1 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

DC MACHINES

Fleming's left hand rule and right hand rule

If a current carrying conductor placed in a magnetic field, it experiences a force due to the

magnetic field. On the other hand, if a conductor moved in a magnetic field, an EMF gets induced

across the conductor (Faraday's law of electromagnetic induction).

John Ambros Fleming originated two rules to determine the direction of motion (in electric motors)

or the direction of induced current (in electric generators). The rules are called as, Fleming's left

hand rule (for motors) and Fleming's right hand rule (for generators).

Fleming's left hand rule

Fleming's left hand rule is applicable for

electric motors. Whenever a current carrying

conductor is placed in a magnetic field, the

conductor experiences a force. According to

Fleming's left hand rule, if the thumb, fore-finger

and middle finger of left hand are stretched

perpendicular to each other as shown the figure

above, and if fore finger represents the direction of

magnetic field, the middle finger represents the

direction of current, then the thumb represents the direction of force.

Fleming's right hand rule

Fleming's right hand rule is applicable for

electrical generators. As per Faraday's law of

electromagnetic induction, whenever a conductor is

moved in an electromagnetic field, and closed path is

provided to the conductor, current gets induced in it.

According to Fleming's right hand rule, the thumb,

fore finger and middle finger of right hand are

stretched perpendicular to each other as shown in the

figure at right, and if thumb represents the direction of

the movement of conductor, fore-finger represents direction of the magnetic field, then the middle

finger represents direction of the induced current.

1. Explain the Construction of a DC machine with proper diagram?

A DC generator can be used as a DC motor without any constructional changes and vice

versa is also possible. Thus, a DC generator or a DC motor can be broadly termed as a DC machine.

These basic constructional details are also valid for the construction of a DC motor. Hence, let's

call this point as construction of a DC machine instead of just 'construction of a dc generator'.

2 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

The above figure shows the constructional details of a simple 4-pole DC machine. A DC machine

consists two basic parts; stator and rotor. Basic constructional parts of a DC machine are described

below.

1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It

not only provides mechanical strength to the whole assembly but also carries the magnetic

flux produced by the field winding.

2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They

carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i)

they support field coils and (ii) spread out the flux in air gap uniformly.

3. Field winding: They are usually made of copper. Field coils are former wound and placed

on each pole and are connected in series. They are wound in such a way that, when

energized, they form alternate North and South poles.

4. Armature core: Armature core is the rotor of the machine. It is cylindrical in shape with

slots to carry armature winding. The armature is built up of thin laminated circular steel disks

for reducing eddy current losses. It may be provided with air ducts for the axial air flow for

cooling purposes. Armature is keyed to the shaft.

3 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

5. Armature winding: It is usually a former wound copper coil which rests in armature slots.

The armature conductors are insulated from each other and also from the armature core.

Armature winding can be wound by one of the two methods; lap winding or wave winding.

6. Commutator and brushes: The function of a commutator, in a dc generator, is to collect the

current generated in armature conductors. The voltage generated in the armature, placed in a

rotating magnetic field, of a DC generator is alternating in nature. The commutation in DC

machine or more specifically commutation in DC generator is the process in which generated

alternating current in the armature winding of a dc machine is converted into direct current

after going through the commutator and the stationary brushes.Brushes are usually made

from carbon or graphite. They rest on commutator segments and slide on the segments when

the commutator rotates keeping the physical contact to collect or supply the current.

2. Explain with neat diagram the working principle of D.C. Motor.

A DC motor in simple words is a device that converts direct current(electrical energy) into

mechanical energy. In Simple words working principle of motor is when any current

carrying conductor placed in magnetic field experience a force.

The very basic construction of a dc

motor contains a current carrying

armature which is connected to the

supply end through commutator

segments and brushes and placed

within the north south poles of a

permanent or an electro-magnet as

shown in the diagram.

If current carrying conductor is placed in magnetic field and

having direction of current inside plane . Then the magnetic

lines of force are more right to conductor and less left to

conductor so forcing it leftside. It is rather like the effect of a

stretched rubber sheet on an object put on it.

There are two conditions which are necessary to produce a

force on the conductor. The conductor must be carrying

current, and must be within a magnetic field. When these two

conditions exist, a force will be applied to the conductor, which will attempt to move the conductor

in a direction perpendicular to the magnetic field. This is the basic theory by which all DC motors

operate. The force exerted upon the conductor can be expressed as follows.

F= B I l Newton (1)

where B is the density of the magnetic field, l is the length of conductor, and I the value of

current flowing in the conductor. The direction of motion can be found using Fleming’s Left Hand

Rule.

Fleming’s left hand rule is used to determine the

direction of force acting on the armature conductors of

dc motor. Fleming’s left hand rule says that if we extend the

index finger, middle finger and thumb of our left hand in

such a way that the current carrying conductor is placed in a

4 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

magnetic field (represented by the index finger) is perpendicular to the direction of current

(represented by the middle finger), then the conductor experiences a force in the direction

(represented by the thumb) mutually perpendicular to both the direction of field and the current in

the conductor.

3. Explain the different types of DC motors?

DC generators can be classified in two main categories, viz; (i) Separately excited and (ii)

Self-excited.

(i) Separately excited: In this type, field coils are energized from an independent external DC

source.

(ii) Self excited: In this type, field coils are energized from the current produced by the generator

itself. Initial EMF generation is due to residual magnetism in field poles. The generated emf causes a

part of current to flow in the field coils, thus strengthening the field flux and thereby increasing

EMF generation. Self excited dc generators can further be divided into three types -

(a) Series wound - field winding in series with armature winding

(b) Shunt wound - field winding in parallel with armature winding

(c) Compound wound - combination of series and shunt winding

Each DC machine can act as a generator or a motor. Hence, this classification is valid for

both: DC generators and DC motors. DC machines are usually classified on the basis of their

field excitation method. This makes two broad categories of dc machines; (i) Separately excited

and (ii) Self-excited.

• Separately excited: In separately excited dc machines, the field winding is supplied from a

separate power source. That means the field winding is electrically separated from the armature

circuit. Separately excited DC generators are not commonly used because they are relatively

expensive due to the requirement of an additional power source or circuitry.

• Self-excited: In this type, field winding and armature winding are interconnected in various

ways to achieve a wide range of performance characteristics (for example, field winding in series

or parallel with the armature winding).

5 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

In self-excited type of DC generator, the field winding is energized by the current produced by

themselves. A small amount of flux is always present in the poles due to the residual magnetism.

So, initially, current induces in the armature conductors of a dc generator only due to the residual

magnetism. The field flux gradually increases as the induced current starts flowing through the

field winding.

Self-excited machines can be further classified as –

o Series wound – In this type, field winding is connected in series with the armature winding.

Therefore, the field winding carries whole load current (armature current). That is why series

winding is designed with few turns of thick wire and the resistance is kept very low (about 0.5

Ohm).

o Shunt wound – Here, field winding is connected in parallel with the armature winding. Hence,

the full voltage is applied across the field winding. Shunt winding is made with a large number

of turns and the resistance is kept very high (about 100 Ohm). It takes only small current which

is less than 5% of the rated armature current.

o Compound wound – In this type, there are two sets of field winding. One is connected in series

and the other is connected in parallel with the armature winding. Compound wound machines

are further divided as -

▪ Short shunt – field winding is connected in parallel with only the armature winding

▪ Long shunt – field winding is connected in parallel with the combination of series field winding

and armature winding

1. Discuss different types of D.C. motor with suitable circuit diagram, voltage equation

and application.

The DC motor has a lot of application in today’s field of engineering and technology. Starting from an

electric shaver to parts of automobiles, in all small or medium sized motoring applications DC motors

come handy. And because of its wide range of application different functional types of dc motor are

available in the market for specific requirements. The types of DC motor can be listed as follows

6 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Separately exited dc motor:

Applications Of DC motor:

Types of

motors

characteristics Applications

Shunt motor 1. Almost constant speed

2. Adjustable speed

3. Starting torque is

medium

For driving constant speed line shafting,

lathes, centrifugal pumps, reciprocating

pumps, Machine tools.

Series motor 1. Variable speed

2. Adjustable varying speed

3. Starting torque is very

high

For traction ( Electric trains locomotives)

Lift, crane, hoists, conveyor belts, trolleys car.

Cumulative

motor

1. variable speed

2. adjustable variable speed

3. higher starting torque

Punch, shears, rolling mills, planning

machines in workshop, elevators.

𝑣 = 𝐸𝑏 + 𝐼𝑎𝑅𝑎

𝐼𝐿 = 𝐼𝑎 + 𝐼𝑓

𝐼𝑓 =𝑉

𝑅𝑆𝐻

𝑣 = 𝐸𝑏 + 𝐼𝑎(𝑅𝑎 + 𝑅𝑠𝑒)

𝐼𝑙 = 𝐼𝑎 = 𝐼𝑓

𝑣 = 𝐸𝑏 + 𝐼𝑎(𝑅𝑎 + 𝑅𝑠𝑒)

𝐼𝐿 = 𝐼𝑎 + 𝐼𝑓

𝐼𝑎 = 𝐼𝑠𝑒 ;

𝐼𝑓 =𝑉

𝑅𝑆𝐻

𝑣 = 𝐸𝑏 + 𝐼𝑎𝑅𝑎

𝐼𝐿 = 𝐼𝑎

𝐼𝑓 =𝑉

𝑅𝑆𝐻

7 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

4. What is back EMF and explain its significance?

When the armature of a d.c. motor rotates under the influence of the driving torque, the

armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a

generator. The induced e.m.f. acts in opposite direction to the applied voltage V (Lenz’s law) and in

known as back or counter e.m.f. Eb.

The back emf Eb(= PΦZN/60 A) is always less than the applied voltage V, although this

difference is small when the motor is running under normal conditions.

A shunt wound dc motor shown in figure below. When

dc voltage V is applied across the motor terminals, the

field magnets are excited and armature conductors

are supplied with current. Therefore, driving torque

acts on the armature which begins to rotate. As the

armature rotates, back emf Eb is induced which

opposes the applied voltage V.

The applied voltage V has to force current through the

armature against the back emf Eb. The electric work

done in overcoming and causing the current to flow against Eb is converted into mechanical energy

developed in the armature. It follows, therefore, that energy conversion in a d.c. motor is

only possible due to the production of back emf Eb.

The presence of back emf makes the d.c. motor a self-regulating machine i.e., it makes the motor to

draw as much armature current as is just sufficient to develop the torque required by the load.

Armature current, Ia=(V−Eb) / Ra

Back emf in a d.c. motor regulates the flow of armature current i.e., it automatically changes the

armature current to meet the load requirement.

5. Explain the different Characteristics of DC motors?

Generally, three characteristic curves are considered for DC motors which are, (i) Torque vs.

armature current (Ta - Ia), (ii) Speed vs. armature current and (iii) Speed vs. torque. These are

explained below for each type of DC motor. These characteristics are determined by keeping

followingtwo relations in mind.

Ta αΦ.Ia and N α Eb/Φ

Characteristics of DC series motors

1. Torque vs. armature current (Ta-Ia)

This characteristic is also known as electrical

characteristic. We know that torque is directly

proportional to armature current and flux,

Ta α Φ.Ia.

Thus, before magnetic saturation of the field,

Φ α Ia. Therefore, before magnetic saturation

Ta α Ia2.

Therefore, the Ta-Ia curve is parabola for smaller

8 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

values of Ia.

Therefore After magnetic saturation of the field winding, flux Φ is independent of armature

current Ia., the torque varies proportional to Ia only, T α Ia.

Therefore, after magnetic saturation, Ta-Ia curve becomes straight line.

The shaft torque (Tsh) is less than armature torque (Ta) due to stray losses.

In DC series motors, (prior to magnetic saturation) torque increases as the square of armature

current, these motors are used where high starting torque is required

2. Speed vs. armature current (N-Ia)

We know the relation, N α Eb/Φ and Φ α Ia so N α 1/ Ia

Thus, for small currents speed is inversely proportional

to Φ. As we know, Φ α Ia speed is also inversely

proportional to Ia.

When armature current is very small the speed

becomes dangerously high. That is why a series

motor should never be started without some

mechanical load.

But, at heavy loads, armature current Ia is large. And

hence speed is low which results in decreased back emf

Eb. Due to decreased Eb, more armature current is allowed.

Speed vs. torque (N-Ta)

This characteristic is also called as mechanical characteristic. From the above two characteristics

of DC series motor, it can be found that when speed is high, torque is low and vice versa.

T α ΦIa and N α Eb/Φ

Φ α Ia so N α 1/ Ia

Characteristics of DC shunt motors

Torque vs. armature current (Ta-Ia)

In case of DC shunt motors we can assume the field flux Φ

to be constant. T α ΦIa so T α Ia

Hence the Ta-Ia characteristic for a dc shunt motor will be a

straight line through origin.

Since, heavy starting load needs heavy starting current,

shunt motor should never be started on a heavy load.

The shaft torque (Tsh) is less than armature torque (Ta) due

to stray losses

9 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Speed vs. armature current (N-Ia)

As flux Φ is assumed constant, we can say N α Eb. But,

back emf is also almost constant, the speed remains

constant. But practically, Φ as well as Eb decreases with

increase in load. But, the Eb decreases slightly more than

Φ, and hence the speed decreases slightly. Generally, the

speed decreases by 5 to 15% of full load speed only. And

hence, a shunt motor can be assumed as a constant speed

motor.

𝑣 = 𝐸𝑏 + 𝐼𝑎𝑅𝑎 ; 𝐼𝐿 = 𝐼𝑎 + 𝐼𝑓 ; 𝐼𝑓 =𝑉

𝑅𝑆𝐻

Speed vs. Torque (N-Ta)

Characteristics of DC compound motor

6. Necessity of Starters for dc motors?

Basic operational voltage equation of a DC motor is given as

V = Eb + IaRa and hence Ia = (V - Eb) / Ra

Now, when the motor is at rest, obviously, there is no back emf Eb, hence armature current will be

high at starting. Ia = V / Ra because Ra is small.

This excessive current will-

1. blow out the fuses and may damage the armature winding and/or commutator brush arrangement.

10 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

2. produce very high starting torque (as torque is directly proportional to armature current), and this

high starting toque will produce huge centrifugal force which may throw off the armature windings.

Thus to avoid above two drawbacks, starters are used for starting of DC machine.

Thus, to avoid the above dangers while starting a DC motor, it is necessary to limit the

starting current. For that purpose, starters are used to start a DC motor. There are various starters

like, 3 point starter, 4 point starter, No load release coil starter, thyristor starter etc.

The main concept behind every DC motor starter is, adding external resistance to the

armature winding at starting.

7. Explain the 3 point starters of dc shunt motors?

A 3 point starter in simple words is a device that helps in the starting and running of a shunt

wound DC motor or compound wound DC motor. The general motor emf equation V = Eb + Ia.Ra,

at starting is modified to V = Ia.Ra as at starting Eb = 0. Ia= V/ Ra

that the current will be dangerously high at

starting (as armature resistance Ra is small).

So to limit stating current starters are used.

Construction of 3 Point Starter

A starter is a variable resistance, integrated

into number of sections as shown in the

figure beside. The contact points of these

sections are called studs and are shown

separately as OFF, 1, 2,3,4,5, RUN. Other

than that there are 3 main points, referred to

as

1. 'L' Line terminal. (Connected to positive

of supply.)

2. 'A' Armature terminal. (Connected to the

armature winding.)

3. 'F' Field terminal. (Connected to the field

winding.) And from there it gets the name 3

point starter.

The point 'L' is connected to an electromagnet called overload release (Coil) (OLC) as shown in

the figure. The other end of 'OLR' is connected to the lower end of conducting lever of starter handle

where a spring is also attached with it and the starter handle contains also a soft iron piece housed on

it. This handle is free to move to the other side RUN against the force of the spring. This spring

brings back the handle to its original OFF position under the influence of its own force. Another

parallel path is derived from the stud '1', given to the another electromagnet called No Volt Coil

(NVC) which is further connected to terminal 'F'. The starting resistance at starting is entirely in

series with the armature. The OLR and NVC acts as the two protecting devices of the starter.

Working of Three Point Starter

(i) To start with, the d.c. supply is switched on with handle in the OFF position.

(ii) The handle is now moved clockwise to the first stud. As soon as it comes in contact with the first

stud, the shunt field winding is directly connected across the supply, while the whole starting

11 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

resistance is inserted in series with the armature circuit.

(iii) As the handle is gradually moved over to the final stud, the starting resistance is cut out of the

armature circuit in steps. The handle is now held magnetically by the no-volt release coil which is

energized by shunt field current.

Working of No Voltage Coil :--- If the supply voltage is suddenly interrupted or if the field

excitation is accidentally cut, the no-volt release coil is demagnetized and the handle goes back to

the OFF position under the pull of the spring. If no-volt release coil were not used, then in case of

failure of supply, the handle would remain on the final stud. If then supply is restored, the motor will

be directly connected across the supply, resulting in an excessive armature current.

Working of overload coil :-- If the motor is over-loaded (or a fault occurs), it will draw excessive

current from the supply. This current will increase the ampere-turns of the over-load release coil

and pull the lever , due to tringular part of lever , short-circuiting the no- volt release coil. The no-

volt coil is demagnetized and the handle is pulled to the OFF position by the spring. Thus, the motor

is automatically disconnected from the supply.

Drawback:

In a three-point starter, the no-volt release coil is connected in series with the shunt field

circuit so that it carries the shunt field current. While exercising speed control through field

regulator, the field current may be weakened to such an extent that the no-volt release coil may not

be able to keep the starter arm in the ON position. This may disconnect the motor from the supply

when it is not desired. This drawback is overcome in the four point starter.

8. Explain 4 point starter of dc shunt motor?

Construction and Operation of Four

Point Starter

A 4 point starter as the name suggests

has 4 main operational points, namely

1.'L' Line terminal. (Connected to positive of

supply.)

2. 'A' Armature terminal. (Connected to the

armature winding.)

3. 'F' Field terminal. (Connected to the field

winding.) Like in the case of the 3 point

starter, and in addition to it there is,

4. A 4th point N. (Connected to the No

Voltage Coil)

The remarkable difference in case of a 4

point starter is that the No Voltage Coil is

connected independently across the supply

through the fourth terminal called 'N' in

addition to the 'L', 'F' and 'A'. As a direct

consequence of that, any change in the field

supply current does not bring about any difference in the performance of the NVC. Thus it must be

ensured that no voltage coil always produce a force which is strong enough to hold the handle in its

'RUN' position, against force of the spring, under all the operational conditions. Such a current is

12 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

adjusted through No Voltage Coil with the help of fixed resistance R connected in series with the

NVC using fourth point 'N' as shown in the figure above.

Considering that supply is given and the handle is taken stud No.1, then the circuit is

complete and line current that starts flowing through the starter. In this situation we can see that the

current will be divided into 3 parts, flowing through 3 different points. i) 1 part flows through the

starting resistance (R1+ R2+ R3…..) and then to the armature. ii) A 2nd part flowing through the field

winding F. iii) And a 3rd part flowing through the no voltage coil in series with the protective

resistance R.

Working of No Voltage Coil :--- If the supply voltage is suddenly interrupted or if the field

excitation is accidentally cut, the no-volt release coil is demagnetized and the handle goes back to

the OFF position under the pull of the spring. If no-volt release coil were not used, then in case of

failure of supply, the handle would remain on the final stud. If then supply is restored, the motor will

be directly connected across the supply, resulting in an excessive armature current.

Working of overload coil :-- If the motor is over-loaded (or a fault occurs), it will draw excessive

current from the supply. This current will increase the ampere-turns of the over-load release coil

and pull the lever , due to tringular part of lever , short-circuiting the no- volt release coil. The no-

volt coil is demagnetized and the handle is pulled to the OFF position by the spring. Thus, the motor

is automatically disconnected from the supply.

9. Explain speed control methods for dc motor.

We know that the expression of speed control dc motor is given as,

The different methods of speed control in DC shunt motors are explained below,

1. Flux Control Method

2. Armature Control Method

3. Voltage Control Method

o Multiple Voltage Control

o Ward Leonard System

1) field control methods and 2) armature control methods.

1. Field Control of DC Shunt Motor :- We know, back emf of a DC motor Eb is the induced emf due

to rotation of the armature in magnetic field. Thus value of the

Eb can be given by

the

Eb = PØNZ/60A

(where, P= no. of

poles, Ø=flux/pole,

N=speed in rpm,

Z=no. of armature

conductors,

A=parallel paths)

13 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Eb can also be given as, Eb = V- IaRa N = Eb 60A/PØZ but, for a DC motor A, P and Z are constant

N α K Eb/Ø (where, K=constant)

thus, it shows speed is directly proportional to back emf and inversely proportional to the flux per

pole. 1. Field rheostat control of DC Shunt Motor: N α 1 /Ø Ø α If

In this method , speed variation is accomplished by means of a variable resistance inserted in series

with the shunt field . An increase in controlling resistances reduces the field current with a

reduction in flux and an increase in speed. This method of speed control is independent of load on

the motor. Power wasted in controlling resistance is very less as field current is a small value.

Advantages

1. It provides relatively smooth and easy control.

2. Speed control above rated speed is possible.

3. As the field winding resistance is high, the field current is small. Hence power loss (R) in the

external resistance is very small, which makes the method more economical and efficient.

4. As the field current is small, the size of rheostat required is small.

Disadvantages

1. The speed control below normal rated speed is not possible as flux can be increased only upto its

rated value.

2. As flux reduces, speed increases. But high speed affects the commutation making motor operation

unstable. So there is limit to the maximum speed above normal, possible by this method.

2. Armature Control of DC Shunt Motor

In this method armature circuit is provided with a variable resistance. Field is directly connected

across the supply so flux is not changed due to variation of series resistance. This is applied for dc

shunt motor. This method is used in printing press, cranes, hoists where speeds lower than rated

is used for a short period only.

We know, back emf of a DC motor Eb is the induced emf due to

rotation of the armature in magnetic field. Thus value of the Eb can be given by the

Eb = PØNZ/60A (where, P= no. of poles, Ø=flux/pole, N=speed in rpm, Z=no. of armature conductors,

A=parallel paths)

Eb can also be given as, Eb = V- IaRa N = Eb 60A/PØZ but, for a DC motor A, P and Z are constant

N α K Eb/Ø (where, Kand Ø =constant) So N α

Eb Speed of the motor is directly proportional to the back emf Eb and Eb = V- IaRa.. Thus if

we add resistance in series with armature, Ia decreases and hence speed decreases.

14 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Greater the resistance in series with armature, greater the decrease in speed.

Advantages

1. Easy and smooth speed control below normal is possible.

Disadvantages

1. As the entire armature current passes through the external resistance, there are tremendous

power losses.

2. As armature current is more than field current, rheostat required is of large size and

capacity.

3. Speed above rated is not possible by this method.

4. Due to large power losses, the method is expensive, wasteful and less efficient.

5. The method needs expensive heat dissipation arrangemenr

10. Compare dc motor and generator.

DC Motor Generators

• The electric motor converts electrical

energy into mechanical energy

• A generator converts mechanical energy

into electrical energy.

• The shaft of a motor is driven by the

magnetic forces developed between the

armature and field

•A shaft attached to the rotor is driven by a

mechanical force.

•Current has to be supplied to the armature

winding.

• Electric current is produced in the armature

windings.

•Motors obey Fleming`s Left Hand Rule

•Generators obey Fleming’s Right Hand

Rule

It is based on the principle that current

carrying conductor placed in magnetic field

exerience the force.

It based on the principle of electromagnetic

induction.

11. Losses in Dc machine.

a The losses in a dc machine (generator or motor) may be divided into three classes viz

(i) Copper losses (ii) Iron or core losses and (iii) Mechanical losses.

All these losses appear as heat and thus raise the temperature of the machine. They also lower the

efficiency of the machine.

Copper losses: These losses occur due to currents in the various windings of the machine.

1. Armature copper loss = Ia2Ra

2. Shunt field copper loss = Ish2Rsh

3. Series field copper loss = Ise2Rse

Note. There is also brush contact loss due to brush contact resistance (i.e., resistance between the

surface of brush and surface of commutator). This loss is generally included in armature copper loss.

15 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Iron or Core losses: These losses occur in the armature of a d.c. machine and are due to the

rotation of armature in the magnetic field of the poles. They are of two types viz., (i) hysteresis

loss (ii) eddy current loss.

(i) Hysteresis loss

Hysteresis loss occurs in the armature of the d.c. machine since any

given part of the armature is subjected to magnetic field reversals as

it passes under successive poles.

Figure shows an armature rotating in two-pole machine. Consider a small piece ab of the armature.

When the piece ab is under N-pole, the magnetic lines pass from a to b. Half a revolution later, the

same piece of iron is under S-pole and magnetic lines pass from b to a so that magnetism in the iron

is reversed. In order to reverse continuously the molecular magnets in the armature core, some

amount of power has to be spent which is called hysteresis loss. It is given by Steinmetz formula.

This formula is

Hysteresis loss Ph = η Bmax1.6 f V

where Bmax = Maximum flux density in armature

f = Frequency of magnetic reversals

= NP/120 where N is in r.p.m.

V = Volume of armature in m3

h = Steinmetz hysteresis co-efficient

In order to reduce this loss in a d.c. machine, armature core is made of such materials which have a

low value of Steinmetz hysteresis co-efficient e.g.,silicon steel.

(ii) Eddy current loss

In addition to the voltages induced in the armature conductors, there are also voltages induced in

the armature core. These voltages produce circulating currents in the armature core as shown

in Fig. These are called eddy currents and power loss due to their flow is called eddy current loss.

The eddy current loss appears as heat which raises the temperature of the machine and lowers its

efficiency.

If a continuous solid iron core is used, the resistance to eddy current path will be small due to large

cross-sectional area of the core. Consequently, the magnitude of eddy current and hence eddy

current loss will be large. The magnitude of eddy current can be reduced by making core

resistance as high as practical. The core resistance can be greatly increased by constructing the

core of thin, round iron sheets called laminations. The laminations are insulated from each other

with a coating of varnish. The insulating coating has a high resistance, so very little current flows

from one lamination to the other. Also, because each lamination is very thin, the resistance to current

flowing through the width of a lamination is also quite large. Thus laminating a core increases the

core resistance which decreases the eddy current and hence the eddy current loss.

Eddy current loss Pe = Ke Bmax f2 t2 V

where Ke = Constant depending upon the electrical resistance of core and system of units used

Bmax = Maximum flux density in Wb/m2

f = Frequency of magnetic reversals in Hz

t = Thickness of lamination in m

V = Volume of core in m3

16 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

It may be noted that eddy current loss depends upon the square of lamination thickness. For this

reason, lamination thickness should be kept as small as possible.

Mechanical losses

These losses are due to friction and windage.

(i) friction loss e.g., bearing friction, brush friction etc.

(ii) windage loss i.e., air friction of rotating armature.

These losses depend upon the speed of the machine. But for a given speed, they are practically

constant.

Note. Iron losses and mechanical losses together are called stray losses.

12. Explain different tests performed on dc motor.

There are several tests that are conducted for testing a dc machine (generator or motor) to judge

its performance. The most important performance tests to be conducted on dc machine are:

1. The magnetization or open-circuit test.

2. The load characteristic.

3. The determination of efficiency curve.

4. The temperature rise test.

This article mainly deals with the determination of efficiency curve. The efficiency of a d.c. machine

depends upon its losses. The smaller the losses, the greater is the efficiency of the machine and vice-

versa.

The consideration of losses in a dc machine is important for two principal reasons.

• First, losses determine the efficiency of the machine and appreciably influence its operating

cost.

• Secondly, losses determine the heating of the machine and hence the power output that may

be obtained without undue deterioration of the insulation.

Efficiency of a D.C. Machine

The determination of efficiency of DC machine is important for the testing of DC machines. The

power that a dc machine receives is called the input and the power it gives out is called the output.

Therefore, the efficiency of a d.c. machine, like that of any energy-transferring device, is given by;

Efficiency = Output / Input -------------------------------①

Output = Input - Losses and Input = Output + Losses

Therefore, the efficiency of a d.c. machine can also be expressed in the following forms:

Efficiency = (Input - Losses) / Input -----------------------②

Efficiency = Output / (Output + Losses) --------------------③

The most obvious method of determining the efficiency of a d.c. machine is to directly load it and

measure the input power and output power. Then we can use Eq ① to determine the efficiency of

the machine. This method suffers from three main drawbacks.

• First, this method requires the application of load on the machine.

17 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

• Secondly, for machines of large rating, the loads of the required sizes may not be available.

• Thirdly, even if it is possible to provide such loads, large power will be dissipated, making it

an expensive method.

The most common method of measuring the efficiency of a d.c. machine is to determine its losses

(instead of measuring the input and output on load). We can then use Eq ② or Eq ③ to determine

the efficiency of the machine. This method has the obvious advantage of convenience and economy.

Methods for determining Efficiency

There are three different methods of determining the efficiency of a dc machine, namely, direct

method, indirect method and regenerative method.

Direct Method

By this method the efficiency and losses of only small machines can be determined. In this method

full load is applied to the machine and output is directly measured.

• Though this method is very simple in looking but involves complication in the measurement

of mechanical power input in the case of generator, and output in the case of a motor.

• In the case of a generator, the input mechanical power is measured by connecting some form

of dynamo-meter to the prime mover and water resistance load is applied.

• This method is wasteful, since all the input is to be wasted and can only be used in case of

small machines due to the-difficulty of having enough power and suitable brake

arrangements in case of larger machines.

Indirect Method

By Indirect method of testing of dc machines, efficiency of shunt and compound dc machines can be

determined.

• This method enables the determination of losses without actually loading the machine. The

power is required to supply the losses only, so there is no difficulty in applying this method

even to very large machines.

• Although the efficiency can be calculated with fair accuracy from the results obtained with

this method, the disadvantage of this method is that the machine is run light during the test

which gives no indication as to the temperature rise on load or to the commutating qualities

of the machines.

Regenerative Method

Regenerative method requires two identical machines, one of them operates as a motor and drives

the other, which is mechanically coupled to it.

• The other machine ,operates as a generator and feed back power into the supply. Thus the

total power drawn from the supply is only for supplying the internal losses of the two

machines.

• Thus very large machines may be tested as the power required is small.

Important Tests of DC Machines

The important Tests of DC machines are classified as under

18 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

1. Efficiency By Direct Loading

o Brake test

o Using calibrated generator

2. Swinburne’s Method ( Indirect Method )

3. Regenerative or Hopkinson’s-Test ( Regenerative Method )

4. Retardation or Running down Test

13. Explain no load test of dc motor.(Swinburne’s test)

Swinburne’s test is the simplest indirect method of testing dc machines. In this method, the dc

machine (generator or motor) is run as a motor at no-load and losses of the machine are determined.

Once the losses of the machine are known, its efficiency at any desired load can be determined in

advance.

It may be noted that this method is applicable to those machines in which flux is practically

constant at all loads e.g., shunt and compound machines.

The machine is run as a motor on no-load with supply voltage adjusted to the rated voltage

i.e. voltage stamped on the nameplate. The speed of the motor is adjusted to the rated speed with the

help of field regulator R as shown in figure

Let V = Supply voltage

Io = No-load current read by ammeter A1

Ish = Shunt-field current read by ammeter A2.

∴ No-load armature current, Ia0 = Io - Ish No-load input power to motor = V Io

No-load power input to armature = V Iao = V(Io - Ish)

Since the output of the motor is zero, the no-load input power to the armature supplies

(a) iron losses in the core (b) friction loss

(c) windage loss (d) armature Cu loss [ Iao2Ra or (Io - Ish)

2Ra .

Constant losses, Wc = Input to motor - Armature Cu loss

Wc = V Io - (Io - Ish)2Ra

Since constant losses are known, the efficiency of the machine at any other load can be determined.

Suppose it is desired to determine the efficiency of the machine at load current I. Then,

19 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Armature current, Ia = I - Ish ... if the machine is motoring

= I + Ish ... if the machine is generating

Efficiency when running as a motor

Input power to motor = VI

Armature Cu loss = Ia2Ra = (I - Ish)

2Ra

Constant losses = Wc found above

Total losses = (I - Ish)2Ra + Wc

∴ Motor efficiency, ηm = (Input - Losses)/Input = [VI - (I - Ish)2Ra + Wc]/VI

Advantages of Swinburne’s test

The following are the advantages of swinburne's test

• The power required to carry out the test is small because it is a no-load test. Therefore, this

method is quite economical.

• The efficiency can be determined at any load because constant losses are known.

• This test is very convenient.

Disadvantages of Swinburne's test

The disadvantages of swinburne's test are given below

• It does not take into account the stray load losses that occur when the machine is loaded.

• This test does not enable us to check the performance of the machine on full-load. For

example, it does not indicate whether commutation on full load is satisfactory and whether

the temperature rise is within the specified limits.

•

• This test does not give quite accurate efficiency of the machine. It is because iron losses

under actual load are greater than those measured. This is mainly due to armature reaction

distorting the field.

14. Explain brake load test on dc motor.

Brake test is also a method of finding efficiency of dc motors.We took dc shunt motor as running

machine.Brake test also called as direct loading test of testing the motor. Because loading will be

applied directly on shaft of the motor by means of a belt and pulley arrangement.

Test Requirements:

1. DC shunt motor

2. Water-cooled pulley

3. Spring balance

Procedure of Brake Test On DC Shunt Motor

1. By adjusting the handle of the pulley take different readings of the spring balance.

2. The tension in the belt can be adjusted using the handle. The tension in kg can be obtained from

the spring balance readings.

20 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

3. Adjusting the load step by step till full load, number of readings can be obtained.By increasing the

load is slowly, adjust to get rated load current.

4. The power developed gets wasted against the friction between belt and shaft. Due to the braking

action of belt the test is called brake test.

5. The speed can be measured by tachometer. Thus all the motor characteristics can be plotted.

You can refer figure for the experimental setup for performing brake test on a d.c. shunt motor,belt

& pulley arrangement mounted on the shaft of the motor.

Calculation Of Brake Test On DC Shunt Motor

Let R (or) r= Radius of pulley in meters

N = Speed in r.p.m.

W1 = Spring balance reading on tight side in kg

W2 = Spring balance reading on slack side in kg

So net pull on the belt due to friction at the pulley is the difference between the two spring balance

readings.

Net pull on the rope = (W - S) kg = (W - S) X 9.81 newtons......(1)

As radius R and speed N are known, the shaft torque developed can be obtained as,

Tsh = Net pull X R=(W - S) X 9.81 X R .....(2)

Hence the output power can be obtained as,Say speed of the pulley is N r.p.m., then,The above

equation shows the output power of dc shunt motor in brake test.

Now let, V = Voltage applied in volts

I = Total line current drawn in amps.

As we know V,I are input parameters of dc motors in brake test.then,

Pin=V.I Watts .....(3)

We have output and input.Then why late go and find the efficiency of dc shunt motor.

21 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Efficiency (η)=Output/Input [No units]

From equation (2) & (3)

Advantages Of Brake Test On DC Shunt Motor

1. Actual efficiency of the motor under working conditions can be found out.

2. Brake test is simple and easy to perform.

3. It is not only for dc shunt motor, also can be performed on any type of d.c. motor.

Disadvantages Of Brake Test On DC Shunt Motor

1. In brake test due the belt friction lot of heat will be generated and hence there is large dissipation

of energy.

2. Cooling arrangement is necessary to minimize the heat. Mostly in our laboratories we use water as

cooling liquid.

3. Convenient only for small rated machines due to limitations regarding heat dissipation

arrangements.

4.Power developed gets wasted hence brake test method is little expensive.

5. The efficiency observed is on lower side.

15. Explain electric braking .(dynamic,plugging and Regenerative braking)

An electric motor can be brought to rest quickly by using usual mechanical brakes or electric brakes.

With the mechanical brakes it is not possible to achieve a smooth stop and also linings, levers and

mechanical arrangement is necessary. It also depends on the skill of the operator.

As against this electric braking is easy, reliable which brings motor to rest very quickly. Three

types of electric braking methods are available i) Rheostatic or dynamic braking ii) Plugging and iii)

Regenerative braking. Though motor is brought to rest electrically, to maintain its state of rest a

mechanical brake is must.

Advantage of Electrical Braking over Mechanical Braking:

• Electric braking is fast and cheap.

• In electric braking there is no maintenance cost like replaced brake shoes periodically.

• By using electic braking the capacity of the system( like higher speeds, heavy loads) can be

increased.

• A part of energy is returned to the supply consequently the running cost is reduced.

• In electric braking negligible amount of heat is generated whereas in mechanical braking

enormous heat is produced at brake shoes which leads to failure of brakes.

22 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Disadvantages, Limitations of Electrical Braking:

• Electric braking can not be used for hold the machine after coming to the rest.

So a mechanical braking is required additionally.

• For example in the case of the train going up hill, after the train has been stopped using

electric braking, it has to be prevented from moving down hill.

In such cases mechanical brakes also should be provided for holding.

During the braking period, the motor has to function as generator. So it must have suitable

braking characteristics. ie, the choice of motor is limited.

Let us see these methods as applied to shunt and series motor.

1.1 Electric Braking of D.C. Shunt Motor

(a) Rheostatic or Dynamic Braking :

The fig shows that the dynamic or rheostat braking of a DC shunt motor. In this the running motor

armature is cut off from the source and it’s connected across an adjustable resistance. But the field

winding of the motor is left connected from the supply. When the armature slowing down, rotates in

a strong magnetic field and therefore works as a generator and sending a high current through

resistance R. This reasons the energy influenced by the revolving armature to be dissipated rapidly

in the form of heat in the resistance. Because of this the motor is taken to standstill immediately.

By varying the adjustable resistance R the braking torque can be controlled. The motor speed

reduces once the value of resistance R decreases, and then the breaking torque might be controlled at

a large value. When the motor speed is low, the braking torque becomes low and the motor attains to

stop because of friction. Generally this dynamic braking type is used widely in connection with the

control of hoists and elevators. Also this type is used where the motor should be stopped, started and

reversed repeatedly.

The disadvantages of this method is that in case of failure of electric supply, this method is

ineffective.

Let us see how the baking torque of motor depends upon the speed.

Let us see how the baking torque of motor depends upon the speed.

Referring to the figure-b above;

Armature Current Ia = Eb / (R+Ra)

= (K1.NΦ) / (R+Ra) (∵ Eb∝N)

Braking Torque TB = K2IaΦ

= K2Φ (K1.NΦ / R+Ra)

= K3NΦ2Where,

K2 and K3 are constant

For Shunt Motor, Φ is constant.

23 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

Therefore Braking torque TB∝N

i.e. Braking torque decrease the motor speed is also decrease.

(b) Plugging :

This involves the reversing of the armature connections of the motor. We have seen that if the

direction of current through armature or field is reversed. In case of plugging, motor is subjected to

the reversely directed torque. Under such condition the back e.m.f. and the voltage act in the same

direction, so the current flowing in the circuit is produced by a voltage V + Eb i.e. approximately 2

V. To limit high inrush of current an additional resistance R is connected. In this method, motor tries

to accelerate in the other direction, after coming to rest. So some auxiliary device is required to cut-

off the supply as soon as motor comes to rest. In case of failure of supply this method is ineffective.

This method gives more braking torque that the rheostatic braking. The method applied to a shunt

motor is shown in the Fig. 2(a) and (b).

The method is generally used in braking elevators, rolling mills, machine tools etc.

Now will see how braking torque depends on the motor speed.

From the above fig (plugging)

Armature Current Ia = (V+Eb) / (R=Ra)

Ia = (V/R+Ra) + (K1NΦ/R+Ra)

∵ Eb ∝ NΦ

Braking Torque TB = K2IaΦ

TB = K2Φ {(V/R+Ra) + (K1NΦ / R+Ra)}

∴ TB = K3NΦ + K4NΦ2

For a DC Shunt Motor, Φ is constant.

Thus Braking torque ∴ TB = K5 + K6N

Therefore braking torque reduces as the motor speed slows down. Even when the motor speed

reduced to zero there will be certain braking torque (TB = K5) (c) Regenerative braking :

In this method, instead of being disconnected from the supply, it remains connected and returns

the braking energy to the line. Consider a shunt motor running as shown in Fig. 3(a). Suppose the

load causes the speed to be increased above normal, the field current remaining the same then the

back e.m.f. becomes greater than the supply voltage (Eb > V).

24 | Mrs. V. S. Patki (M.E.(Electrical machines and Drives))

W.I.T. Solapur (Electronics Department)

The current gets reversed and power will be supplied to the line, tending to prevent any further

increase in the speed. Alternatively same effect can be obtained by increasing the field current where

motor quickly slowed down to the speed corresponding to the new value of the field current. Due to

reversal of direction of armature current as Eb > V, armature torque is reversed and speed falls until

Eb becomes less than V.

The method is used while lowering the cage of a hoist or the downgrade motion of an

electric train.