introduction to electrical drives

7/29/2019 Introduction to Electrical Drives

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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

movement or motion and energy that is used to provide the motion can come from varioussources: diesel engines, petrol engines, hydraulic motors, electric motors etc.

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 motorcan 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.

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 andrequire regular maintenance. In the past, induction and synchronous machines were used for

constant speed applications this was mainly because of the unavailability of variable frequencysupply.

Figure 1 Conventional variable speed electrical drive system

With the advancement of power electronics, microprocessors and digital electronics, typicalelectric 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 whereonly AC source is available, however, it can also be used when the power source available is DC

or vice versa

Figure 2 Modern Electric drive system employing power electronic converters

ACmotor

DCgenerator

variableDC DC

motor

variablespeed

Load

fixedspeed

If

Ia

Power

Source

Control

feedback

Power

Processor(Power electronic

Converters)

ControlUnit

Motor Load


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

electric drives as shown in Figure 3.

Figure 3 Multi-disciplinary nature of electric drive system

Components of Electrical DrivesThe 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 tomechanical - therefore can be regarded as energy converters. In braking mode, the flow ofpower is reversed. Depending upon the type of power converters 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 onapplications, cost, environmental factors and also the type of sources available.. Broadly,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 magnetBrushless DC motor require power electronic converters

Stepper motors require power electronic converters

Synchronous reluctance motors or switched reluctance motor require power electronicconverters

b) Power processor or power modulatorSince the electrical sources are normally uncontrollable, it is therefore necessary to be ableto 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

Machine design Speed sensorless Machine theory

Non-linear control Real-time control DSP application PFC

Speed sensorless Power electronic converters

Utility interface

Renewable energy


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of the motors can be changed at will. Power electronic converters have several advantagesover classical methods of power conversion, such as : More efficient since ideally no losses occur in power electronic converters

Flexible voltage and current can be shaped by simply controlling switching functions of

the power converter Compact smaller, compact and higher ratings solidstate 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) theavailable sources to suit the load i.e. motors. These converters are efficient because the

switches operate in either cut-off or saturation modes

Several conversion are possible:

AC to DC

DC to AC

DC to DC

AC to AC

Dioderectifier

DC-DCconverter

control

Controlledrectifier

control

Inverter(PWM)

control

DC-DCconverter

control

Inverter(six-step)

control

DC-DCConverter

control

ControlledRectifier

control

Inverter

(six-step)

control

DiodeRectifier

control

Inverter(PWM)

MatrixConverter

control


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c) 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 canbe 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/microp.,complex estimations and observers can be easily implemented.

d) SourceElectrical 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 convertersPower sources can be of AC or DC in nature and normally are uncontrollable, i.e. their

magnitudes or frequencies are fixed or depend on the sources of energy such as solar orwind. 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 ofoperation 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

Comparison between DC and AC drives

Motors : DC require maintenance, heavy, expensive, speed limited by mechanical construction AC less maintenance, light, cheaper, robust, high speed (esp. squirrelcage type)

Control unit: DC drives: Simple control decoupling torque and flux by mechanical commutator the

controller can be implemented using simple analog circuit even for high performancetorque control cheaper.

AC drives, the types of controllers to be used depend on the required drive performance obviously, cost increases with performance. Scalar control drives technique does notrequire fast processor/DSP whereas in FOC or DTC drives, DSPs or fast processors are

normally employed.

Performance:

In DC motors, flux and torque components are always perpendicular to one another

thanks to the mechanical commutator and brushes. The torque is controlled via thearmature current while maintaining the field component constant. Fast torque and

decouple control between flux and torque components can be achieved easily. In AC machines, in particular the induction machines, magnetic coupling between

phases and between stator and rotor windings makes the modeling and torque controldifficult and complex. Control of the steady state operating conditions is accomplished

by controlling the magnitude and the frequency of the applied voltage; which is known


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as the scalar control technique. This is satisfactory in some applications. The transientstates or the dynamics of the machine can only be controlled by applying the vectorcontrol technique whereby the decoupling between the torque and flux components is

achieved through frame transformations. Implementation of this control technique is

complex thus requires fast processors such as Digital Signal Processors (DSPs).

Overview of AC and DC drives

The advancement in electric drive system is very much related to the development in the powersemiconductor devices technology. The introduction of the Silicon-Controlled Rectifier (SCR) in

1957 has initiated the application of solid state devices in power converters. The development ofthe electrical drives systems can be divided into three stages

Before power semiconductor devices were introduced:

AC drives were used for fixed speed operation. Generating an AC voltage with variablefrequency was only possible by using rotary converters, which are bulky and inflexible. Although

it is possible to use variable voltage with fixed frequency sources to control the speed of AC

motors, the efficiency of the drive system will be very poor especially at low speeds. On the otherhand, variable DC supply can be produced using multi-machine configuration and hence could

be used to control the armature voltage of the DC motors. Consequently, DC drives are widelyused for variable speed operation, whereas AC machines were used mainly for fixed speed

applications.

After power semiconductor devices were introduced in 1950sAlthough self turnoff devices (Bipolar Junction Transistor BJT) were available in the

1950s their voltage ratings were too low which make them inappropriate to be used in power

circuit. Silicon-Controlled Rectifier (SCR) was introduced in 1957. The higher ratings of SCRcompared to the solid state transistor at that time, has made it possible for it to be used in

static frequency converters or inverters. Speed control with AC motor can be performed because

variable frequency AC supply can be generated using inverters. However, since the switchingfrequency of an SCR was low which require commutation circuit in order to turn off, square

wave inverters were mainly used in AC drive system. In early 1960s, the improvement in the

fabrication of BJT along with the introduction of pulse width modulation (PWM) controltechnique has significantly contributed to the improvement in the AC motor drives. Transient

torque control to some extend, was nearly achieved to the expense of a very complex algorithmwith numerous approximations. The true high performance torque control similar to DC drives

was still not achievable due to the complex magnetic coupling between phases in the stator androtor of the AC machines. Nevertheless, DC drives were gradually being replaced with AC drives

in medium performance variable speed applications. Applications requiring precise and fast

torque control were still dominated by DC drives.

After semiconductor devices were introduced in 1980s

In 1972, Prof. Blashke published his approach of AC motor control, to what is nowknown as Field Oriented Control (FOC) or vector control. FOC control basically transformed the

control of AC motors to the one similar to DC motor control. In other words, the highperformance torque control can be achieved using AC motors. This is possible through complexframe transformations and algorithm. However not until in the early 80s, where faster

microprocessors were available, the algorithm used for FOC was not practically realizable. In1980s, increasing number of applications utilizing FOC control could be found in industries.Applications which were previously possible only with DC drives were gradually being replaced

with FOC of AC drives. It was predicted that the AC drives will eventually replace the DC drives

in the near future.


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Torque Equations For Rotating Systems

The Newtons Law states that, the net force acting on a body of mass M equals to the rate of

change of its mechanical momentum, which is the product of its mass and its velocity in the

direction of the net force. In the equation form, this is given by

(1)

where F is the net force acting on the body, M is the mass of the body and v is its velocity. This

is illustrated by Figure 4.

Figure 4 Translational motion

With constant mass, (1) can be written as

For rotational motion (which is the case for rotating electrical machines), the force, the mass

and the linear velocity is equivalent to the torque, the moment of inertia and the angular

velocity, respectively. Equation (1) can therefore be written as

(3)

where T is the net torque, J is the moment of inertia and is the angular velocity. The rotational

system which is analogous to the translational system of Figure 4 is shown in Figure 5.

Figure 5 Rotational motion

For most of the cases, J is constant thus reducing (3) to

(4)

In terms of the angular position, , this can be written as

M

x

v

FpFf

, TeTL

J


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(5)

For rotating electrical machines, the net torque is given by

(6)

where Te is the internal electrical torque produced by the motor, Tl is the load torque and/or the

internal friction of the motor. T is the available torque at the shaft and is responsible foraccelerating the inertia of the motor. T is also known as the dynamic torque and it only existsduring the transient (i.e. acceleration and deceleration). In order to accelerate in forward

direction, Te Tl must be positive; which means that the applied electrical torque must be larger

than the load torque. In order to decelerate, the net torque must be negative; the electricaltorque must be made smaller than the load torque and the motor operates in braking mode

more on this later. Note that the speed is always continuous. A discontinuity in speed (i.e. step

change in speed) theoretically will require an infinite torque. This is analogous to the voltage andcurrent across a capacitor in which discontinuity in capacitor voltage is not allowed as it

correspond to an infinite capacitor current.

Equation (4) relates the torque and the mechanical speed (or position) of the machine. For agiven electrical torque profile, with the known moment of inertia and the load torque, the speed

profile of the drive system can be determined. In a torque-controlled drive system, the speed isgoverned by the load. If the load torque comprise of only the frictional torque which is

proportional to the speed, (4) can be written as

(7)

Equation (7) can be easily simulated using SIMULINK as shown in Figure 6. In the simulation, a

square wave torque is applied.

Figure 6 Dynamic simulation of mechanical system

Usually in a cascaded closed-loop control system in which the speed is to be controlled, thereference torque will be generated by the speed controller. In such cases, the torque will be

governed by the speed.

If we multiply (7) with the angular speed, we obtain an equation describing the power balance,

(8)

torque

speed

position


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Where pD = mTe is the driving power, pL= mTl is the load power and is the change in

kinetic energy. Integrating the equation with time and setting the initial speed (0) = 0, we

obtain the following:

(9)

The last term of (9) is the stored kinetic energy of the system. It is analogous to the energy

stored in a capacitor or an inductor . Similar to a capacitor voltage or an

inductor current, an angular velocity must be continuous. An abrupt (discontinuous) change

in will results in an infinite power.

Relation between translational and rotational motionsIn most applications of the drive systems, the translational and rotational motions are related.

An example of a typical system is shown in Figure 7.

Figure 7 Translational and rotational motions

The relation between the torques and the linear forces are given by

Tl = rFl, Tm = rFm .Also,

V = r

If the mass M is constant, we can write

(10)

Equation (10) states that the equivalent moment of inertia of the translational motion referred tothe axis of the pulley is given by Jequ = Mr2

M

FmFl

rr

v

Tm

Tl


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System with gearsIt was found out that machines designed to operate at low speeds are large in size compared tothe ones which are designed to operate at high speeds. In order to avoid the unnecessary large

size machines, high speed operations are normally preferred. However, in some applications,

slow motion with high torque is required. Consequently for such applications, gears whichreduce speed but amplify the torque, are commonly employed. An example of the hoist drive

employing gears is shown in Figure 8.

Figure 8 Hoist drive with gears

The hoist drive system shown in Figure 8 can be represented by an equivalent system similar to

Figure 5. In order to do that, we need to obtain the equivalent moment of inertia and loadtorque. If the mass M3 is considered being moved upwards, with the negligible frictional torque,it can be shown that the torque equation for the equivalent system is given by

(11)

where

Steady state operating speedThe characteristics of the motor and load are normally described based on their torque versus

speed graph or T- characteristics. The T- characteristic of a motor corresponds to the

variation of its torque versus its speed, with all other variables, including the voltage (or current)

and frequency (for AC motor) are kept constant. Typical shape of T- characteristics of different

motors are shown in Figure 9.

The loads on the other hand will have their own T- characteristics. It is the intersection

between the motor and the load T- characteristics that determines the steady state speed. This

can be seen from (6) where at steady state d/dt = 0 and Te = Tl.

J1

J2

M3

J3

1, Tm 2

3

2r3Loss-

free

gear

Synchronous motor Separately excitedDC motor

Induction motor Series DC motor


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The steady state torque-speed characteristic of the motor depends on the applied voltage orcurrent. Hence, by changing the point of intersections between the motor and load torque-speedcurves, different steady-state speeds can be achieved.

Figure 9 Different steady state speeds (Tl = Te) for different motors T- characteristics

It should be noted that the graph in Figure 9 only displayed the steady state characteristics ofthe load and motor. The transient responses before these steady state speeds are reached have

to be dealt with using the dynamic characteristics of the load and motor.

Components of Load Torque, Tl

In general, the load torque Tl can be classified into two types: the passive load torque (frictional

torque) and the active load torque. Frictional toque exists only when there is motion and italways opposes the driving torque. Active load torque on the other hand, is independent of the

direction of motion.

Frictional torque

Moving parts of the motor and load constitute the frictional torque. There are several types of

frictional as described in Figure 4 and explained below:

Coulomb friction exists in bearings, gears, coupling and brakes. It is almostindependent of speed.

Viscous friction exist in lubricated bearings due to the laminar flow of the lubricant. Itis directly proportional to the speed.

Windage friction occurs due the turbulent flow of air or liquid. It is directly proportional

to the square of speed

In practical drive system consisting of load and motor, all components of friction describedabove exist simultaneously. However, in most of the cases, only one or two components are

dominating. For instance, a fan or a propeller will typically have the windage friction

dominating, whereas in paper mill and machine tools, the dominating one could be the viscousfriction.

Torque

speed

Torque-speed characteristic ofthe load, Tl

Different steady-state torque-speedcharacteristics of the motor, Te

1 2 3

Different motor

speeds


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Constant torqueThe direction of constant load torque is independent of speed it retains the direction even

when the direction of rotation reverses or changes, e.g. gravity, tension or compressionundergone by elastic body. This type of torque is capable of driving the motor under equilibriumand is said to be an active torque.

Thermal considerations

The losses in the machines contribute to the temperature increase in the machine. The variousparts of the machine have different temperature limits. Particularly important is the insulation

used for the windings which give rise to the different classes of machines. If the temperaturegoes beyond the allowable temperature, it will cause an immediate breakdown (short circuit inthe winding) or it will deteriorate the quality and hence reduces the lifetime of the insulation

material. Allowable power losses are higher for materials which can withstand higher

temperature which translates to higher costs. The classes of the insulator used for the windingin electrical machines are shown in Table 1.

T

Viscous

Coulomb

Windage

Speed

Torque

Gravitationaltorque

FL

TL

gM

TL = rFL = r g M sin

Te

Figure 10 Frictionaltorque

Figure 11 Constant load torque: gravitational force


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Table 1 Classification of the insulators

Class Max safe temp. oC

V 90

A 105E 120

B 130

F 155

H 180C >180

Three main cause of power losses are:

Conductor losses (i2R)Exist in the windings, cables, brushes, slip rings, commutator, and etc.

Core lossesMainly due to eddy current and hysteresis losses

Friction and windage losses

Mainly due to ball bearings, brushes, ventilation losses

The constructions of the machines are very complex; normally built from various types ofmaterials (heterogeneous) with complex geometrical shapes. To exactly predict the heat flow andhence 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 operatingconditions, the motors can therefore adequately modeled as homogeneous bodies. Obviously,

this assumption cannot determine the specific internal thermal conditions for the motors.

Figure 12 Homogeneous body

Let us assume that a homogeneous body shown in Figure 12 represents a motor which has athermal 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, isrepresented 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 powerequals the output power; this is when the steady state temperature is reached. The equation

describing the power balance is given by

(12)

Thermal capacity, C (Ws/oC)Surface A, (m2)

Surface temperature, T (oC)

Ambient temperature, To

p1

INPUT POWER(losses)

p2

OUTPUT POWER(convection)


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The heat dissipated by convection is given by

p2= A (T To) (13)

where is the coefficient of heat transfer.

If we let T = T To , equation (12) can be written as

or

(14)

where T= C/(A) is the thermal time constant. With T(0)=0 and a step change in the power

input p1 from 0 to ph at t=0, the solution for T is

(15)

At steady state, T() = ph/(A)

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

ambient temperature.

(16)

t

t

Heatingtransient

Coolingtransient


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Figure 13 Heating and cooling transients

The thermal time constant depends on the coefficient of heat transfer which in turn depends

on the velocity of the cooling air. Machines which are self-ventilated will have larger cooling timeconstants compared to their heating (assumed moving) time constants. On the other hand

machines with forced ventilation system will have a cooling and heating time constants of more

or less equal. It should be noted that the thermal time constant of electrical machines aretypically much larger than their mechanical or electrical time constants. It may vary from fewminutes few hours.

If the thermal time constant is large, a temporary overload is therefore possible withoutexceeding the temperature limits. Three typical modes of operation are:

- Continuous duty

- Short time intermittent duty- Periodic intermittent duty

(i) Continuous dutyThe motor is loaded continuously. Obviously the rating of the motor must at least equal the

continuous loading of the machine. Normally, motor with next higher power rating from

commercial available rating is selected.

(ii) Short time intermittent duty

The time of operation is considerably less than the thermal time constant. The motor is allowed

to cool to ambient temperature before the new load cycle is applied. The motor is allowed to beoverloaded provided that the maximum temperature is not exceeded. However, the application of

much higher power than the rated power is subject to the available torque of the machine. ForDC machine this is limited due the sparking between the brushes and the commutator. In

induction machine, this is limited by its pull-out torque.

(iii)Periodic intermittent duty

The load cycle is repeated periodically. The machine is not allowed to cool to ambient when thenext load cycle is applied. The temperature will fluctuate and the mean value will eventually

settle to a steady state value. The machine can be overloaded and amount of overloadingdepends on the duty cycle of the load. The heating and cooling time constant may be different

depending whether the machine is self-cooled or forced-cooled.


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Four-quadrant operation of a drive system

The T plane with motors shaft cross sectional area is shown:

Figure 14 Four-quadrant operation of a drive system

The positive or forward speed is arbitrarily chosen in counterclockwise direction (it can also bechosen as clockwise). The positive torque is in the direction that will produce acceleration in

forward speed, as shown above.

The plane is divided into 4 quadrants , thus 4 modes of operation. The quadrants are marked as

I, II, III and IV

Quadrant IBoth torque and speed are positive the motor rotates in forward direction, which is in the same

direction as the motor torque. The power of the motor is the product of the speed and torque (P

= Te), therefore the power of the motor is positive. Energy is converted from electrical form to

mechanical form, which is used to rotate the motor. The mode of operation is known as forwardmotoring.

Quadrant II

The speed is in forward direction but the motor torque is in opposite direction or negative value.The torque produced by the motor is used to brake the forward rotation of the motor. The

mechanical energy during the braking, is converted to electrical energy thus the flow of energy

is from the mechanical system to the electrical system. The product of the torque and speed isnegative thus the power is negative, implying that the motor operates in braking mode. The

mode of operation is known as forward braking.

Quadrant III

The speed and the torque of the motor are in the same direction but are both negative. The

reverse electrical torque is used to rotate the motor in reverse direction. The power, i.e. theproduct of the torque and speed, is positive implying that the motor operates in motoring mode.

The energy is converted from electrical form to mechanical form. This mode of operation is

known as reverse motoring.

T

III

III IV

TeTe

TeTe


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Quadrant IVThe speed is in reverse direction but the torque is positive. The motor torque is used to brakethe reverse rotation of the motor. The mechanical energy gained during the braking is converted

to electrical form thus power flow from the mechanical system to the electrical system. The

product of the speed and torque is negative implying that the motor operates in braking mode.This mode of operation is known as reverse braking.

Ratings of converters and motors

In order to accelerate to a given reference value, the motor torque has to be larger than the loadtorque. According to (1), the difference between Tl and Te determines how fast the angularacceleration 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.

Figure 7 Speed response with different torque limit settings

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. Thismomentary high torque is possible due to the large thermal capacity of the motor with suitableinsulators used for the winding. The converter, which conducts the motor current, must be able

to sustain this condition. However since the thermal capacity of a switching device is small, the

current cannot be higher than its rated value even for a short time. Consequently, the currentrating 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 whereasthe 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 meansthat sustained overloads on the motor has to be protected by an additional thermal protection

mechanism. Above the base speed, b, the toque is limited by the maximum 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.


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Steady-state stability

The motor will operate at the steady-state speed (point where Tl = 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 thestable 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

mathematically that the condition for stable equilibrium is:

(17)

Figure 9 Steady state stability

Te Tl

Motor will decelerate

back to equilibrium

since Tl > Te

Motor will accelerate

away from equilibriumsince Te > Tl

Torque

speed

Torque

speed

Tl Te

Torque

Speed

Power limit fortransient torque

Power limit forcontinuoustorque

Transienttorque limit

Continuoustorque limit

Maximumspeed limit

b- b

Figure 8 Limits for torque, speedand power for drive system


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References

G.K. Dubey, Fundamental of Electrical Drives, Narosa, 1994.

W. Leonhard, Control of Electrical Drives, Springer-Verlag, 2001

introduction to electrical drives

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