INDUCTION MOTOR

Principle of DC Generator
• A dc generator produces direct power. Both of these
generators produce electrical power, based on same
fundamental principle of Faraday’s law of electromagnetic
induction.
• when an conductor moves in a magnetic field it cuts magnetic
lines force, due to which an emf is induced in the conductor.
• The most basic two essential parts of a generator are
a) a magnetic field and
b) conductors which move inside that magnetic field.
1

Single Loop DC Generator
Rectangular loop of conductor is ABCD which rotates inside the
magnetic field about its won axis ab.
As during this movement two sides, i.e. AB and CD of the loop
cut the flux lines there will be an emf induced in these both of
the sides
2

The loop is opened and connect it with a split ring as shown in
the figure below.
3

It is seen that in the first half of the revolution current flows always
along ABLMCD i.e. brush no 1 in contact with segment a.
In the next half revolution, in the figure the direction of the induced
current in the coil is reversed.
But at the same time the position of the segments a and b are also
reversed which results that brush no 1 comes in touch with that
segment b.
Hence, the current in the load resistance again flows from L to M.
4

The position of the brushes is so arranged that the change over
of the segments a and b from one brush to other takes place
when the plane of rotating coil is at right angle to the plane of
the lines of force.
5

Construction of DC Generator
A DC Generator has the following parts
1) Yoke
2) Pole of Generator
3) field winding
4) Armature of dc generator
5) Brushes of generator
6) Bearing
6

Yoke of DC Generator
(i) It holds the magnetic pole cores of the generator and acts
as cover of the generator.
(ii) It carries the magnetic field flux.
• In small generator, yoke are made of cast iron.
• But for large construction of DC generator, where weight of
the machine is concerned, lighter cast steel or rolled steel is
preferable for constructing yoke of dc generator.
7

Pole cores and pole shoes of DC Generator
There are mainly two types of construction available.
One: Solid pole core, where it made of a solid single piece of cast
iron or cast steel.
Two: Laminated pole core, where it made of numbers of thin, plates
of annealed steel.
The construction of magnetic poles basically comprises of two parts
namely,
the pole core and
the pole shoe, stacked together and then attached to the yoke.
8

These two structures are assigned for different purposes,
the pole core is of small cross sectional area and its function is to
just hold the pole shoe over the yoke,
whereas the pole shoe having a relatively larger cross-sectional
area spreads the flux produced over the air gap
Pole cores and pole shoes of DC Generator
9

Armature Core of DC Generator
• The purpose of armature core is to hold the armature
winding and provide low reluctance path for the flux
• Although a dc generator provides direct current but induced
current in the armature is alternating in nature.
• That is why, cylindrical or drum shaped armature core is
build up of circular laminated sheet.
• In every circular lamination, slots are either die – cut or
punched on the outer periphery and the key way is located
on the inner periphery
10

Armature Winding of DC Generator
• Armature winding are generally formed wound.
• Various conductors of the coils are insulated from each other.
• The conductors are placed in the armature slots, which are
lined with tough insulating material.
11

Commutator of DC Generator
• The commutator plays a vital role in dc generator.
• It collects current from armature and sends it to the load as
direct current.
• It actually takes alternating current from armature and
converts it to direct current and then send it to external load.
• It is cylindrical structured and is build up of wedge – shaped
segments of high conductivity, hard drawn or drop forged
copper.
• Each segment is insulated from
the shaft
12

Brushes of DC Generator
• The brushes are made of carbon.
• These are rectangular block shaped.
• The only function of these carbon brushes of dc generator is
to collect current from commutator segments.
• The brushes are housed in the rectangular box shaped brush
holder.
13

• If the airgap is of uniform length, the e.m.f. generated in a
conductor remains constant while it is moving under a pole face,
• and then decreases rapidly to zero when the conductor is midway
between the pole tips of adjacent poles.
DC generator
14

• Three coils, 1–1′, 2–2′ and 3–3′, are
arranged in the slots so that their end
connections overlap one another
• we find that the direction of the e.m.f.
generated in conductor 1 is towards the
paper whereas that generated in
conductor 1′ is outwards from the paper.
• The distance between coil sides 1 and 1′
is called a pole pitch.
• In practice, the coil span must be a
whole number and is approximately
equal to
DC generator
15

Armature windings can be divided into two groups, depending
upon the manner in which the wires are joined to the
commutator, namely:
1. Lap windings.
2. Wave windings.
• In lap windings the two ends of any one coil are taken to
adjacent segments
• In wave windings the two ends of each coil are bent in
opposite directions and taken to segments some distance
apart
• if a machine has p pairs of poles
• No. of parallel paths with a lap winding = 2p
• and No. of parallel paths with a wave winding = 2
Armature winding
16

• When an armature is rotated through one revolution, each
conductor cuts the magnetic flux emanating from all the N poles
and also that entering all the S poles.
• if Φ is the useful flux per pole, in webers, entering or leaving
the armature, p the number of pairs of poles and Nr the speed
in revolutions per minute
Calculation of e.m.f. generated in an armature winding
18

19

If Z is the total number of armature conductors, and c the
number of parallel paths through winding between positive and
negative brushes
The number of conductors in series in each of the parallel paths
between the brushes remains practically constant; hence total
e.m.f. between brushes is
Average e.m.f. per conductor × no. of conductors in series per
path
20

A four-pole wave-connected armature has 51 slots with 12
conductors per slot and is driven at 900 r/min. If the useful flux
per pole is 25 mWb, calculate the value of the generated e.m.f.
21

Total number of conductors = Z = 51 ×
12 = 612; c = 2; p = 2;
N = 900 r/min; Φ=0.025 Wb
Using expression , we have
22

An eight-pole armature is wound with 480 conductors. The
magnetic flux and the speed are such that the average e.m.f.
generated in
each conductor is 2.2 V, and each conductor is capable of
carrying a
full-load current of 100 A. Calculate the terminal voltage on no
load,
the output current on full load and the total power generated
on
full load when the armature is
(a) lap-connected;
(b) wave-connected.
23

Copper Losses
• Copper loss is the power lost as heat in the windings; it is caused by the flow of
current through the coils of the DC armature or DC field.
• This loss varies directly with the square of the current in the armature or field and
the resistance of the armature or field coils.
Armature: Ia
2 Ra
Field: If
2 Rf
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2
0 
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26

Armature reaction
shows the distribution of flux when
there is no armature current, the flux in
the gap being practically radial and
uniformly distributed.
shows the distribution of the flux set
up by current flowing through the
armature winding
shows the resultant distribution of
the flux due to the combination of
the fluxes
27

• Over the trailing halves of the pole faces the cross flux is in
opposition to the main flux, thereby reducing the flux density,
whereas over the leading halves the two fluxes are in the
same direction, so that the flux density is strengthened.
• One important consequence of this distortion of the flux is
that the magnetic neutral axis is shifted through an angle θ
from AB to CD
Armature reaction
28

DC motor
• A DC motor is a mechanically commutated electric motor
powered from direct current (DC).
• DC motors have a rotating armature winding but non-rotating
armature magnetic field and a static field winding (winding
that produce the main magnetic flux) or permanent magnet.
• The speed of a DC motor can be controlled by changing the
voltage applied to the armature or by changing the field
current
29

Construction of DC Motor….????
30

Counter-electromotive force/ Back EMF
• The counter-electromotive force also known as back
electromotive force (abbreviated counter EMF, or CEMF) is the
voltage, or electromotive force, that pushes against the current
which induces it.
• Back electromotive force is a voltage that occurs in electric
motors where there is relative motion between the armature
of the motor and the external magnetic field.
• In dc motor, the armature conductors are revolving in the
magnetic field and emf is induced in the armature conductors.
• The direction of the induced emf is in opposite direction to the
applied voltage as per Lenz’s law. So, the induced emf in motor
is called as back emf or counter emf.
31

Torque Equation of DC Motor
The equation of torque is given by
τ = F.R.sinθ ————-(1)
Pm = τ.ω
Where F is force in linear direction.
R is radius of the object being rotated,
and θ is the angle, the force F is making with R vector
32

if E is the supply voltage, Eb is the back emf produced and Ia, Ra
are the armature current and armature resistance respectively
then the voltage equation is given by.
E = Eb + Ia.Ra ————-(2)
multiply both sides of equation
(2) by Ia.
∴ E Ia = Eb Ia + Ia
2.Ra————-(3)
33

• Now Ia
2.Ra is the power loss due to heating of the armature
coil, and
• the true effective mechanical power that is required to produce
the desired torque of dc machine is given by
Pm = Eb.Ia ————-(4)
The mechanical power Pm is related to the electromagnetic
torque Tg as
Pm = Tg.ω ————-(5)
Where ω is speed in rad/sec.
34

Now equating equation (4) & (5) we get
Eb.Ia = Tg.ω
Now for simplifying the torque equation of dc motor we
substitute
Where, P is no of poles
φ is flux per pole
Z is No. of conductors
A is No. of parallel paths
and N is the speed of the D.C. Motor.
35

N is the speed of the D.C. Motor.
Substituting equation (6) and (7) in equation (4), we get:
36

The torque we so obtain, is known as the electromagnetic torque
of dc motor, and subtracting the mechanical and rotational losses
from it we get the mechanical torque.
∴ Tm = Tg – mechanical losses.
This is the torque equation of dc motor. It can be further
simplified as:
Tg = ka φIa
Which is constant for a particular machine and therefore the
torque of dc motor varies with only flux φ and armature current
Ia
37

The Torque equation of a dc motor can also be explained
Torque Equation of DC Motor: 2
38

Current / conductor Ic = Ia / A
∴ force per conductor = fc = B L Ia/A
Now torque Tc = fc.r = BLIa.r/A
Hence the total torque developed of a dc machine is
39
Where, P is no of poles
φ is flux per pole
Z is No. of conductors
A is No. of parallel paths
and N is the speed

This torque equation of dc motor can be further simplified as:
Tg = ka φIa
Which is constant for a particular machine and therefore the
torque of dc motor varies with only flux φ and armature current Ia
40

The types of DC motor can be listed as follows
Types of DC motor
41

Separately Excited DC Motor
• As the name suggests, in case of a separately excited DC Motor
the supply is given separately to the field and armature
windings.
• Armature current does not flow through the field windings, as
the field winding is energized from a separate external source
of dc current
42

Separately Excited DC Motor
• From the torque equation of dc motor we know
• Tg = Ka φ Ia
• So the torque in this case can be varied by varying field
flux φ, independent of the armature current Ia
43

Permanent Magnet DC Motor
• The permanent magnet DC motor consists of an armature winding
as in case of an usual motor, but does not necessarily contain the
field windings.
• The construction of these types of DC motor are such that, radially
magnetized permanent magnets are mounted on the inner
periphery of the stator core to produce the field flux.
• The rotor on the other hand has a conventional dc armature with
commutator segments and brushes.
44

The torque equation of dc motor suggests Tg = Ka φ Ia.
Here φ is always constant, as permanent magnets of
required flux density are chosen at the time of construction
and can’t be changed there after.
Permanent Magnet DC Motor
45

Self Excited DC Motor
In case of self excited dc motor, the field winding is connected
either in series or in parallel or partly in series, partly in parallel to
the armature winding, and on this basis its further classified as:-
i) Shunt Wound DC Motor
ii) Series Wound DC Motor
iii) Compound Wound DC Motor
46

Shunt Wound DC Motor
In case of a shunt wound dc motor or more specifically shunt
wound self excited dc motor, the field windings are connected in
parallel to the armature winding
47

48
lets consider the basic voltage equation given by
E = Eb + Ia Ra ————–(1)
[Where E, Eb, Ia, Ra are the supply voltage, back emf,
armature current and armature resistance respectively]
Now Eb = ka φ ω ————–(2)
[since back emf increases with flux φ and angular speed ω]
Now substituting Eb from equation (2) to equation (1) we get,
E = ka φ ω + Ia Ra

49
The torque equation of a dc motor resembles Tg = Ka φIa—————(4)
Substituting the torque equation in equation (3) we get
This is similar to the equation of a straight line, and we can graphically
representing the torque speed characteristic of a shunt wound self
excited dc motor as

50
The shunt wound dc motor is a constant speed motor, as the
speed does not vary here with the variation of mechanical load on
the output.

51
In case of a series wound self excited dc motor or simply series
wound dc motor, the entire armature current flows through the
field winding as its connected in series to the armature winding.
Series Wound DC Motor

52
lets get to the torque speed equation.
From the circuit diagram we can see that the voltage equation
gets modified to
E = Eb + Ia (Ra + Rs) ———(5)
Where as back emf remains Eb = kaφω
Neglecting saturation we get φ = K1.If = K1.Ia
[ since field current = armature current]
∴ Eb = ka K1Iaω = KsIaω————–(6)
Series Wound DC Motor

53
From equation (5) & (6)
In a series wound dc motor, the speed varies with load. And
operation wise this is its main difference from a shunt wound dc
motor.

54
Compound Wound DC Motor
The compound wound self excited dc motor or simply compound
wound dc motor essentially contains the field winding connected
both in series and in parallel to the armature winding

55
The excitation of compound wound dc motor can be of two
types depending on the nature of compounding
Cumulative Compound DC Motor
When the shunt field flux assists the main field flux, produced by
the main field connected in series to the armature winding then its
called cumulative compound dc motor.
φtotal = φseries + φshunt

56
Differential compound dc motor
In case of a differentially compounded self excited dc motor i.e.
differential compound dc motor, the arrangement of shunt and
series winding is such that the field flux produced by the shunt field
winding diminishes the effect of flux by the main series field
winding.
φtotal = φseries – φshunt

57
The net flux produced in Differential compound dc motor
is lesser than the original flux and hence does not find much of a
practical application.

58
Speed Control of DC Motor
Speed control is a different concept from speed regulation where
there is natural change in speed due change in load on the shaft.
the expression of speed control dc motor is given as
N = (E – IaRa) / kφ
Therefore speed (N ) of 3 types of dc motor – SERIES, SHUNT AND
COMPOUND can be controlled by changing the quantities on RHS of
the expression. So speed can be varied by changing
(i) terminal voltage of the armature V ,
(ii) external resistance in armature circuit R and
(iii) flux per pole φ .

59
The first two cases involve change that affects armature circuit
and the third one involves change in magnetic field. Therefore
speed control of dc motor is classified as
1) armature control methods and
2) field control methods.
Speed Control of DC Motor

60
Armature Control of DC Series Motor
• We know that, the supply voltage is constant for DC Motors
and changing the supply voltage of Motor is quite difficult.
• So, the armature voltage is controlled instead of directly
controlling the supply voltage.
• A variable resistance is provided with the armature in series.
The armature voltage is controlled by varying the variable
resistance

61
This method of speed control is used for both DC series motor and
DC shunt motor.

62
• Let us take a resistance Re be the resistance ( variable resistor or
rheostat) placed in series with the armature circuit. Then,
speed,
N= {E-Ia(Ra+Rse)}/kφ.
• The speed of the motor decreases as the series resistance Rse is
being increased.

63
• The chief disadvantage of this method is that there is
considerable power dissipation in the series resistance.
• This method is useful where reduction in speed is required for
a short period.

64
Speed Control of DC Motor: Field Current Control
• The main field flux is produced by the field current.
• So, the speed of DC Motor is easily controlled by varying the
field current.
• They are of following types
1. Field Rheostat
2. Field Diverter
3. Tapped field method

65
• Field Rheostat: This method is mainly used to control the speed of DC
Shunt motor. Here an additional resistance is connected in series with
the shunt field circuit.
• The field current is controlled by varying the rheostat.
• This is the most satisfactory and economical way to control the speed
of D.C. motors.
• Since the field current is usually very small, the losses in the
resistance may be considered negligible.

66
Field Diverter: In this case of a series motor, a diverter i.e. a variable
resistance is connected in parallel with the series field.
This will divert a part of the load current and thus the field is
weakened with a result that the speed of the motor is increased.
The parallel resistor is termed as diverter.

67
• Tapped field method: In this method, the ampere turns are
varied by changing the field turns The motor runs at its
minimum speed when the full winding is effective.
• Cutting of the field turns in steps increases the speed of the
motor.
• By this method full and half speed may be obtained without any
rheostatic loss.

68
Starting of DC Motor
• A dc motor (unlike other types of motor)
has a very high starting current that has
the potential of damaging the internal
circuit of the armature winding of dc
motor if not restricted to some limited
value.
• This limitation to the starting current of
dc motor is brought about by means of
the starter.
• Starter: A device containing a variable
resistance connected in series to the
armature winding so as to limit the
starting current of dc motor to a desired
optimum value taking into consideration
the safety aspect of the motor.

69
Why the dc motor has such high starting current ???

70
• Let us take into consideration the basic operational voltage
equation of the dc motor given by,
E = Eb + Ia Ra
• Where E is the supply voltage, Ia is the armature current, Ra is
the armature resistance. And the back emf is given by Eb
• Now the back emf, in case of a dc motor, is very similar to the
generated emf of a dc generator as it’s produced by the
rotational motion of the current carrying armature conductor
in presence of the field.
• This back emf of dc motor is given by

71
From this equation we can see that Eb is directly proportional to
the speed N of the motor. Now since at starting N = 0, Eb is also
ZERO, and under this circumstance the voltage equation is
modified to
E = 0 + Ia Ra

72
For all practical practices to obtain optimum operation of the
motor the armature resistance is kept very small usually of the
order of 0.5 Ω and the bare minimum supply voltage being 220
volts.
Under these circumstance the starting current, Ia is as high as
220/0.5 amp = 440 amp.

73
• Such high starting current of dc motor creates two major
problems.
• 1) Firstly, current of the order of 400 amp has the potential of
damaging the internal circuit of the armature winding of dc
motor at the very onset.
• 2) Secondly, since the torque equation of dc motor is given by
• Very high electromagnetic starting torque of DC motor is
produced by virtue of the high starting current, which has the
potential of producing huge centrifugal force capable of flying
off the rotor winding from the slots.

74
Starting Methods of DC Motor
The main principal of this being the addition of external resistance
R ext to the armature winding, so as to increase the effective
resistance to Ra + Rext, thus limiting the armature current to the
rated value.
The new value of starting armature current is desirably low

75
• Now as the motor continues to run and gather speed, the back
emf successively develops and increases, countering the supply
voltage, resulting in the decrease of the net working voltage.
Thus now
• At this moment to maintain the armature current to its rated
value, Rext is progressively decreased unless its made zero, when
the back emf produced is at its maximum.
• This regulation of the external resistance in case of the starting
of dc motor is facilitated by means of the starter.

76
• Starters can be of several types and requires a great deal of
explanation and some intricate level understanding.
• But on a brief over-view the main types of starters used in the
industry today can be illustrated as:
1. 3 point starter.
2. 4 point starter.
3. Series wound dc motor‘s starter using no load release coil.
• First two are used for the starting of shunt wound dc motor
and compound wound dc motor

80
Three Phase Induction Motor
• An induction or asynchronous motor is an AC electric motor
in which the electric current in the rotor needed to produce
torque is induced by electromagnetic induction from the
magnetic field of the stator winding.
• An induction motor does not require mechanical
commutation, separate-excitation or self-excitation for all or
part of the energy transferred from stator to rotor, as in
universal, DC and large synchronous motors.

81
Working Principle of Three Phase Induction Motor
• An electrical motor is such an electromechanical device which
converts electrical energy into a mechanical energy.
• This Motor consists of two major parts:
• Stator: Stator of three phase induction motor is made up of
numbers of slots to construct a 3 phase winding circuit which
is connected to 3 phase AC source.
• The three phase windings are arranged in such a manner in
the slots that they produce a rotating magnetic field after AC is
given to them.

83
Rotor: Rotor of three phase induction motor consists of cylindrical
laminated core with parallel slots that can carry conductors.
Conductors are heavy copper or aluminum bars which fits in each
slots & they are short circuited by the end rings.
Working Principle of Three Phase Induction Motor

85
Normalized slip or slip is expressed in fraction as fraction of synchronous speed
When the motor runs the frequency of the rotor depends on the relative speed or slip
speed
Frequency of the rotor is given by following relation

86
A 6 pole induction motor is fed from 50 Hz supply. If the frequency of the rotor emf at
full load is 2 Hz find the full load slip and speed

88
The three phase induction motor is found for 4 poles and is supplied for from 50 Hz
Calculate
a) Synchronous speed
b) Speed of rotor when slip is 4%
c) Rotor frequency when the speed of the rotor is 600 rpm

90
Types of 3 phase induction motors :
There are 2 types of 3 phase induction motors based on the rotor
windings
1. Squirrel-cage induction motor
2. Slip ring induction motor (wound-rotor induction motor)

91
Squirrel-cage induction motor :
• In this motor the rotor looks like a squirrel cage that's why the
name squirrel cage induction motor.
• Squirrel cage rotor consists of thick copper bars, which are
slightly longer than the rotor, which are pushed into the slots.

92
• The conductors are often skewed slightly along the length of
the rotor to smooth out torque fluctuations that might result at
some speeds due to interactions with the pole pieces of the
stator.
Squirrel-cage induction motor :

93
Slip ring induction motor (wound-rotor induction motor)
In this the rotor will be having Conventional 3-phase windings made
of insulated wire, which produce a wound rotor.
A wound rotor has a 3-phase winding, similar to the stator winding.
The rotor winding terminals are connected to three slip rings which
turn with the rotor. The slip rings/brushes allow external resistors to
be connected in series with the winding.

94
The rotor consists of numbers of slots and rotor winding are placed
inside these slots.
The rotor also carries star or delta winding similar to that of stator
winding.
The three end terminals are connected together to form star
connection.
As its name indicates three phase slip ring induction motor consists
of slip rings connected on same shaft as that of rotor.

95
• The external resistance can be easily connected through the
brushes and slip rings and hence used for speed control and
improving the starting torque of three phase induction motor.
• The brushes are used to carry current to and from the rotor
winding.
• These brushes are further connected to three phase star
connected resistances.
• At starting, the resistance are connected in rotor circuit and is
gradually cut out as the rotor pick up its speed.

96
Due to presence of slip rings and brushes the rotor construction
becomes somewhat complicated therefore it is less used as
compare to squirrel cage induction motor.
Application:
Slip ring induction motor are used where high starting torque is
required i.e in hoists, cranes, elevator etc

97
SLIP RING OR PHASE WOUND
INDUCTION MOTOR
SQUIRREL CAGE INDUCTION
MOTOR
Construction is complicated due to
presence of slip ring and brushes
Construction is very simple
The rotor winding is similar to the
stator winding
The rotor consists of rotor bars
which are permanently shorted
with the help of end rings
We can easily add rotor resistance
by using slip ring and brushes
Since the rotor bars are
permanently shorted, its not
possible to add external resistance
Due to presence of external
resistance starting torque can be
controlled
Staring torque cannot be improved
Slip ring and brushes are present Slip ring and brushes are absent

98
Frequent maintenance is required
due to presence of brushes
Less maintenance is required
The construction is complicated
and the presence of brushes and
slip ring makes the motor more
costly
The construction is simple and
robust and it is cheap as compared
to slip ring induction motor
This motor is rarely used, only 10
% industry uses slip ring induction
motor
Due to its simple construction and
low cost. The squirrel cage
induction motor is widely used
Rotor copper losses are high and
hence less efficiency is less
Less rotor copper losses and hence
high efficiency
Speed control by rotor resistance
method is possible
Speed control by rotor resistance
method is not possible
Slip ring induction motor are used
where high starting torque is
required i.e in hoists, cranes,
elevator etc
Squirrel cage induction motor is
used in lathes, drilling machine,
fan, blower printing machines etc

99
Star-Delta Starter
• Most induction motors are started directly on line, but when
very large motors are started that way, they cause a
disturbance of voltage on the supply lines due to large starting
current surges.
• To limit the starting current surge, large induction motors are
started at reduced voltage and then have full supply voltage
reconnected when they run up to near full rotation speed.
• Two methods are used for reduction of starting voltage are
• star delta starting and
• auto transformer stating.

100
Working Principal of Star-Delta Starter:
• This is the reduced voltage starting method.
• Voltage reduction during star-delta starting is achieved by
physically reconfiguring the motor windings.
• During starting the motor windings are connected in star
configuration and this reduces the voltage across each winding
by a factor of 3.
• This also reduces the torque by a factor of three.
• After a period of time the winding are reconfigured as delta and
the motor runs normally.

101
• Traditionally in many supply regions, there has been a
requirement to fit a reduced voltage starter on all motors
greater than 5HP (4KW).
• The Star/Delta (or Wye/Delta) starter is one of the lowest cost
electromechanical reduced voltage starters that can be applied.
Star-Delta Starter

INDUCTION MOTOR

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INDUCTION MOTOR