steady state, it

implies that if this characteristic is to be measured experimentally, we need to look at the

torque for a given speed after all transients have died down. One cannot, for example, try

Where! S is the synchronous speed in radians per second and s is the slip. Further, this is the

torque produced per phase. Hence the overall torque is given by


Torque, Nm to obtain this curve by directly starting the motor with full voltage applied

to the terminals and measuring the torque and speed dynamically as it runs up to steady speed.

Another point to note is that the equivalent circuit and the values of torque predicted is

valid when the applied voltage waveform is sinusoidal. With non-sinusoidal voltage

waveforms, the procedure is not as straight forward.

With respect to the direction of rotation of the air-gap flux, the rotor maybe driven to

higher speeds by a prime mover or may also be rotated in the reverse direction. The torque-

speed relation for the machine under the entire speed range is called the complete speed-

torque characteristic. A typical curve is shown in fig for a four-pole machine, the synchronous

speed being 1500 rpm. Note that negative speeds correspond to slip values greater than 1, and

speeds greater than 1500 rpm correspond to negative slip. The plot also shows the operating

modes of the induction machine in various regions. The slip axis is also shown for

convenience.


Substituting ^s into the expression for torque gives us the value of the stalling torque ^ T

the negative sign being valid for negative slip.

The expression shows that ^ Te is the independent of R0 r, while ^s is directly

proportional to R0 r. This fact can be made use of conveniently to alter ^s. If it is possible to

change R0 r, then we can get a whole series of torque-speed characteristics, the maximum

torque remaining constant all the while. But this is a subject to be discussed later.

The value of the stalling torque may be obtained by differentiating the expression for

torque with respect to zero and setting it to zero to find the value of ^s. Using this method,

Restricting ourselves to positive values of slip, we see that the curve has a peak point.

This is the maximum torque that the machine can produce, and is called as stalling torque. If

the load torque is more than this value, the machine stops rotating or stalls. It occurs at a slip

^s, which for the machine of fig is 0.38. At values of slip lower than ^s, the curve falls steeply

down to zero at s = 0. The torque at synchronous speed is therefore zero. At values of slip

higher than s = ^s, the curve falls slowly to a minimum value at s =1. The torque at s = 1

(speed = 0) is called the starting torque.


We may note that if R is chosen equal to becomes unity, which p means that the maximum

torque occurs at starting. Thus changing of R r, wherever possible can serve as a means to

control the starting torque.

While considering the negative slip range, (generator mode) we note that the maximum

torque is higher than in the positive slip region (motoring mode).

Operating Point

Consider a speed torque characteristic shown in fig. For an induction machine, having the

load characteristic also superimposed on it. The load is a constant torque load i.e., the torque

required for operation is fixed irrespective of speed. The system consisting of the motor and

load will operate at a point where the two characteristics meet. From the above plot, we note

that there are two such points. We therefore need to find out which of these is the actual

operating point.

To answer this we must note that, in practice, the characteristics are never fixed; they

change slightly with time. It would be appropriate to consider a small band around the curve

drawn where the actual points of the characteristic will lie. This being the case let us considers

that the system is operating at point 1, and the load torque demand increases slightly. This is

shown in fig, where the change is exaggerated for clarity. This would shift the point of

operation to a point 10 at which the slip would be less and the developed torque higher.

The difference in torque-developed 4Te, being positive will accelerate the machine. Any

overshoot in speed as it approaches the point 10 will cause it to further accelerate since the

developed torque is increasing. Similar arguments may be used to show that if for some reason

the developed torque becomes smaller the speed would drop and the effect is cumulative.

Therefore we may conclude that 1 is not a stable operating point.

Let us consider the point 2. If this point shifts to 20, the slip is now higher (speed is

lower) and the positive difference in torque will accelerate the machine. This behavior will

tend to bring the operating point towards 2 once again. In other words, disturbances at point 2

will not cause a runaway effect. Similar arguments may be given for the case where the load

characteristic shifts down. Therefore we conclude that point 2 is a stable operating point.


The reader is referred to fig which shows the complete speed-torque characteristic of the

induction machine along with the various regions of operation.

Let us consider a situation where the machine has just been excited with three phase

supply and the rotor has not yet started moving. A little reaction on the definition of the slip

indicates that we are at the point s = 1. When the rotating magnetic field is set up due to stator

currents, it is the induced emf that causes current in the rotor, and the interaction between the

two causes torque. It has already been pointed out that it is the presence of the non-zero slip

that causes a torque to be developed. Thus the region of the

Modes of Operation

torque, Nm From the foregoing discussions, we can say that the entire region of the

speed-torque characteristic from s = 0 to s = ^s is an unstable region, while the region from s =

^s to s = 0 is a stable region. Therefore the machine will always operate between s = 0 and s =

^s.


Figure: Stability of operating point

s = 0 and s = 1 is the region where the machine produces torque to rotate a passive load

and hence is called the motoring region. Note further that the direction of rotation of the rotor

is the same as that of the air gap flux.

Suppose when the rotor is rotating, we change the phase sequence of excitation to the

machine. This would cause the rotating stator field to reverse its direction the rotating stator

mmf and the rotor are now moving in opposite directions. If we adopt the convention that

positive direction is the direction of the air gap flux, the rotor speed would then be a negative

quantity. The slip would be a number greater than unity. Further, the rotor as we know should

be "dragged along" by the stator field. Since the rotor is rotating in the opposite direction to

that of the field, it would now tend to slow down, and reach zero speed.

Therefore this region (s > 1) is called the braking region. (What would happen if the

supply is not cut-off when the speed reaches zero?) . There is yet another situation. Consider a

situation where the induction machine is operating from mains and is driving an active load (a

load capable of producing rotation by itself). A typical example is that of a windmill, where

the fan like blades of the windmill are connected to the shaft of the induction machine.

Rotation of the blades may be caused by the motoring action of the machine, or by wind

blowing. Further suppose that both acting independently cause rotation in the same direction.

Now when both grid and wind act, a strong wind may cause the rotor to rotate faster than the

mmf produced by the stator excitation. A little reaction shows that slip is then negative.

Further, the wind is rotating the rotor to a speed higher than what the electrical supply

alone would cause. In order to do this it has to contend with an opposing torque generated by

the machine preventing the speed build up. The torque generated is therefore negative. It is

this action of the wind against the torque of the machine that enables wind-energy generation.

The region of slip s > 1 is the generating mode of operation. Indeed this is at present the most

commonly used approach in wind-energy

curve between


generation. It may be noted from the torque expression of equation that torque is negative

for negative values of slip.

Braking of D.C. shunt motor: basic idea

It is often necessary in many applications to stop a running motor rather quickly. We

know that any moving or rotating object acquires kinetic energy. Therefore, how fast we can

bring the object to rest will depend essentially upon how quickly we can extract its kinetic

energy and make arrangement to dissipate that energy somewhere else. If you stop pedaling

your bicycle, it will eventually come to a stop eventually after moving quite some distance.

The initial kinetic energy stored, in this case dissipates as heat in the friction of the road.

However, to make the stopping faster, brake is applied with the help of rubber brake shoes on

the rim of the wheels.

Thus stored K.E now gets two ways of getting dissipated, one at the wheel-brake shoe

interface (where most of the energy is dissipated) and the other at the road-tier interface. This

is a good method no doubt, but regular maintenance of brake shoes due to wear and tear is

necessary.

If a motor is simply disconnected from supply it will eventually come to stop no doubt,

but will take longer time particularly for large motors having high rotational inertia. Because

here the stored energy has to dissipate mainly through bearing friction and wind friction. The

situation can be improved, by forcing the motor to operate as a generator during braking. The

idea can be understood remembering that in motor mode electromagnetic torque acts along the

direction of rotation while in generator the electromagnetic torque acts in the opposite

direction of rotation. Thus by forcing the machine to operate as generator during the braking

period, a torque opposite to the direction of rotation will be imposed on the shaft, thereby

helping the machine to come to stop quickly. During braking action, the initial K.E stored in

the rotor is either dissipated in an external resistance or fed back to the supply or both.

Rheostat braking

Consider a d.c shunt motor operating from a d.c supply with the switch S connected to

position 1 as shown in figure. S is a single pole double throw switch and can be connected

either to position 1 or to position 2. One end of an external resistance Rb is connected to

position 2 of the switch S as shown.

Let with S in position 1, motor runs at n rpm, drawing an armature current Ia and the back

emf is


opposition with the supply voltage. Also note Te and n have same clockwise direction.

since Ia has changed direction but has notAs time passes after switching, n decreases reducing

K.E and as a consequence both Eb and Ia decrease. In other words value of braking torque

will be highest at t = 0+, and it decreases progressively and becoming zero when the machine

finally come to a stop.

Plugging or dynamic braking

This method of braking can be understood by referring to figures 39.25 and 39.26. Here

S is a double pole double throw switch. For usual motoring mode, S is connected to positions

1 and 1'.

Now if S is suddenly thrown to position 2 at t = 0, the armature gets disconnected from

the supply and terminated by Rb with field coil remains energized from the supply. Since

speed of the rotor cannot change instantaneously, the back emf value Eb is still maintained

with same polarity prevailing at t = 0-. Thus at t = 0+, armature current will be Ia = Eb/(ra +

Rb) and with reversed direction compared to direction prevailing during motor mode at t = 0-.

Obviously for t > 0, the machine is operating as generator dissipating power to Rb and

now the electromagnetic torque Te must act in the opposite direction to that of n


Across terminals 2 and 2', a series combination of an external resistance Rb and supply

voltage with polarity as indicated is connected. However, during motor mode this part of the

circuit remains inactive.

To initiate braking, the switch is thrown to position 2 and 2' at t = 0, thereby

disconnecting the armature from the left hand supply. Here at t = 0+, the armature current will

be Ia = (Eb + V)/(ra + Rb) as Eb and the right hand supply voltage have additive polarities by

virtue of the connection. Here also Ia reverses direction-producing Te in opposite direction to

n. Ia decreases as Eb decreases with time as speed decreases.

However, Ia cannot become zero at any time due to presence of supply V. So unlike

rheostatic braking, substantial magnitude of braking torque prevails. Hence stopping of the

motor is expected to be much faster then rheostat breaking.

But what happens, if S continuous to be in position 1' and 2' even after zero speed has

been attained? The answer is rather simple; the machine will start picking up speed in


the reverse direction operating as a motor. So care should be taken to disconnect the right

hand supply, the moment armature speed becomes zero.

Regenerative braking

A machine operating as motor may go into regenerative braking mode if its speed

becomes sufficiently high so as to make back emf greater than the supply voltage i.e., Eb > V.

Obviously under this condition the direction of Ia will reverse imposing torque which is

opposite to the direction of rotation. The situation is explained in figures 39.27 and 39.28. The

normal motor operation is shown in figure 39.27 where armature motoring current Ia is drawn

from the supply and as usual Eb < V. Since

The question is how speed on its own become large enough to make Eb < V causing

regenerative braking. Such a situation may occur in practice when the mechanical load itself

becomes active. Imagine the DC motor is coupled to the wheel of locomotive which is moving

along a plain track without any gradient as shown in figure. Machine is running as a motor at a

speed of n1 rpm. However, when the track has a downward gradient (shown in figure 39.28),

component of gravitational force along the track also appears which will try to accelerate the

motor and may increase its speed to n2

such that Eb. In such a scenario, direction of Ia reverses, feeding power back to supply.

Regenerative braking here will not stop the motor but will help to arrest rise of dangerously

high speed.


STARTING OF D.C. MACHINES:

For the machine to start, the torque developed by the motor at zero speed must exceed

that demanded by the load. Then TM _ TL will be positive so also is di=dt, and the machine

accelerates. The induced emf at starting point is zero as the i = 0 The armature current with

rated applied voltage is given by V=Ra where Ra is armature circuit resistance.

Normally the armature resistance of a d.c. machine is such as to cause 1 to 5 percent

drop at full load current. Hence the starting current tends to rise to several times the full load

current. The same can be told of the torque if full flux is already established. The machine

instantly picks up the speed. As the speed increases the induced emf appears across the

terminals opposing the applied voltage. The current drawn from the mains thus decreases, so

also the torque. This continues till the load torque and the motor torque are equal to each

other. Machine tends to run continuously at this speed, as the acceleration is zero at this point

of operation. The starting is now discussed with respect to specific machines.

DC shunt motor

UNIT-III

STARTING METHODS


If armature and field of DC shunt motor are energized together, large current is drawn

at start but the torque builds up gradually as the field flux increases gradually. To improve the

torque per ampere of line current drawn it is advisable to energize the field first. The starting

current is given by V=Ra and hence to reduce the starting current to a safe value, the voltage

V can be reduced or armature circuit resistance Ra can be increased. Variable voltage V can

be obtained from a motor generator set. This arrangement is called Ward-Leonard

arrangement. A schematic diagram of Ward- Leonard arrangement is shown in Fig. By

controlling the field of the Ward-Leonard generator one can get a variable voltage at its

terminals, which is used, for starting the motor. The second method of starting with increased

armature circuit resistance can be obtained by adding additional resistances in series with the

armature, at start. The current and the torque get reduced. The torque speed curve under these

conditions is shown in Fig.(a). It can be readily seen from this graph that the unloaded

machine reaches its final speed but a loaded machine may crawl at a speed much below the

normal speed. Also, the starting resistance wastes large amount of power. Hence the starting

resistance must be reduced to zero at the end of the starting process. This has to be done

progressively, making sure that the current does not jump up to large values. Starting of series

motor and compound motors are similar to the shunt motor. Better starting torques are

obtained for compound motors as the torque per ampere is more. Characteristics for series

motors are given in fig.

At the instant of starting, rotor speed n = 0, hence starting armature current is since,

armature resistance is quite small, starting current may be quite high (many times larger than

the rated current). A large machine, characterized by large rotor inertia (J), will pick up speed

rather slowly. Thus the level of high starting current may be maintained for quite some time so

as to cause serious damage to the brush or commutator and to the armature winding. Also the

source should be capable of supplying this burst of large current. The other loads already

connected to the same source, would experience a dip in the terminal voltage, every time a

D.C motor is attempted to start with full voltage. This dip in supply voltage is caused due to

sudden rise in voltage drop in the source's internal resistance. The duration for which this drop

in voltage will persist once again

We know armature current in a d.c motor is given by

1. Problems of starting with full voltage

Starting of D.C shunt motor

If the starting resistor is reduced in uniform steps then the current peaks reached as we

cut down the resistances progressively increase. To ascertain that at no step does

Grading of starting resistance for a shunt motor .


Depends on inertia (size) of the motor. Hence, for small D.C motors extra precaution may not

be necessary during starting as large starting current will very quickly die down because of

fast rise in the back emf. However, for large motor, a starter is to be used during starting.

2. A simple starter

To limit the starting current, a suitable external resistance R ext is connected in

series (Figure (a)) with the armature so that at the time of starting, to have sufficient

starting torque, field current is maximized by keeping the external field resistance Rf,

to zero value.

As the motor picks up speed, the value of R ext is gradually decreased to zero so that

during running no external resistance remains in the armature circuit. But each time one has to

restart the motor, the external armature resistance must be set to maximum value by moving

the jockey manually.

Now if the supply goes off (due to some problem in the supply side or due to load

shedding), motor will come to a stop. All on a sudden, let us imagine, supply is restored. This

is then nothing but full voltage starting. In other words, one should be constantly alert to set

the resistance to maximum value whenever the motor comes to a stop. This is one major

limitation of a simple rheostat starter.

3. 3-point starter

A “3-point starter” is extensively used to start a D.C shunt motor. It not only overcomes

the difficulty of a plain resistance starter, but also provides additional protective features such

as over load protection and no volt protection. The diagram of a 3-point starter connected to a

shunt motor is shown in figure . Although, the circuit looks a bit clumsy at a first glance, the

basic working principle is same as that of plain resistance starter The starter is shown enclosed

within the dotted rectangular box having

Imagine, the motor to be running with R ext = 0 (Figure (b)).


three terminals marked as A, L and F for external connections. Terminal A is connected to one

armature terminal Al of the motor. Terminal F is connected to one field terminal F1 of the

motor and terminal L is connected to one supply terminal as shown. F2 terminal of field coil is

connected to A2 through an external variable field resistance and the common point connected

to supply (-ve). The external armatures resistances consist of several resistances connected in

series and are shown in the form of an arc. The junctions of the resistances are brought out as

terminals (called studs) and marked as 1,2,.. .12. Just beneath the resistances, a continuous

copper strip also in the form of an arc is present.

There is a handle which can be moved in the clockwise direction against the spring

tension. The spring tension keeps the handle in the OFF position when no one attempts to

move it. Now let us trace the circuit from terminal L (supply +ve). The wire from L passes

through a small electro magnet called OLRC, (the function of which we shall discuss a little

later) and enters through the handle shown by dashed lines. Near the end of the handle two

copper strips are firmly connected with the wire. The furthest strip is shown circular shaped

and the other strip is shown to be rectangular. When the handle is moved to the right, the

circular strip of the handle will make contacts with resistance terminals 1, 2 etc. progressively.

On the other hand, the rectangular strip will make contact with the continuous arc copper strip.

The other end of this strip is brought as terminal F after going through an electromagnet coil

(called NVRC). Terminal F is finally connected to motor field terminal Fl.


4. Working principle

Let us explain the operation of the starter. Initially the handle is in the OFF position.

Neither armature nor the field of the motor gets supply. Now the handle is moved to stud

number 1. In this position armature and all the resistances in series gets connected to the

supply. Field coil gets full supply as the rectangular strip makes contact with arc copper strip.

As the machine picks up speed handle is moved further r to stud number 2. In this position the

external resistance in the armature circuit is less as the first resistance is left out. Field

however, continues to get full voltage by virtue of the continuous arc strip. Continuing in this

way, all resistances will be left out when stud number 12 (ON) is reached. In this position, the

electromagnet (NVRC) will attract the soft iron piece attached to the handle. Even if the

operator removes his hand from the handle, it will still remain in the ON position as spring

restoring force will be balanced by the force of attraction between NVRC and the soft iron

piece of the handle. The no volt release coil (NVRC) carries same current as that of the field

coil. In case supply voltage goes off, field coil current will decrease to zero. Hence NVRC will

be reenergized and will not be able to exert any force on the soft iron piece of the handle.

Restoring force of the spring will bring the handle back in the OFF position.

The starter also provides over load protection for the motor. The other electromagnet,

OLRC overload release coil along with a soft iron piece kept under it, is used to achieve this.

The current flowing through OLRC is the line current IL drawn by the motor. As the motor is

loaded, Ia hence IL increases. Therefore, IL is a measure of loading of the motor. Suppose we

want that the motor should not be over loaded beyond rated current. Now gap between the

electromagnet and the soft iron piece is so adjusted

that forth iron piece will not be pulled up. However, if rated I I force of attraction will be

sufficient to pull up iron piece. This upward movement of the iron piece of OLRC is utilized

to de-energize NVRC. To the iron a copper strip (Ä shaped in figure) is attached. During over

loading condition, this copper strip will also move up and put a short circuit between two

terminals B and C. Carefully note that B and C are nothing but the two ends of the NVRC. In

other words, when over load occurs a short circuit path is created across the NVRC. Hence

NVRC will not carry any current now and gets reenergized. The moment it gets reenergized,

spring action will bring the handle in the OFF position thereby disconnecting the motor from

the supply.

Three-point starter has one disadvantage. If we want to run the machine at higher speed

(above rated speed) by field weakening (i.e., by reducing field current), the strength of NVRC

magnet may become so weak that it will fail to hold the handle in the ON position and the

spring action will bring it back in the OFF position. Thus we find that a false disconnection of

the motor takes place even when there is neither over load nor any sudden disruption of

supply.

DIFFERENT TYPES OF STARTERS FOR 3 PHASE INDUCTION MOTOR (IM)

INSTRUCTIONAL OBJECTIVES


• Need of using starters for Induction motor

• Two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage Induction

motor

• Starter using additional resistance in rotor circuit, for Wound rotor (Slip-ring) Induction

motor

Introduction

Induction motors can be started Direct-on-Line (DOL), which means that the rated

voltage is supplied to the stator, with the rotor terminals short-circuited in a wound rotor (slip-

ring) motor. For the cage rotor, the rotor bars are short circuited via two end rings. Neglecting

stator impedance, the starting current in the stator windings

Direct-on-Line (DOL) Starters

Direct-on-Line (DOL) starter, Star-delta starter, auto-transformer starter, rotor resistance

starter, starting current, starting torque, starters for squirrel cage and wound rotor induction

motor, need for starters.

In the previous, i.e. fourth, lesson of this module, the expression of gross torque

developed, as a function of slip (speed), in IM has been derived first. The sketches of the

different torque-slip (speed) characteristics, with the variations in input (stator) voltage and

rotor resistance, are presented, along with the explanation of their features. Lastly, the

expression of maximum torque developed and also the slip, where it occurs, have been

derived. In this lesson, starting with the need for using starters in IM to reduce the starting

current, first two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage IM

and then, the starter using additional resistance in rotor circuit, for Wound rotor (Slip-ring)

IM, are presented along with the starting current drawn from the input (supply) voltage, and

also the starting torque developed using the above starters.


Need for Starters in IM

torque is which shows that, as the starting current increases, the starting torque also increases.

This results in higher accelerating torque (minus the load torque and the torque component of

the losses), with the motor reaching rated or near rated speed quickly.

The input voltage per phase to the stator is equal to the induced emf per phase in the stator

winding, as the stator impedance is neglected (also shown in the last lesson (#32)). In the

formula for starting current, no load current is neglected. It may be noted that the starting

current is quite high, about 4-6 times the current at full load, may be higher, depending on the

rating of IM, as compared to no load current. The starting


Starters for Cage IM

the motor (stator), i.e.

The starting current in IM is proportional to the input voltage per phase

, where,

The main problem in starting induction motors having large or medium size lies

mainly in the requirement of high starting current, when started direct-on-line (DOL). Assume

that the distribution line is starting from a substation (Fig.), where the supply voltage is

constant. The line feeds a no. of consumers, of which one consumer has an induction motor

with a DOL starter, drawing a high current from the line, which is higher than the current for

which this line is designed. This will cause a drop (dip) in the voltage, all along the line, both

for the consumers between the substation and this consumer, and those, who are in the line

after this consumer. This drop in the voltage is more than the drop permitted, i.e. higher than

the limit as per ISS, because the current drawn is more than the current for which the line is

designed. Only for the current lower the current for which the line is designed, the drop in

voltage is lower the limit. So, the supply authorities set a limit on the rating or size of IM,

which can be started DOL. Any motor exceeding the specified rating, is not permitted to be

started DOL, for which a starter is to be used to reduce the current drawn at starting.

to

as the voltage drop in the

stator impedance is small compared to the input voltage, or if the stator impedance is

neglected. This has been shown earlier. So, in a (squirrel) cage induction motor, the starter is

used only to decrease the input voltage to the motor so as to decrease the starting current. As

described later, this also results in decrease of starting torque.


is of the starting current, if DOL starter is used. The voltage per phase in each stator

winding is now (. 3 / s V ). So, the starting current using star-delta starter is reduced by 33.3%.

As for starting torque, being proportional to the square of the current in each of the stator

windings in two different connections as shown earlier, is also reduced by ( 2 ) 3 / 1 ( 3 / 1 = ),

as the ratio of the two currents is ( 3 / 1 ), same as that (ratio) of the voltages applied to each

winding as shown earlier. So, the starting torque is reduced by 33.3%, which is a disadvantage

of the use of this starter. The load torque and the loss torque, must be lower than the starting

torque, if the motor is to be started using this starter. The advantage is that, no extra

component, except that shown in Fig. 33.2c, need be used, thus making it simple. As shown

later, this is an auto-transformer starter with the voltage ratio as 57.7%. Alternatively, the

starting current in the second case with the stator winding reconnected as star, can be found by

using star-

phase or line current drawn from supply at start (standstill) is

the input current to the motor is which is the current, if the motor is started direct-on-line

(DOL). Now, if the stator winding is connected as star, the

, where is the impedance of the motor per phase at standstill or start (stator impedance

and rotor impedance referred to the stator, at standstill). The line current or

This type is used for the induction motor, the stator winding of which is nominally delta-

connected (Fig. 33.2a). If the above winding is reconnected as star (Fig.33.2b), the voltage per

phase supplied to each winding is reduced by 3(.577). This is a simple starter, which can

be easily reconfigured as shown in Fig. 33.2c. As the voltage per phase in delta connection is

V s, the phase current in each stator winding is

which


delta conversion as given in lesson #18, with the impedance per phase after converting to

delta, found as ( s Z · 3 ), and the starting current now being reduced to (1/3 ) of the starting

current obtained using DOL starter, with the stator winding connected in delta.

Auto-transformer Starter


Let be the starting current, when the motor is started using DOL starter, (i.e.,) applying

rated input voltage. The input current of IM, which is the output current of auto-transformer,

when this starter is used with input voltage. The input current of auto-transformer, which is

the starting current drawn from the supply, is obtained by equating input and output volt-

amperes, neglecting losses and assuming nearly same power factor on both sides. As

discussed earlier, the starting torque, being proportional to the square of the input current to

IM in two cases, with and without auto-transformer (i.e. direct), is also reduced by , as the

ratio of the two currents is same as that (ratio) of the voltages applied to the motor as shown

earlier. So, the starting torque is reduced by the same ratio as that of the starting current.

If the ratio is both starting current and torque are %) 80 ( 8 . 0 = x %) 64 ( 64 . 0) 8 . 0 (

2 2 = = x times the values of starting current and torque with DOL starting, which is nearly 2

times the values obtained using star-delta starter. So, the disadvantage is that starting current is

increased, with the result that lower rated motor can now be started, as the current drawn from

the supply is to be kept within limits, while the advantage is that the starting torque is now

doubled, such that the motor can start against higher load torque. The star-delta starter can be

considered equivalent to an autotransformer starter with the ratio, %) 7 . 57 ( 577 . 0 = x . If

%) 70 ( 7 . 0 = x , both starting current and

An auto-transformer, whose output is fed to the stator and input is from the supply

(Fig. 33.3), is used to start the induction motor. The input voltage of IM is, which is the output

voltage of the auto-transformer, the input voltage being V s. The output voltage/input voltage

ratio is x , the value of which lies between 0.0 and 1.0


torque are times the values of starting current and torque with DOL starting, which is nearly

1.5 times the values obtained using star delta starter.

Rotor Resistance Starters for Slip-ring (wound rotor) IM

In a slip-ring (wound rotor) induction motor, resistance can be inserted in the rotor

circuit via slip rings (Fig. 33.4), so as to increase the starting torque. The starting current in

the rotor winding is

where = Additional resistance per phase in the rotor circuit.

increases, as the total resistance in starting torque the rotor circuit is increased. Though the

starting current decreases, the total resistance increases, thus resulting in increase of starting

torque as shown in Fig. 32.2b, and also obtained by using the expression given earlier, for

increasing values of the resistance in the rotor circuit. If the additional resistance is used only

for starting, being rated for intermittent duty, the resistance is to be decreased in steps, as the

motor speed increases. Finally, the external resistance is to be completely cut out, i.e. to be

made equal to zero (0.0), thus leaving the slip-rings short-circuited. Here, also the additional

cost of the external resistance with intermittent rating is to be incurred, which results in

decrease of starting current, along with increase of starting torque, both being advantageous.

Also it may be noted that the cost of a slip-ring induction is higher than that of IM with cage

rotor, having same power rating. So, in both cases, additional cost is to be incurred to obtain

the above advantages. This is only used in case higher starting torque is needed to

The input (stator) current is proportional to the rotor current as shown earlier. The starting

current (input) reduces, as resistance is inserted in the rotor circuit. But the


start IM with high load torque. It may be observed from Fig. 32.2b that the starting torque

increases till it reaches maximum value,

rotor circuit is increased, the range of total resistance being

The range of external resistance is between

zero (0.0) and

torque is equal to the maximum value, i.e.

inserted is equal to

further,

2 r x -). The starting

if the external resistance

the external resistance in the

if the external resistance in the rotor circuit is increased

the starting torque decreases. This is, because the

STARTING METHODS FOR SINGLE-PHASE INDUCTION MOTOR

Instructional Objectives

• Why there is no starting torque in a single-phase induction motor with one (main) winding in

the stator?

• Various starting methods used in the single-phase induction motors, with the introduction of

additional features, like the addition of another winding in the stator, and/or capacitor in series

with it.

Introduction

In the previous, i.e. fifth, lesson of this module, the direct-on-line (DOL) starter used

in three-phase IM, along with the need for starters, has been described first. Two types of

starters - star-delta, for motors with nominally delta-connected stator winding, and

autotransformer, used for cage rotor IM, are then presented, where both decrease in starting

current and torque occur. Lastly, the rotor resistance starter for slip-ring (wound rotor) IM has

been discussed, where starting current decreases along with increase in starting torque. In all

such

This is, because the starting current decreases at a faster rate, even if the total

resistance in the rotor circuit is increased.

In this lesson - the fifth one of this module, the direct-on-line (DOL) starter used for

IM, along with the need for other types of starters, has been described first. Then, two types of

starters - star-delta and autotransformer, for cage type IM, are presented. Lastly, the rotor

resistance starter for slip-ring (wound rotor) IM is briefly described. In the next (sixth and last)

lesson of this module, the various types of single-phase induction motors, along with the

starting methods, will be presented.


cases, additional cost is to be incurred. In the last (sixth) lesson of this module, firstly it is

shown that there is no starting torque in a single-phase induction motor with only one (main)

winding in the stator. Then, the various starting methods used for such motors, like, say, the

addition of another (auxiliary) winding in the stator, and/or capacitor in series with it.

Keywords:

Single-phase induction motor, starting torque, main and auxiliary windings, starting

methods, split-phase, capacitor type, motor with capacitor start/run.

Single-phase Induction Motor

The winding used normally in the stator (Fig.) of the single-phase induction motor

(IM) is a distributed one. The rotor is of squirrel cage type, which is a cheap one, as the rating

of this type of motor is low, unlike that for a three-phase IM. As the stator winding is fed from

a single-phase supply, the flux in the air gap is alternating only, not a synchronously rotating

one produced by a poly-phase (may be two- or three-) winding in the stator of IM. This type of

alternating field cannot produce a torque

if the rotor is stationery

torque in either direction then immediately a torque is produced in the motor. The motor then

accelerates to its final speed, which is lower than its synchronous speed. This is now explained

using double field revolving theory.

Double field revolving theory

so, a single-phase IM is not self-starting, unlike a three-phase one. However, as shown later, if

the rotor is initially given some


in opposite directions. This is shown in Fig. The first set of figures (Fig. 34.1a (i-iv)) show the

resultant sum of the two rotating fluxes or fields, as the time axis (angle) is changing from Fig.

shows the alternating or pulsating flux (resultant)

varying with time or angle.

When the stator winding (distributed one as stated earlier) carries a sinusoidal current (being

fed from a single-phase supply), a sinusoidal space distributed mmf, whose peak or maximum

value pulsates (alternates) with time, is produced in the air gap. This sinusoidal varying flux is

the sum of two rotating fluxes or fields, the magnitude of

which is equal to half the value of the alternating flux

and both the fluxes rotating synchronously at the speed,


(space

torques produced (Fig. 34.3). If the rotor is stationarythe slip due to forward (anticlockwise)

rotating field is 0 . 1 = sf . Similarly, the slip due to backward rotating field is also sb = 0 .1.

The two torques are equal and opposite, and the resultant torque is 0.0 (zero). So, there is no

starting torque in a single-phase IM.

But, if the motor (rotor) is started or rotated somehow, say in the anticlockwise

(forward) direction, the forward torque is more than the backward torque, with the resultant

torque now being positive. The motor accelerates in the forward direction, with the forward

torque being more than the backward torque. The resultant torque is thus positive as the motor

rotates in the forward direction. The motor speed is decided by the load torque supplied,

including the losses (specially mechanical loss).

Mathematically, the mmf, which is distributed sinusoidal in space, with its peak value

pulsating with time, is described as

The flux or field rotating at synchronous speed, say, in the anticlockwise direction, i.e.

the same direction, as that of the motor (rotor) taken as positive induces emf (voltage) in the

rotor conductors. The rotor is a squirrel cage one, with bars short circuited via end rings. The

current flows in the rotor conductors, and the electromagnetic torque is produced in the same

direction as given above, which is termed as positive (+ve). The other part of flux or field

rotates at the same speed in the opposite (clockwise) direction, taken as negative. So, the

torque produced by this field is negative (-ve), as it is in the clockwise direction, same as that

of the direction of rotation of this field. Two torques are in the opposite direction, and the

resultant (total) torque is the difference of the two


angle) measured from the winding axis. Now,

is distributed both in space and time, i.e. it can be

expressed as,

So, the mmf

which shows that a pulsating field can be considered as the sum of two synchronously rotating

fields

and the backward rotating field is

The forward rotating field is,

Both the fields have the same amplitude equal to

with angular speed the slip due to the forward rotating field is,

is the maximum value of the pulsating mmf along the axis of the winding. When

the motor rotates in the forward (anticlockwise) direction

Similarly, the slip

due to the backward rotating field, the speed of which is

The resultant torque is,

It was earlier shown that, when the rotor is stationary,

with both as

The torque produced by the two fields is in opposite direction.

Therefore, the resultant torque at start is 0.0 (zero).

STARTING METHODS

The single-phase IM has no starting torque, but has resultant torque, when it rotates at

any other speed, except synchronous speed. It is also known that, in a balanced two-phase IM

having two windings, each having equal number of turns and placed at a space angle of 90o

(electrical), and are fed from a balanced two-phase supply, with two voltages equal in

magnitude, at an angle of 90o, the rotating magnetic fields are produced, as in a three-phase

IM. The torque-speed characteristic is same as that of a three-phase one, having both starting

and also running torque as shown earlier. So, in a single-phase IM, if an auxiliary winding is

introduced in the stator, in addition to the main winding, but placed at a space angle of 90o

(electrical), starting torque is produced.


The currents in the two (main and auxiliary) stator windings also must be at an angle of 90o,

to produce maximum starting torque, as shown in a balanced two-phase stator. Thus, rotating

magnetic field is produced in such motor, giving rise to starting torque. The various starting

methods used in a single-phase IM are described here.

Resistance Split-phase Motor

The schematic (circuit) diagram of this motor is given in Fig. As detailed earlier, another

(auxiliary) winding with a high resistance in series is to be added along with the main winding

in the stator. This winding has higher resistance to reactance

placed at a space angle of from the main winding as given earlier. The phasor diagram of the

currents in two windings and the input voltage is shown in Fig. 34.4b. The current (Ia ) in the

auxiliary winding lags the voltage (V ) by an angle,

which is small, whereas the current (Im ) in the main winding lags the

which is nearly 90o. The phase angle between the voltage (V ) by an angle,

two currents is which should be at least 30°. This results in a small amount of starting torque.

The switch, S (centrifugal switch) is in series with the auxiliary winding. It automatically cuts

out the auxiliary or starting winding, when the motor attains a speed close to full load speed.

The motor has a starting torque of 100-200% of full load torque, with the starting current as 5-

7 times the

ratio as compared to that in the main winding and is


full load current. The torque-speed characteristics of the motor with/without auxiliary winding

are shown in Fig. The change over occurs, when the auxiliary winding is switched off as given

earlier. The direction of rotation is reversed by reversing the terminals of any one of two

windings, but not both, before connecting the motor to the supply terminals. This motor is

used in applications, such as fan, saw, small lathe, centrifugal pump, blower, office

equipment, washing machine, etc.

The motor described earlier, is a simple one, requiring only second (auxiliary) winding placed

at a space angle of 90 o from the main winding, which is there in nearly all such motors as

discussed here. It does not need any other thing, except for centrifugal switch, as the auxiliary

winding is used as a starting winding. But the main problem is low starting torque in the

motor, as this torque is a function of, or related to the phase difference (angle) between the

currents in the two windings. To get high starting torque, the phase difference required is

90°(Fig. 34.5b), when the starting torque will be proportional to the product of the magnitudes

of two currents. As the current in the main winding is lagging by

Capacitor-start Motor

is taken as negative (-ve), while is positive (+ve). This can be can be achieved by having a

capacitor in series with the auxiliary winding, which results in additional cost, with the

increase in starting torque. The two types of such motors are described here.

the current in the auxiliary winding has to lead the input voltage by with


Capacitor-start and Capacitor-run Motor

The schematic (circuit) diagram of this motor is given in Fig. It may be observed that a

capacitor along with a centrifugal switch is connected in series with the auxiliary winding,

which is being used here as a starting winding. The capacitor may be rated only for

intermittent duty, the cost of which decreases, as it is used only at the time of starting. The

function of the centrifugal switch has been described earlier. The phasor diagram of two

currents as described earlier, and the torque-speed characteristics of the motor with/without

auxiliary winding, are shown in Fig. respectively. This motor is used in applications, such as

compressor, conveyor, machine tool drive, refrigeration and air-conditioning equipment, etc.


in the first case (starting), while it is for between the two currents is 90° second case

(running). In the second case, the motor is a balanced two phase one, the two windings having

same number of turns and other conditions as given earlier, are also satisfied. So, only the

forward rotating field is present, and the no backward rotating field exists. The efficiency of

the motor under this condition is higher. Hence, using two capacitors, the performance of the

motor improves both at the time of starting and then running. This motor is used in

applications, such as compressor, refrigerator, etc.

In this motor (Fig. 34.6a), two capacitors -Cs for starting, and Cr for running, are used.

The first capacitor is rated for intermittent duty, as described earlier, being used only for

starting. A centrifugal switch is also needed here. The second one is to be rated for continuous

duty, as it is used for running. The phasor diagram of two currents in both cases, and the

torque-speed characteristics with two windings having different values of capacitors, are

shown in Fig. 34.6b and Fig. 34.6c respectively. The phase difference


Shaded-pole Motor

Let the main winding flux be

A typical shaded-pole motor with a cage rotor is shown in Fig. This is a single phase

induction motor, with main winding in the stator. A small portion of each pole is covered with

a short-circuited, single-turn copper coil called the shading coil. The sinusoidal varying flux

created by ac (single-phase) excitation of the main winding induces emf in the shading coil.

As a result, induced currents flow in the shading coil producing their own flux in the shaded

portion of the pole.

Beside the above two types of motors, a Permanent Capacitor Motor (Fig.) with the

same capacitor being utilized for both starting and running, is also used. The power factor of

this motor, when it is operating (running), is high. The operation is also quiet and smooth.

This motor is used in applications, such as ceiling fans, air circulator, blower, etc.


where

As per the above equations, the shading coil current (I sc ) and flux

phasors lag behind the induced emf (Esc ) by angle

while the flux

is in phase with

by the angle

The resultant flux in the shaded pole is given by the phasor sum

lags the flux in the unshaded portion of the pole resulting in a net pole torque, which causes

the rotor to rotate from the unshaded to the shaded portion of the pole. The motor thus has a

definite direction of rotation, which cannot be reversed. The reversal of the direction of

rotation, where desired, can be achieved by providing two shading coils, one on each end of

every pole, and by open-circuiting one set of shading coils and by short-circuiting the other

set. The fact that the shaded-pole motor is single-winding (no auxiliary winding) self-starting

one, makes it less costly and results in rugged construction. The motor has low efficiency and

is usually available in a range of 1/300 to 1/20 kW. It is used for domestic fans, record players

and tape recorders, humidifiers, slide projectors, small business machines, etc. The shaded-

pole principle is used in starting electric clocks and other single-phase synchronous timing

motors.

space as well as have a time phase difference there by producing forward and backward

rotating fields, which produce a net torque. It may be noted that the motor is self-starting

unlike a single-phase single-winding motor. It is seen from the phasor diagram (Fig. 34.8b)

that the net flux in the shaded portion of the

The two sinusoidal varying fluxes

as shown in Fig. and lags the flux

phasor leads the induced emf (Esc ) by 90o. Obviously the phasor


In this lesson - the sixth and last one of this module, firstly, it is shown that, no starting

torque is produced in the single-phase induction motor with only one (main) stator winding, as

the flux produced is a pulsating one, with the winding being fed from single phase supply.

Using double revolving field theory, the torque-speed characteristics of this type of motor are

described, and it is also shown that, if the motor is initially given some torque in either

direction, the motor accelerates in that direction, and also the torque is produced in that

direction. Then, the various types of single-phase induction motors, along with the starting

methods used in each one are presented. Two stator windings - main and auxiliary, are needed

to produce the starting torque. The merits and demerits of each type, along with their

application area, are presented. The process of production of starting torque in shade-pole

motor is also described in brief. In the next module consisting of seven lessons, the

construction and also operation of dc machines, both as generator and motor, will be

discussed.


UNIT-IV

CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES

SPEED CONTROL OF D.C. MOTORS

In the case of speed control, armature voltage control and flux control methods are

available. The voltage control can be from a variable voltage source like Ward-Leonard

arrangement or by the use of series armature resistance. Unlike the starting conditions the

series resistance has to be in the circuit throughout in the case of speed control. That means

considerable energy is lost in these resistors. Further these resistors must be adequately cooled

for continuous operation. The variable voltage source on the other hand gives the motor the

voltage just needed by it and the losses in the control gear is a minimum. This method is

commonly used when the speed ratio required is large, as also the power rating.

Field control or flux control is also used for speed control purposes. Normally field

weakening is used. This causes operation at higher speeds than the nominal speed.

Strengthening the field has little scope for speed control as the machines are already in a state

of saturation and large field mmf is needed for small increase in the flux. Even though flux

weakening gives higher speeds of operation it reduces the torque produced by the machine for

a given armature current and hence the power delivered does not increase at any armature

current. The machine is said to be in constant power mode under field weakening mode of

control. Above the nominal speed of operation, constant ux mode with increased applied

voltage can be used; but this is never done as the stress on the commutator insulation

increases.

Thus operation below nominal speed is done by voltage control. Above the nominal

speed field weakening is adopted. For weakening the field, series resistances are used for

shunt as well as compound motors. In the case of series motors however field weakening is

done by the use of „diverters‟. Diverters are resistances that are connected in parallel to the

series winding to reduce the field current without affecting the armature current.

We know that the speed of shunt motor is given by:

where, Va is the voltage applied across the armature and ö is the flux per pole and is

proportional to the field current If. As explained earlier, armature current Ia is decided by the

mechanical load present on the shaft. Therefore, by varying Va and If we can vary n. For fixed

supply voltage and the motor connected as shunt we can vary Va by controlling an external

resistance connected in series with the armature. If of course can be varied by controlling

external field resistance Rf connected with the field circuit. Thus for shunt motor we have

essentially two methods for controlling speed, namely by:

Speed control of shunt motor


1. varying armature resistance.

2. varying field resistance.

1. Speed control by varying armature resistance

The inherent armature resistance ra being small, speed n versus armature current Ia

characteristic will be a straight line with a small negative slope as shown in figure. In the

discussion to follow we shall not disturb the field current from its rated value. At no

load (i.e., Ia = 0) speed is highest and Note that for shunt motor voltage applied to the field

and armature circuit are same and equal to the supply voltage V. However, as the motor is

loaded, Ia ra drop increases making speed a little less than the no load speed n0. For a well-

designed shunt motor this drop in speed is small and about 3 to 5% with respect to no load

speed. This drop in speed from no load to full load condition expressed as a percentage of no

load speed is called the inherent speed regulation of the motor.

It is for this reason, a d.c shunt motor is said to be practically a constant speed motor

(with no external armature resistance connected) since speed drops by a small amount from no

load to full load condition.

Since for constant operation, Te becomes simply proportional to Ia. Therefore, speed vs.

torque characteristic is also similar to speed vs. armature current characteristic as shown in

figure. The slope of the n vs Ia or n vs Te characteristic can be modified by deliberately

connecting external resistance rext in the armature circuit. One can get a family of speed


vs. armature curves as shown in figures for various values of r ext. From these characteristics

it can be explained how speed control is achieved. Let us assume that the load torque TL is

constant and field current is also kept constant. Therefore, since steady state operation

demands Te = TL, Te = a k I ö too will remain constant; which means Ia will not change.

Suppose rext = 0, then at rated load torque, operating point will be at C and motor speed will

be n. If additional resistance rext1 is introduced in the armature circuit, new steady state

operating speed will be n1 corresponding to the operating point D. In this way one can get a

speed of n2 corresponding to the operating point E, when rext2 is introduced in the armature

circuit. This same load torque is supplied at various speed. Variation of the speed is smooth

and speed will decrease smoothly if r ext is increased.

Obviously, this method is suitable for controlling speed below the base speed and for

supplying constant rated load torque which ensures rated armature current always. Although,

this method provides smooth wide range speed control (from base speed down to zero speed),

has a serious draw back since energy loss takes place in the external resistance r ext reducing

the efficiency of the motor.


2. Speed control by varying field current

In this method field circuit resistance is varied to control the speed of a d.c shunt

motor. Let us rewrite .the basic equation to understand the method.

torque remains constant and flux is reduced to

state is obtained from

value. So from the initial steady condition, we have

speed no corresponding to . However, this method will not be suitable for constant load

torque. To make this point clear, let us assume that the load torque is constant at rated

values and Since no load speed

Speed versus armature current characteristic is shown in figure for two flux

for flux value is than the no load

If we vary If, flux will change, hence speed will vary. To change If an external

resistance is connected in series with the field windings. The field coil produces rated flux

when no external resistance is connected and rated voltage is applied across field coil. It

should be understood that we can only decrease flux from its rated value by adding external

resistance. Thus the speed of the motor will rise as we decrease the field current and speed

control above the base speed will be achieved.

If load

new armature current in the steady

Therefore new armature current is

But the fraction, hence new armature current will be greater than the rated armature

current and the motor will be overloaded. This method therefore, will be suitable for a load

whose torque demand decreases with the rise in speed keeping the output power constant as

shown in figure. Obviously this method is based on flux weakening of the main field.

Therefore at higher speed main flux may become so weakened, that armature reaction effect

will be more pronounced causing problem in commutation.


3. Speed control by armature voltage variation

In this method of speed control, armature is supplied from a separate variable d.c

voltage source, while the field is separately excited with fixed rated voltage as shown in

figure. Here the armature resistance and field current are not varied. Since the no load

Speed the speed versus Ia characteristic will shift parallel as shown in figure for different

values of Va.


As flux remains constant, this method is suitable for constant torque loads. In a way

armature voltage control method is similar to that of armature resistance control method

except that the former one is much superior as no extra power loss takes place in the armature

circuit. Armature voltage control method is adopted for controlling speed from base speed

down to very small speed, as one should not apply across the armature a voltage, which is

higher than the rated voltage.

4. Ward Leonard method: combination of Va and If control

In this scheme, both field and armature control are integrated as shown in figure.

Arrangement for field control is rather simple. One has to simply connect an appropriate

rheostat in the field circuit for this purpose. However, in the pre power electronic era,

obtaining a variable d.c supply was not easy and a separately excited d.c generator was used to

supply the motor armature. Obviously to run this generator, a prime mover is required. A 3-

phase induction motor is used as the prime mover which is supplied from a 3-phase supply.

By controlling the field current of the generator, the generated emf,


hence Va can be varied. The potential divider connection uses two rheostats in parallel

to facilitate reversal of generator field current. First the induction motor is started with

generator field current zero (by adjusting the jockey positions of the rheostats). Field

supply of the motor is switched on with motor field rheostat set to zero. The applied

voltage to the motor Va, can now be gradually increased to the rated value by slowly

increasing the generator field current. In this scheme, no starter is required for the d.c

motor as the applied voltage to the armature is gradually increased. To control the

speed of the d.c motor below base speed by armature voltage, excitation of the d.c

generator is varied, while to control the speed above base speed field current of the d.c

motor is varied maintaining constant Va. Reversal of direction of rotation of the motor

can be obtained by adjusting jockeys of the generator field rheostats. Although, wide

range smooth speed control is achieved, the cost involved is rather high as we require

one additional d.c generator and a 3-phase induction motor of similar rating as that of

the d.c motor whose speed is intended to be controlled.

In present day, variable d.c supply can easily be obtained from a.c supply by

using controlled rectifiers thus avoiding the use of additional induction motor and

generator set to implement Ward Leonard method.

Series motor


In this motor the field winding is connected in series with the armature and the

combination is supplied with d.c voltage as depicted in figure 39.13. Unlike a shunt

motor, here field current is not independent of armature current. In fact, field and

armature currents are equal i.e.,

Now torque produced in a d.c motor is:

Torque vs. armature current characteristic

Since in the linear zone and characteristic is as

shown in figure the saturation zone, the T vs. Ia

Speed vs. armature current

From the KVL equation of the motor, the relation between speed and armature current can

be obtained as follows:

1. Characteristics of series motor

Since torque is proportional to the square of the armature current, starting torque of a

series motor is quite high compared to a similarly rated d.c shunt motor. .


Remember that the value of Ia, is a measure of degree of loading. Therefore, a series

motor should never be operated under no load condition. Unlike a shunt motor, a series motor

has no finite no load speed. Speed versus armature current characteristic is shown in figure

side

The relationship is inverse in nature making speed dangerously high as


Since in the linear zone, the relationship between speed and torque is

1. Speed control below base speed

For constant load torque, steady armature current remains constant, hence flux also

remains constant. Since the machine resistance as r +re is quite small, the back emf Eb is

approximately equal to the armature terminal voltage Va. Therefore, speed is proportional to

Va. If Va is reduced, speed too will be reduced. This Va can be controlled either by

connecting external resistance in series or by changing the supply voltage.

Series-parallel connection of motors

If for a drive two or more (even number) of identical motors are used (as in traction),

the motors may be suitably connected to have different applied voltages across the motors for

controlling speed. In series connection of the motors shown in figure, the applied voltage

across each motor is V/2 while in parallel connection shown in figure, the applied voltage

across each motor is V. The back emf in the former case will be approximately half than that

in the latter case. For same armature current in both the cases (which means flux per pole is

same), speed will be half in series connection compared to parallel connection.

Speed control of series motor

Represent appropriate constants to take into account the proportionality that exist

between current, torque and flux in the linear zone. This relation is also inverse in nature

indicating once again that at light load or no load condition; series motor speed approaches a

dangerously high value. The characteristic is shown in figure. For this reason, a series motor is

never connected to mechanical load through belt drive. If belt snaps, the motor becomes

unloaded and as a consequence speed goes up unrestricted causing mechanical damages to the

motor.


2. Speed control above base speed

Flux or field current control is adopted to control speed above the base speed. In a

series motor, independent control of field current is not so obvious as armature and field coils

are in series. However, this can be achieved by the following methods:

1. Using a diverter resistance connected across the field coil.

In this method shown in figure 39.19, a portion of the armature current is diverted

through the diverter resistance. So field current is now not equal to the armature current; in

fact it is less than the armature current. Flux weakening thus caused, raises the speed of the

motor.


2. Changing number of turns of field coil provided with tapings.

In this case shown figure 39.20, armature and field currents are same. However

provision is kept to change the number of turns of the field coil. When number of turns

changes, field mmf se f N I changes, changing the flux hence speed of the motor.

3. Connecting field coils wound over each pole in series or in. parallel.

Generally the field terminals of a d.c machine are brought out after connecting the

field coils (wound over each pole) in series. Consider a 4-pole series motor where there will

be 4 individual coils placed over the poles. If the terminals of the individual coils are


brought out, then there exist several options for connecting them. The four coils could be

connected in series as in figure 39.21; the 4 coils could be connected in parallel or parallel

combination of 2 in series and other 2 in series as shown in figure 39.22. In figure for series

connection of the coils (figure 39.21) flux produced is proportional to Ia and for series-parallel

connection (figure 39.22) flux produced is proportional to

Therefore, for same armature current Ia, flux will be doubled in the second case and

naturally speed will be approximately doubled as back emf in both the cases is close to supply

voltage V. Thus control of speed in the ratio of 1:2 is possible for series parallel connection.

In a similar way, reader can work out the variation of speed possible between (i) all

coils connected in series and (ii) all coils connected in parallel.


UNIT-V

CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C. DRIVES

SPEED CONTROL OF INDUCTION MACHINES:

We have seen the speed torque characteristic of the machine. In the stable region of

operation in the motoring mode, the curve is rather steep and goes from zero torque at

synchronous speed to the stall torque at a value of slip s = ^s. Normally ^s may be such that

stall torque is about three times that of the rated operating torque of the machine, and hence

may be about 0.3 or less. This means that in the entire loading range of the machine, the speed

change is quite small. The machine speed is quite with respect to load changes. The entire

speed variation is only in the range ns to (1 _ ^s)ns, ns being dependent on supply frequency

and number of poles.

The foregoing discussion shows that the induction machine, when operating from

mains is essentially a constant speed machine. Many industrial drives, typically for fan or

pump applications, have typically constant speed requirements and hence the induction

machine is ideally suited for these. However, the induction machine, especially the squirrel

cage type, is quite rugged and has a simple construction. Therefore it is good candidate for

variable speed applications if it can be achieved.

From the torque equation of the induction machine, we can see that the torque depends

on the square of the applied voltage. The variation of speed torque curves with respect to the

applied voltage is shown in Fig. These curves show that the slip at maximum torque ^s

remains same, while the value of stall torque comes down with decrease in applied voltage.

The speed range for stable operation remains the same.

Further, we also note that the starting torque is also lower at lower voltages. Thus,

even if a given voltage level is sufficient for achieving the running torque, the machine may

not start. This method of trying to control the speed is best suited for loads that require very

little starting torque, but their torque requirement may increase with speed.

1. Speed control by changing applied voltage


2. Rotor resistance control

The reader may recall the expression for the torque of the induction machine. Clearly, it is

dependent on the rotor resistance. Further, that the maximum value is independent of the rotor

resistance. The slip at maximum torque dependent on the rotor resistance. Therefore, we may

expect that if the rotor resistance is changed, the maximum torque point shift to higher slip

values, while retaining a constant torque. Figure shows a family of torque-speed characteristic

obtained by changing the rotor resistance.

Note that while the maximum torque and synchronous speed remain constant, the slip at

which maximum torque occurs increases with increase in rotor resistance, and so does the

starting torque. Whether the load is of constant torque type or fan-type, it is

Figure: Speed-torque curves: voltage variation

Figure also shows a load torque characteristic one that is typical of a fan type of load.

In a fan (blower) type of load, the variation of torque with speed is such that T /! 2. Here one

can see that it may be possible to run the motor to lower speeds within the range ns to (1 _ ^s)

ns. Further, since the load torque at zero speed is zero, the machine can start even at reduced

voltages. This will not be possible with constant torque type of loads. One may note that if the

applied voltage is reduced, the voltage across the magnetizing branch also comes down. This

in turn means that the magnetizing current and hence ux level are reduced. Reduction in the ux

level in the machine impairs torque production (recall explanations on torque production). If,

however, the machine is running under lightly loaded conditions, then operating under rated

flux levels is not required. Under such conditions, reduction in magnetizing current improves

the power factor of operation. Some amount of energy saving may also be achieved. Voltage

control may be achieved by adding series resistors (a lossy, inefficient proposition), or a series

inductor autotransformer (a bulky solution) or a more modern solution using semiconductor

devices. A typical solid-state circuit used for this purpose is the AC voltage controller or AC

chopper. Another use of voltage control is in the so-called `soft-start' of the machine. This is

discussed in the section on starting methods.


evident that the speed control range is more with this method. Further, rotor resistance control

could also be used as a means of generating high starting torque.

For all its advantages, the scheme has two serious drawbacks. Firstly, in order to vary the

rotor resistance, it is necessary to connect external variable resistors (winding resistance itself

cannot be changed). This therefore necessitates a slip-ring machine, since only in that case rotor

terminals are available outside. For cage rotor machines, there are no rotor terminals. Secondly,

the method is not very efficient since the additional resistance and operation at high slips entails

dissipation resistors connected to the slip-ring brushes should have good power dissipation

capability. Water based rheostats may be used for this. A `solid-state' alternative to a rheostat is

a chopper controlled resistance where the duty ratio control of the chopper presents a variable

resistance load to the rotor of the induction machine.

The power drawn from the rotor terminals could be spent more usefully. Apart from

using the heat generated in meaning full ways, the slip ring output could be connected to

another induction machine. The stator of the second machine would carry slip frequency

currents of the first machine, which would generate some useful mechanical power. A still

better option would be to mechanically couple the shafts of the two machines together. This

sort of a connection is called cascade connection and it gives some measure of speed control

as shown below.

Let the frequency of supply given to the first machine be f1, its number poles be p1,

and its slip of operation be s1. Let f2; p2 and s2 be the corresponding quantities for the second

machine. The frequency of currents owing in the rotor of the first machine and hence in the

stator of the second machine is s1f1. Therefore f2 = s1f1. Since the machines are coupled at

the shaft, the speed of the rotor is common for both. Hence, if n is the speed of the rotor

3. Cascade control

Figure: Speed-torque curves: rotor resistance variation


Figure: Generalized rotor control

in radians,

Clearly now, the value of s can be changed by the value of Pr. For Pr = 0, the machine is

like a normal machine with a short circuited rotor. As Pr becomes positive, for all other circuit

conditions remaining constant, s increases or in the other words, speed reduces. As Pr

becomes negative, the right hand side of the equation and hence the slip decreases. The

physical interpretation is that we now have an active source connected on the rotor side, which

is able to supply part of the rotor copper losses. When Pr = _I02 2 R2 the entire copper loss is

supplied by the external source. The RHS and hence the slip is zero. This corresponds to

operation at synchronous speed. In general the circuitry connected to the rotor may not be a

simple resistor or a machine but a power electronic circuit, which can process this power

requirement. This circuit may drive a machine or recover power back to the mains. Such

circuits are called static Kramer drives.

4. Pole changing schemes

The latter expression is for the case where the second machine is connected in opposite

phase sequence to the first. The cascade-connected system can therefore run at two possible

speeds. Speed control through rotor terminals can be considered in a much more general way.

Consider the induction machine equivalent circuit, where the rotor circuit has been terminated

with a voltage source Er.

If the rotor terminals are shorted, it behaves like a normal induction machine. This is

equivalent to saying that across the rotor terminals a voltage source of zero magnitude is

connected. Different situations could then be considered if this voltage source Er had a non-

zero magnitude. Let the power consumed by that source be Pr. Then considering the rotor side

circuit power dissipation per phase

Note that while giving the rotor output of the first machine to the stator of the second,

the resultant stator mmf of the second machine may set up an air-gap flux which rotates in the

same direction as that of the rotor, or opposes it. This results in values for speed as


In case (b) however, the flux lines are out of the poles in A & C. The flux lines will be

then have to complete the circuit by owing into the pole structures on the sides. If, when seen

from the rotor, the pole emanating flux lines is considered as north pole and the pole into

which they enter is termed as south, then the pole configurations produced by these

connections is a two-pole arrangement in fig and a four-pole arrangement. Thus by changing

the terminal connections we get either a two pole air-gap field or a four- pole field. In an

induction machine this would correspond to a synchronous speed reduction in half from case

(a) to case (b). Further note that irrespective of the connection, the applied voltage is balanced

by the series addition of induced emfs in two coils. Therefore the air-gap flux in both cases is

the same. Cases (a) and (b) therefore form a pair of constant torque connections.

Figure: Pole arrangement

Sometimes induction machines have a special stator winding capable of being

externally connected to form two different number of pole numbers. Since the synchronous

speed of the induction machine is given by ns = fs=p (in rev./s) where p is the number of pole

pairs, this would correspond to changing the synchronous speed. With the slip now

corresponding to the new synchronous speed, the operating speed is changed. This method of

speed control is a stepped variation and generally restricted to two steps.

If the changes in stator winding connections are made so that the air gap flux remains

constant, then at any winding connection, the same maximum torque is achievable. Such

winding arrangements are therefore referred to as constant-torque connections. If however

such connection changes result in air gap flux changes that are inversely proportional to the

synchronous speeds, then such connections are called constant-horsepower type. The

following figure serves to illustrate the basic principle.

Consider a magnetic pole structure consisting of four pole faces A, B, C, D in fig.

Coils are wound on A & C in the directions shown. The two coils on A & C may be connected

in series in two different ways | A2 may be connected to C1 or C2. A1 with the other terminal

at C then form the terminals of the overall combination. Thus two connections result as shown

in fig.

Now, for a given direction of current flow at terminal A1, say into terminal A1, the

flux directions within the poles are shown in the figures. In case (a), the flux lines are out of

the pole A (seen from the rotor) for and into pole C, thus establishing a two-pole structure.


Consider, on the other hand a connection as shown in the fig. The terminals T1 and T2

are where the input excitation is given. Note that current direction in the coils now resembles

that of case (b), and hence this would result in a four-pole structure. However, in fig, there is

only one coil-induced emf to balance the applied voltage. Therefore flux in case (c) would

therefore be halved compared to that of case (b) (or case(a), for that matter). Cases (a) and (c)

therefore form a pair of constant horse power connections. It is important to note that in

generating a different pole numbers, the current through one coil (out of two, coil C in this

case) is reversed. In the case of a three-phase machine, the following example serves to

explain this. Let the machine have coils connected as shown [C1 _ C6]

The current directions shown in C1 & C2 correspond to the case where T1; T2; T3 are

supplied with three phase excitation and Ta; Tb & Tc are shorted to each other (STAR point).

The applied voltage must be balanced by induced emf in one coil only (C1 & C2 are parallel).

If however the excitation is given to Ta; Tb& Tc with T1; T2; T3 open, then current

through one of the coils (C1 & C2) would reverse. Thus the effective number of poles would

increase, thereby bringing down the speed. The other coils also face similar conditions.

5. Stator frequency control

The expression for the synchronous speed indicates that by changing the stator

frequency also it can be changed. This can be achieved by using power electronic circuits

called inverters, which convert dc to ac of desired frequency. Depending on the type of


control scheme of the inverter, the ac generated may be variable-frequency-fixed-

amplitude or variable-frequency- variable-amplitude type. Power electronic control

achieves smooth variation of voltage and frequency of the ac output. This when fed to the

machine is capable of running at a controlled speed. However, consider the equation for

the induced emf in the induction machine.

where N is the number of the turns per phase, _m is the peak flux in the air gap and f is the

frequency. Note that in order to reduce the speed, frequency has to be reduced. If the

frequency is reduced while the voltage is kept constant, thereby requiring the amplitude of

induced emf to remain the same, flux has to increase. This is not advisable since the machine

likely to enter deep saturation. If this is to be avoided, then flux level must be maintained

constant which implies that voltage must be reduced along with frequency. The ratio is held

constant in order to maintain the flux level for maximum torque capability.

Actually, it is the voltage across the magnetizing branch of the exact equivalent circuit

that must be maintained constant, for it is that which determines the induced emf. Under

conditions where the stator voltage drop is negligible compared the applied voltage, is valid.

In this mode of operation, the voltage across the magnetizing inductance in the 'exact'

equivalent circuit reduces in amplitude with reduction in frequency and so does the inductive

reactance. This implies that the current through the inductance and the flux in the machine

remains constant. The speed torque characteristics at any frequency may be estimated as

before. There is one curve for every excitation frequency considered corresponding to every

value of synchronous speed. The curves are shown below. It may be seen that the maximum

torque remains constant.

Figure: Pole change example: three phase


Equation shows that this maximum value is independent of the frequency. Further ^s! is

independent of frequency. This means that the maximum torque always occurs at a speed

lower than synchronous speed by a fixed difference, independent of frequency. The overall

effect is an apparent shift of the torque-speed characteristic as shown in fig. Though this is the

aim, E is an internal voltage, which is not accessible. It is only the terminal voltage V that we

have access to and can control. For a fixed V, E changes with operating slip (rotor branch

impedance changes) and further due to the stator impedance drop. Thus if we approximate E=f

as V=f, the resulting torque-speed characteristic shown in fig. is far from desirable.

If this equation is differentiated with respect to s and equated to zero to find the slip at

maximum torque ^s, we get ^s = _R0 r=(! L0 lr). The maximum torque is obtained by

substituting this value into eqn.

Figure: Torque-speed curves with E=f held constant

This may be seen mathematically as follows. If E is the voltage across the magnetizing branch

and f is the frequency of excitation, then E = kf, where k is the constant of proportionality. If!

= 2_f, the developed torque is given by


Figure: Torque-speed curves with V=f constant

At low frequencies and hence low voltages the curves show a considerable reduction

in peak torque. At low frequencies (and hence at low voltages) the drop across the stator

impedance prevents sufficient voltage availability. Therefore, in order to maintain sufficient

torque at low frequencies, a voltage more than proportional needs to be given at low speeds.

Another component of compensation that needs to be given is due to operating slip.

With these two components, therefore, the ratio of applied voltage to frequency is not a

constant but is a curve such as that shown in fig. With this kind of control, it is possible to get

a good starting torque and steady state

performance. However, under dynamic conditions, this control is insufficient. Advanced

control techniques such as field- oriented control (vector control) or direct torque control

(DTC) are necessary.

Figure: Voltage boost required for V=f control


Department of Electrical & Electronics Engineering

Electrical Drives and Control – EE 6351

(For Mechanical Engineering)

Third Semester

2 marks (Questions & Answers)

UNIT – I

INTRODUCTION

1. Define Drive and Electric Drive.

Drive: A combination of prime mover, transmission equipment and mechanical

working load is called a drive Electric drive: An Electric Drive can be defined as an electromechanical device for converting electrical energy to mechanical energy to impart motion to different

machines and mechanisms for various kinds of process control.

I.C Engines, Steam engine, Turbine or electric motors.

4. Give some examples of Electric Drives.

i. ii. iii.

iv.

Driving fans, ventilators, compressors and pumps. Lifting goods by hoists and cranes. Imparting motion to conveyors in factories, mines and warehouses Running excavators & escalators, electric locomotives trains, cars trolley buses, lifts & drum winders etc.

5. What are the types of electric drives?

Group electric drives (Shaft drive), I

iii.

ii.

i. Availability of electric drives over a wide range of power a few

watts to mega watts. Ability to provide a wide range of torques over wide range of speeds. Electric motors are available in a variety of design in order to make them compatible to any type of load.

3. List out some advantages of electric drives.

2. List out some examples of prime movers.


Individual Drives, Multi motor electric drives.

Classify electric drives based on the means of control. Manual,Semiautomatic, Automatic. 7. What is a Group Electric Drive (Shaft Drive)?

This drive consists of single motor, which drives one or more line shafts supported on bearings. The line shaft may be fitted with either pulleys & belts or gears, by means of which a group of machines or mechanisms may be operated.

6.

Disadvantages:

2. What is an individual electric drive? Give some examples.

In this drive, each individual machine is driven by a separate motor. This motor also imparts motion to various other parts of the machine. Single spindle drilling machine, Lathe machines etc.

3. What is a multi motor electric drive? Give some examples.

In this drive, there are several drives, each of which serves to activate on of the working parts of the driven mechanisms.

There is no flexibility, Addition of an extra machine to the main shaft is difficult. The efficiency of the drive is low, because of the losses occurring in several transmitting mechanisms. The complete drive system requires shutdown if the motor, requires servicing or repair.

The system is not very safe to operate The noise level at the work spot is very high.

Advantages: A single large motor can be used instead of a number of small motors. The rating of the single motor may be appropriately reduced taking into account the diversity factor of loads.

1. What are the advantages and disadvantages of Group drive (Shaft drive)?


Metal cutting machine tools, paper making machines, rolling mills, traction drive, Traveling cranes etc.,

4. Write about manual control, semiautomatic control & Automatic control?

Manual control: The electric drives with manual control can be as simple as a room fan, incorporating on switch and a resistance for setting the required speed. Semiautomatic control: This control consists of a manual device for giving a certain command (Starting, braking, reversing, change of speed etc.,) and an automatic device that in response to command, operates the drive in accordance with a preset sequence or order.

Automatic control: The electric drives with automatic control have a control gear, without manual devices

Power Supply

Motor

Geared Coupling

Mechanical Laod

A load diagram is the diagram which shows graphically the variation of torque acting on the electric drive. The motor of the electric drive has to overcome the load torque expressed as a function of time.

Types:

One for the static or steady state process Other for the dynamic process, when the dynamic components of torque are induced by the inertia of the motor & load. (Instantaneous speed, acceleration, Torque & power) as a function of time are required to draw…..

14. What are the types Drive systems?

13. What is a load diagram? What are its types? What are required to draw a load diagram?

Speed & Torque Control

12. What are the Typical elements of an Electric Drive?


Electric Drives Electromechanical Drives

Mechanical Drives Hydraulic drives.

15. Give an expression for the losses occurring in a machine.

The losses occurring in a machine is given by W = Wc + x2 Wv Where Wc = Constant losses Wv = Variable losses at full load X = load on the motor expressed as a function of rated load.

ii.

iii.

17. What are the factors that influence the choice of electrical drives? 1. Shaft power & speed11. Speed range 2. Power range12. Efficiency 3. Starting torque13.Influence on the supply network 4. Maintenance14. Special competence 5. Total purchase cost15. Cost of energy losses 6. Influence on power supply16. Environment 7. Availability17. Accessibility 8. Nature of electric supply18. Nature of load 9. Types of drive19. Electrical Characteristics 10.Service cost20. Service capacity & rating

18. Indicate the importance of power rating & heating of electric drives.

Power rating: Correct selection of power rating of electric motor is of economic interest as it is associated with capital cost and running cost of drives. Heating: For proper selection of power rating the most important consideration is the heating effect

i. The machine is considered to be a homogeneous body having a uniform temperature gradient. All the points at which heat generated have the same temperature. All the points at which heat is dissipated are also at same temperature. Heat dissipation taking place is proportional to the difference of temperature of the body and surrounding medium. No heat is radiated. The rate of dissipation of heat is constant at all temperatures.

16. What are the assumptions made while performing heating & cooling calculation of an electric motor?


of load. In this connection various forms of loading or duty cycles have to be considered.

19. How heating occurs in motor drives?

The heating of motor due to losses occurring inside the motor while converting the electrical power into mechanical power and these losses occur in steel core, motor winding & bearing friction.

20. What are the classes of duties? 1. Continuous duty 2. Short time duty operation of motor Main classes of duties 3. Intermittent periodic duty 4. Intermittent periodic duty with starting 5. Intermittent periodic duty with starting & braking 6. Continuous duty with intermittent periodic loading 7. Continuous duty with starting & braking 8. Continuous duty with periodic load changes

23. Why the losses at starting is not a factor of consideration in a continuous duty motor? While selecting a motor for this type of duty it is not necessary to give importance to the heating caused by losses at starting even though they are more than the losses at rated load. This is because the motor does not require frequent starting it is started only once in its duty cycle and the losses during starting do not have much influence on heating.

What is meant by “short time rating of motor”? Any electric motor that is rated for a power rating P for continuous operation can be loaded for a short time duty (Psh) that is much higher than P, if the temperature rise is the consideration.

24.

22. List out some applications for which continuous duty is required. Centrifugal pumps, fans, conveyors & compressors

1. 2. 3. 4. 5. 6.

Reversible &non reversible in controlled constant speed Reversible and non reversible step speed control Reversible and non reversible smooth speed control Constant predetermined position control Variable position control Composite control.

21. How will you classify electric drives based on the method of speed control?


What is meant by “load equalization”? In the method of “load Equalization” intentionally the motor inertia is increased by adding a flywheel on the motor shaft, if the motor is not to be reversed. For effectiveness of the flywheel, the motor should have a prominent drooping characteristic so that on load there is a considerable speed drop.

26. How a motor variable load ? 1. Method of 2. Method of 3. Method of 4. Method of

rating is determined in a continuous duty and

Average losses equivalent power equivalent current equivalent Torque

25.

27. Draw the heating & cooling curve of an electric motor.

29. Write down the heat balance equation. Heat balance equation is given by Ghd0 + S0 .dt = p.dt

28. What are the various function performed by an electric drive?

1. Driving fans, ventilators, compressors & pumps etc., 2. Lifting goods by hoists & cranes 3. Imparting motion to conveyors in factories, mines & warehouses and 4. Running excavators & escalators, electric locomotives, trains, cars, trolley buses and lifts etc.


UNIT – II ELECTRICAL MOTOR CHARACTERISTICS

1. Why a single phase induction motor does not self start?

When a single phase supply is fed to the single phase induction motor. Its stator winding produces a flux which only alternates along one space axis. It is not a synchronously revolving field, as in the case of a 2 or 3phase stator winding, fed from 2 or 3 phase supply.

2. What is meant by plugging?

The N-T curve moves towards the right when the voltage is increased. This can be achieved by means of additional resistance in the armature circuit or by using thyristor power converter.

6. Compare electrical and mechanical braking.

Mechanical Brakes require frequent maintenance Not smooth Can be applied to hold the system at any position torque.

Electrical very little maintenance smooth cannot produce holding

5. What is the effect of variation of armature voltage on N-T curve and how it can be achieved?

Dynamic or Rheostatic braking, Counter current or plugging and Regenerative braking

4. What are the different types of electric braking?

Shunt : driving constant speed, lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating pumps Series : electric locomotives, rapid transit systems, trolley cars, cranes and hoists, conveyors Compound : elevators, air compressors, rolling mills, heavy planners.

3. Give some applications of DC motor.

The plugging operation can be achieved by changing the polarity of the motor there by reversing the direction of rotation of the motor. This can be achieved in ac motors by changing the phase sequence and in dc motors by changing the polarity.


7. When does an induction motor behave to run off as a generator?

When the rotor of an induction motor runs faster than the stator field, the slip becomes negative. Regenerative braking occurs and the K.E. of the rotating parts is return back to the supply as electrical energy and thus the machine generates power.

8. Define slip.

S = Ns – Nr Ns Where, Ns Nr S

9. Define synchronous speed.

In the regenerative braking operation, the motor operates as a generator, while it is still connected to the supply here, the motor speed is grater that the synchronous speed. Mechanical energy is converter into electrical energy, part of which is returned to the supply and rest as heat in the winding and bearing.

6. Give some applications of DC motor.

Shunt: driving constant speed, lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating pumps Series: electric locomotives, rapid transit systems, trolley cars, cranes and hoists, conveyors Compound: elevators, air compressors, rolling mills, heavy planners.

5. What is meant by regenerative braking?

When a single phase supply is fed to the single phase induction motor. Its stator winding produces a flux which only alternates along one space axis. It is not a synchronously revolving field, as in the case of a 2 or 3phase stator winding, fed from 2 or 3 phase supply.

10. Why a single phase induction motor does not self start?

It is given by Ns = 120f / p rpm. WhereNs = synchronous speed, p = no. of stator poles, f = supply frequency in Hz

= synchronous speed in rpm. = rotor speed in rpm = Slip


7. Compare electrical and mechanical braking.

Mechanical Brakes require frequent maintenance Not smooth Can be applied to hold the system at any position torque.

Electrical very little maintenance smooth cannot produce holding

8. Differentiate cumulative and differential compound motors.

Cumulative The orientation of the series flux aids the shunt flux shunt flux

9. What is meant by mechanical characteristics?

A curve drawn between the parameters speed and torque.

differential series flux opposes

2. Mention the Starters used to start an Induction motor.

D.O.L Starter (Direct Online Starter) Star-Delta Starter Auto Transformer Starter Reactance or Resistance starter Stator Rotor Starter (Rotor Resistance Starter)

3. What are the protective devices in a DC/AC motor Starter.

Over load Release (O.L.R) or No volt coil Hold on Coil Thermal Relays Fuses(Starting /Running) Over load relay

Two point Starter Three point Starter Four point Starter

1. Mention the Starters used to start a DC motor.

UNIT – III

STARTING METHODS


4. Is it possible to include/ Exclude external resistance in the rotor of a Squirrel cage induction motor?. Justify

No it is not possible to include/ Exclude external resistance in the rotor of a Squirrel cage induction motor because, the rotors bars are permanently short circuited by means of circuiting rings (end rings) at both the ends. i.e. no slip rings to do so.

5. Give the prime purpose of a starter for motors.

when induction motor is switched on to the supply, it takes about 5 to 8 times full load current at starting. This starting current may be of such a magnitude as to cause objectionable voltage drop in the lines. So Starters are necessary

6. Why motor take heavy current at starting?

To include resistance in the rotor circuit there by reducing the induced rotor current at starting. This can be implemented only on a slip ring induction motor.

9. Why squirrel cage induction motors are not used for loads requiring high starting torque?

Squirrel cage motors are started only by reduced voltage starting methods which leads to the development of low starting torque at starting. This is the reason Why squirrel cage induction motors are not used for loads requiring high starting torque.

10. How reduced voltage starting of Induction motor is achived?.

8. What is the objective of rotor resistance starter (stator rotor starter)?

7. What are the methods to reduce the magnitude of rotor current (rotor induced current) at starting?. By increasing the resistance in the rotor circuit By reducing the magnitude of rotating magnetic field i.e by reducing the applied voltage to the stator windings.

Since rotor of an induction motor behaves as a short circuited secondary of a transformer whose primary is stator winding, heavy rotor current will require corresponding heavy stator balancing currents. Thus motor draws heavy current at starting

When 3 phase supply is given to the stator of an induction motor, magnetic field rotating in space at synchronous speed is produced. This magnetic field is cut by the rotor conductors, which are short circuited. This gives to induced current in them.


D.O.L Starter (Direct Online Starter) Star-Delta Starter Auto Transformer Starter Reactance or Resistance starter

11. Give the relation between line voltage and phase voltage in a (i) Delta connected network (ii) Star connected network Delta connected network: Vphase = Vline Star connected network: Vphase = Vline / √3

14. Compare the Induction motor starters

% of line voltag e applie d

100%

57.7%

80%

Starting current (Is)compared with

D.O.L current(Idol)

Is = Idol

Is = (1/√3)2 Idol

Is =(0.8)2 Idol

Full load current(I )

Is = 6I

Is = 2I

Is = 3.84 I

Starting torque (Ts)compared with

D.O.L Torque(Tdol)

Ts = Tdol

Ts = (1/√3)2 Tdol

Ts = =(0.8)2 Tdol

Full load torque(T)

Ts = 6T

Ts = 2/3T

Ts = 1.28 T

Description of Starter

D.O.L Starter

Star Delta starter

Auto transformer

First stage reduction is due to reduced applied voltage Second stage reduction is due to reduced number of turns

13. Explain double stage reduction of line current in an Auto transformer starter.

Disadvantages: The inrush current of large motors may cause excessive voltage drop in the weak power system The torque may be limited to protect certain types of loads.

Advantages: Highest starting torque Low cost Greatest simplicity

12. Give some advantages and disadvantages of D.O.L starter.


starter 60%

40%

Reactance- resistance starter

64%

Is =(0.6)2 Idol

Is =(0.4)2 Idol

Is = (0.64) Idol 2

Is = 2.16 I

Is = 0.96 I

Is = 2.5 I

Ts = =(0.6)2 Tdol

Ts = =(0.4)2 Tdol

Ts =(0.425)2 Tdol

Ts = 0.72 T

Ts = 0.32 T

Ts = 0.35T

15. Draw the Speed-Torque characteristics of an Induction motor with various values of Rotor Resistance.

Rotor Resistance Increasing

Torque

Speed N = k (V-IaRa)

where V = Terminal Voltage in volts Ia = Armature current in Amps Ra = Armature resistance in ohms flux per pole.

2. What are the ways of speed control in dc motors?

Field control -by varying the flux per pole. Armature control- by varying the terminal voltage

-for above rated speed -for below rated speed

1. Give the expression for speed for a DC motor.

UNIT – IV

CONVENTIONAL SPEED CONTROL

Speed

Tmax


3. Give the Limitation of field control

a. Speed lower than the rated speed cannot be obtained. b. It can cope with constant kW drives only. c. This control is not suitable to application needing speed reversal.

4. Compensating winding can be used to increase the speed range in field control method

5. What are the 3 ways of field control in DC series motor?

6. What are the main applications of Ward-Leonard system?

7. What are the merits and demerits of rheostat control method?

Impossible to keep the speed constant on rapidly changing loads.

A large amount of power is wasted in the controller resistance.

Loss of power is directly proportional to the reduction in speed. Hence

efficiency is decreased. Maximum power developed is diminished in the same ratio as speed. It needs expensive arrangements for dissipation of heat produced in the controller resistance. It gives speed below normal, not above.

8. What are the advantages of field control method?

More economical, more efficient and convenient.

It is used for colliery winders. Electric excavators In elevators Main drives in steel mills and blooming and paper mills.

Field diverter control Armature diverter control Motor diverter control Field coil taps control Series-parallel control


It can give speeds above normal speed.

9. Compare the values of speed and torque in case of motors when in parallel and in series.

The speed is one fourth the speed of the motor when in parallel.

The torque is four times that produced by the motor when in parallel.

10. Mention the speed control method employed in electric traction.

Series-parallel speed control.

11. What is the effect of inserting resistance in the field circuit of a dc shunt motor on its speed and torque?

Applied voltage should be maintained constant so as to maintain field strength

12. While controlling the speed of a dc shunt motor what should be done to achieve a constant torque drive?

For a constant supply voltage, flux will decrease, speed will increase and torque will increase.


UNIT – V

SOLID STATE SPEED CONTROL

1. What is a controlled rectifier?

A controlled rectifier is a device which is used for converting controlled dc power from a control voltage ac supply.

2. What is firing angle?

The control of dc voltage is achieved by firing the thyristor at an adjustable angle with respect to the applied voltage. This angle is known as firing angle.

3. Give some applications of phase control converters.

4. What is the main purpose of free wheeling diode?

6. What is natural or line commutation?

The commutation which occurs without any action of external force is called natural or line commutation.

7. What is forced commutation?

The commutation process which takes place by the action of an external force is called forced commutation.

8. What is a chopper?

A chopper is essentially an electronic switch that turns on the fixed-voltage dc source for a short time intervals and applies the source potential to motor terminals in series of pulses.

A full converter is a tow quadrant converter in which the voltage polarity of the output can reverse, but the current remains unidirectional because of unidirectional thyristors.

5. What is a full converter?

Free wheeling diode is connected across the motor terminal to allow for the dissipation of energy stored in motor inductance and to provide for continuity of motor current when the thyristors are blocked.

Phase control converters are used in the speed control of fractional kW dc motors as well as in large motors employed in variable speed reversing drives for rolling mills. with motors ratings as large as several MW‟s.

.


9. What are the two main difficulties of variable frequency system?

Control of Va requires variation of chopper frequency over a wide range. Filter design for variable frequency operation is difficult.

10. At low voltage, a large value of toff makes the motor current discontinuous.

Classify commutation.

Voltage commutation

Current commutation.

12. What is current commutation?

13. What is load commutation?

14. What are the different means of controlling induction motor?

15. What are the two ways of controlling the RMS value of stator voltage?

16. Mention the two slip-power recovery schemes.

Stator voltage control.

Frequency control

Pole changing control.

Slip power recovery control.

Phase control

Integral cycle control

The load current flowing through the thyristor either becomes zero (as in natural or line commutation employed in converters) or is transferred to another device from the conducting thyristor. This is known as load commutation.

A current pulse is forced in the reverse direction through the conducting thyristor. As the net current becomes zero, the thyristor is turned OFF. This is known as current commutation.

A charged capacitor momentarily reverse-bias the conducting thyristor to turn it off. This is known as voltage commutation.

11. What is voltage commutation?


Static Kramer drive scheme. 17. Give the basic difference between the two slip-power recovery schemes.

The slip is returned to the supply network in scherbius scheme and in Kramer scheme, it is used to drive an auxiliary motor which is mechanically coupled to the induction motor shaft.

18. Write short notes on inverter rectifier.

The dc source could be converted to ac form by an inverter, transformed to a suitable voltage and then rectified to dc form. Because of two stage of conversion, the setup is bulky, costly and less efficient.

Static scherbius scheme

What is electrical power supply system?

The generation, transmission and distribution system of electrical power is called electrical power supply system.

21. What are the 4 main parts of distribution system?

Feeders,

Distributors and

Service mains.

22. What are feeders?

Since a static Kramer system possesses no line commutated inverter, it causes less reactive power and smaller harmonic contents of current than a static scherbius.

20. What are the advantages of static Kramer system,, over static scherbius system?

This drive provides a constant torque control.

If max. slip is denoted by S max, then power rating of diode, inverter and

transformer can be just Smax times motor power rating resulting in a low cost drive.

The scheme has applications in large power fan and pump drives which requires

speed control in anrrow range only.

19. Give the special features of static scherbius scheme.


Feeders are conductors which connect the stations (in some cases generating stations) to the areas to be fed by those stations.

23. What are the advantages of high voltage dc system over high voltage ac system?

It requires only tow conductors for transmission and it is also possible to transmit the power through only one conductor by using earth as returning conductor, hence much copper is saved.

No inductance, capacitance, phase displacement and surge problem.

There is no skin effect in dc, cross section of line conductor is fully utilized.

24. What do you mean by the term earthing?

25. What are the different methods of providing neutral earthing?

Reactance earthing

Arc suppression coil or Peterson coil earthing.

Resistance earthing

Solid earthing

The term “earthing” means connecting the non-current carrying parts of electrical equipment to the neutral point of the supply system to the general mass of earth in such a manner that at all time an immediate discharge of electrical energy takes place without danger.


ee 6351 electrical drives and control 3 0 0 100 …

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