52629729

A
Presentation On
Working Principle Of
D.C. Motors
By
VIKAS MITTAL
1

MOTOR
Definition of Motor

Machine that converts other forms of
energy into mechanical energy and so
imparts motion is called a motor.

2

MOTOR
Different types of Motors
Following are some types of motor:
Pneumatic motor :
It converts pneumatic energy into mechanical
energy.
Hydraulic motor :
It converts kinetic energy of fluid into
mechanical energy.
3

MOTOR
Different types of Motors
Engine:
It converts thermal energy into mechanical
energy.
Electric motor:
It converts electrical energy into mechanical
energy.
Turbine:
It converts kinetic energy of water / Gas /
Steam into mechanical energy. 4

ELECTRIC MOTOR

Machine that converts electrical
energy into mechanical energy is
called an electric motor.
5

ELECTRIC MOTOR

TYPES OF ELECTRIC MOTORS
Depending upon the source of electrical energy,
there are two types of electric motors:
A.C. Motors :
Motor that converts alternating current energy
into mechanical energy is called A.C. motor.
D.C. Motors :
Motor that converts direct current energy into
mechanical energy is called D.C. motor.
6

DIRECT CURRENT MOTOR

CONSTRUCTION OF A DC
MOTOR
DC Motor Consists of Four Main Parts
1. Field Magnets
2. Armature
3. Commutator
4. Brush and Brush Gears
7


PHOTOGRAPHIC VIEW OF A DC MOTOR
8


1. Field system
The function of the field system is to
create a uniform magnetic field within
which the armature rotates. Electro-
magnets are preferred in comparison
with permanent magnets on accounts of
its greater magnetic effect and its field
strength regulation, which can be
achieved by controlling the magnetising
current.
9


Field magnet consists of four parts given below :
1. Yoke or Frame
2. Pole cores
3. Pole shoes
4. Magnetising coils.
5. Interpoles
PHOTOGRAPHIC VIEW OF
STATOR OF A DC MOTOR10


PHOTOGRAPHIC VIEW OF A
FIELD COIL OF DC MOTOR
11


2. ARMATURE
It is a rotating part of a dc machine and is built up in
a cylindrical or drum shape. The purpose of
armature is to rotate the conductors in the uniform
magnetic field. It consists of coils of insulated wires
wound around an iron core. In addition , the most
important function of the armature is to provide a
path of very low reluctance to the magnetic flux. The
armature core is made up of high permeability
silicon steel stampings, each stamping, being
separated from its neighbouring one by thin coating
of varnish as insulation.
Armature continues…….12


Armature continues…….
The use of high grade steel is made (a) to keep
hysteresis loss low, which is due to cyclic change
of magnetisation caused by the rotation of core
in the magnetic field and (b) to reduce eddy
current in the core which are induced by the
rotation of the core in the magnetic field. By
using stampings or laminations, the path of the
eddy current is cut into several units. Each
lamination is about 0.3 to 0.6 mm thick.
13

PHOTOGRAPHIC VIEW OF AN
ARMATURE OF DC MOTOR
14


3. COMMUTATOR
The commutator is a form of rotating switch
placed between the armature and the external supply
source and so arranged
that it will reverse the
direction of the current
flowing through the
armature conductors
during each rotation
of the armature in
case of motor.
commutator continues……
PHOTOGRAPHIC VIEW
OF COMMUTATOR 15


It is a very important part of a dc machine and
serves the following purpose:
1. It provides the electrical connections between
the rotating armature coils and the stationary
external circuit.
2. As the armature rotates, it performs a switching
action to change the direction of flow of current
in armature conductors so that the armature
may be able to run in the same direction
3. It also keeps the rotor or armature mmf
stationary in space.
commutator continues……16


The commutator is essentially of cylindrical
structure and is built up of wedge shaped
segments of high conductivity hard drawn
copper or drop forged copper .These
segments are insulated from each other by
thin layers of mica ( usually of 0.5 to 1.0mm
thickness) or micanite. The segments are held
together by means of two V-shaped rings that
fit into the V- grooves cut into the segments.
17


The winding ends are soldered with copper
lugs or risers. The risers have air space
between them so that air is drawn across the
the commutator thereby keeping the
commutator cool.
18


The commutator is pressed on to the armature
shaft, and the outer periphery is then
machined to provide smooth surface with
which a stationary carbon (or graphite or
copper) brush can maintain continuous
contact at the armature and commutator
rotate. Great care is taken in building the
commutator because even slight eccentricity
will cause the brushes to bounce, causing
undue sparking.
19


4. BRUSHES AND BRUSH GEARS
The function of brushes is send current from
external supply source to armature conductor i.e.
armature winding. Brushes are rectangular in
shape and rest on the commutator. Brushes are
manufactured in variety of compositions and
degrees of hardness to suit the commutation
requirement. They may be classified roughly as
carbon, carbon graphite, graphite, metal
graphite.
20


PHOTOGRAPHIC VIEW OF BRUSH AND
BRUSH GEARS SYSTEM OF DC MOTOR
21

WORKING PRINCIPLE:
The principle upon which a dc motor works is -
If a current carrying conductor is placed in a
magnetic field, mechanical force is experienced
on the conductor, the direction of which is given
by Fleming’s left hand rule (also called motor
rule) and hence the the conductor moves in the
direction of force.

22


The magnitude of the mechanical force experienced on the
conductor is given by
F = B.I.L newtons
Where, B is the flux density in teslas ( Wb/m2
)
I is the current flowing through conductor in amperes
L is the is length of conductor in meters.
In general if the conductor lies at an angle θ with a magnetic
field of flux density B webers/meter2
, the mechanical force
experienced on a conductor is given by
F = B.I.L. sin θ newtons

23


FLEMING’S LEFT HAND RULE:
This rule states that if the thumb, forefinger and
middle finger of the left hand are stretched in such
a way that they are at right angle to each other
mutually and forefinger points towards the
direction of magnetic field, the middle finger
towards the direction of flow of current then the
thumb will point the direction of force acting on
the conductor.
24


MOTION
MAGNETIC
FIELD
DIRECTION
OF CURRENT
25


FORCE ON A CURRENT CARRYING
CONDUCTOR LYING IN THE MAGNETIC
FIELD:
To understand the force on a current carrying conductor
lying in the magnetic field, let us see the following three
figures:
Figure (a) shows the uniform magnetic field between
the two opposite poles
SN
Fig.(a) Main magnetic field 26


In figure (b) the cross section of a conductor
carrying current in inward direction placed
between two magnets, the field being temporarily
removed, is shown. By applying the right hand
thumb rule, the direction of the field around the
conductor is found to be clockwise
Field due to the current in the conductor
Fig (b) N S
27


If the current carrying conductor shown in figure
(b) is placed in the magnetic field shown in figure
(a), the resultant magnetic field would be similar to
that shown in figure (c)
FORCE
FIG. (c)
N S
28


The lines of force above the conductor are
strengthened, since they are in the same direction,
but the lines of force below the conductor are
weakened because the two fields below the
conductor are opposite in directions and hence
tends to destroy each other.
FORCE
FIG. (c)
N S
29


Magnetic lines like rubber bands have a tendency to
strengthen out and, therefore, a force is experienced
on the conductor in the downward direction, as
shown in the fig.( C ).
FORCE
FIG. (c)
N S
30


If the direction of current is reversed in the conductor ,
as shown in the Fig. (d), the direction of force
experienced is reversed. In this case the lines of force
above the conductor are weakened while those below
the conductor are strengthened.
Fig. (d)
FORCE
N S
31


Hence it is observed that when a current carrying
conductor is placed at right angle to the direction of
magnetic field, a mechanical force is Experienced on
the conductor in a direction perpendicular to both the
direction of magnetic field and flow of current.
Fig. (d)
FORCE
N S
32


Magnetic Field in DC Motor
33


Current in DC Motor
34


Force in DC Motor
35


Torque in DC Motor
36


DC Motor Operation
37


COMMUTATOR ACTION IN A DC
MOTOR
In the case of dc motor, it is necessary that the
current through the coils of armature winding be
reversed as a particular coil leaves one pole (say,
north pole), crosses the neutral line and comes
under the influence next pole which is of opposite
polarity (i. e. the south pole) The operation of
commutator, that serves the above purpose, is
given below:
Consider a single turn coil, whose leads are
soldered to cummutator segment (a) and (b), each
carrying a brush as illustrated in fig shown in next
slide Commutator action continues….38

COMMUTATOR ACTION IN A DC MOTOR
Commutator action continues….
... ..
II
1
.
.
2
+
_
a
b
..
2
1
+
_
b a
III
+
.
.
.
.
2
1
_
a b
I
39


The positive side of the supply line is connected
to left hand brush and negative side to the right
side brush. In position I, the line current
arrives at the commutator segment (a), flows
through the bottom side 1 of the coil away from
the reader and the upper side 2 of the coil
towards the reader, reaches the commutator
segment (b) and flows again into the line
through the brush. The coil will tend to rotate
in clock-wise direction, as determined by
Fleming’s left hand rule.
Commutator action continues…. 40


In position II, the coil is on the magnetic neutral
line; there is no contact between the commutator
segments and the brushes, there is no flow of
current through the coil. The coil crosses the
neutral line by inertia. In case of multi-turn coil,
the remaining turns of the coil will supply the
necessary torque.
In position III, the two sides of the coil, 1 and 2,
have changed poles, and the current flowing them
has reversed. The commutator segments, however,
have also changed contact with the brushes. Thus
the coil will continue to rotate in the same direction
as before, i.e. clock-wise. 41


BACK EMF IN A DC MOTOR AND ITS
IMPORTANCE
When the armature of motor continues to rotate due
to motoring action, the armature conductors cut the
magnetic flux and therefore emfs are induced in
them. The direction of this induced emf, known as
back emf, is such that it opposes the applied voltage.
Back emf continues…..
42


Since the back emf is induced due to the generator
action, the magnitude of it is, therefore, given by
the expression,
Back emf, Eb = ØZNP …..(1.1)
60 x A
Where, Ø is flux per pole in webers, Z is the total
number of armature conductors or coil sides on
the armature, P is the number of poles, A is the
number of parallel paths in the armature, N is the
rotational speed of the armature in rpm
43


The equivalent circuit of a dc motor is shown in fig
below. The armature circuit is equivalent to a source
of emf, Eb in series with a resistance, Ra put across a
dc supply mains of V volts. It is evident from fig that
the applied voltage V must be
large enough to balance both
the voltage drop in armature
resistance and the back emf
at all times i.e.
V = Eb + Ia Ra
or Ia = V-Eb …….(1.2)
Ra
V
Eb
V
Ia
Ra
Eb
+
_
Ia
Ia
+
_
Equivalent circuit of a
motor armature 44


V = Eb + Ia Ra
or Ia = V-Eb …….(1.2)
Ra
Where,
V is the applied voltage
across the armature
Eb is the induced voltage
in the armature
Ia is the armature current
Ra is the armature resistance
It is obvious from the expressions (1.1)
and (1.2) that the induced emf in the
armature of a motor, Eb
Back emf
V
Eb
V
Ia
Ra
Eb
+
_
Ia
Ia
+
_
Equivalent circuit of a
motor armature
45


Depends among other factors upon the
armature speed and the armature current
depends upon the back emf Eb for a constant
applied voltage and armature resistance.
If the armature speed is high, back emf will be
large and therefore, armature current small. If
the speed of the armature is low, then back emf
will be less and armature current Ia will be
more resulting in development of large torque.
46


POWER RELATIONSHIP IN A DC MOTOR
Voltage equation for a motor is:
V = Eb + Ia Ra
Multiplying each term of the voltage equation by Ia
we get
VIa = Eb Ia + Ia2
Ra …….(1.3)
The equation (1.3) is known as power equation of
the dc motor
The term VIa represents the power supplied to
the motor armature, Ia2
Ra represents the power
lost in the armature, Eb Ia represents developed in
the armature causing rotation of the armature.
Power relationship continues……47


Power relationship continues……
The power developed ‘Eb Ia’ is not all available
at the shaft since some it is used to overcome the
mechanical power losses of the motor
As mentioned above the mechanical power
developed in the motor is given by
P m = Eb Ia watt ……(1.4)
Power relationship continues……
48


TYPES OF DC MOTORS
Different types of dc motors are:
1. Permanent magnet dc motors
2. Separately excited dc motors
3. Series wound dc motors
4. Shunt wound dc motors
5. Compound wound dc motors
49


1. Permanent magnet dc motors:
It consists of an armature and one or several
permanent magnets encircling the armature.
Field coils are not usually required. However,
some of these motors do have coils wound on
the poles. If they exist,
these coils are Intended
only for recharging the
magnets in the event
that they loose their
strength.
ARMATURE
+
_
N S
Fig. Permanent Magnet Motor
50


2. Separately excited dc motors:
These motors have field coils similar to those of a
shunt wound dc motor, but the field coils and
armature are fed from different supply source.
In a separately excited dc motor,
Armature current,
Ia = Line current, IL
Back emf developed,
Eb = V – I Ra
Separately Excited dc
motors continues…….
DC SUPPLY
MAINS
DC SUPPLY
MAINS
+
-
I
+
-
ARMATURE
FIELD
If
+
-
RHEOSTAT
Separately excited dc motor
51


Separately Excited dc
motors continues…….
Power drawn from the mains, P = VI
Mechanical power developed,
Pm = Power input to armature
- power lost in armature
= VI – I2
Ra
= I (V – I Ra )
= Eb I watt
52


3. Series wound dc motors:
As the name implies, the field coils, consisting of a
few turns of thick wires, are connected in series
with the armature as shown in the fig. The cross-
sectional of the wire for the coils has to be
fairly to carry the armature
current, but owing to large
current, the number of
turns of wire in them
need not be large.
DC Series Motors
ARMATURE
+
_
Series field
DC SUPPLY
MAINS
+
_
Series wound dc motors
53


DC Series Motors continues…….
In a dc series motor,
Armature current, Ia = series field current, Ise
= Line current, IL = I (say)
Armature current, Ia = series field current, Ise =
Line current, IL = I (say)
Back emf developed, Eb = V – I (Ra + Rse )
Power drawn from the mains, P = VI
= VI – I2
(Ra + Rse)
= I [V – I ( Ra + Rse) ]
= Eb I watts 54

4. Shunt wound dc motors:
The word “ shunt ’’ means “ parallel ’’ . In these
motors , the field coils are connected in parallel
with the armature. The field winding consists of a
large number of turns of thin
wire so as to provide
large resistance.
The field current is
much less than the
armature current,
sometimes as low as 5%.
DC Shunt Motors continues…….
DC SUPPLY
MAINS_
_
Shunt wound dc motors
Shuntfield
ARMATURE
Ish
IshIa
IL
+
+
55

The current supplied to the motor is divided into
two paths, one from the field winding and second
through the armature i.e.
Input line current, IL = Ia + Ish
Where Ia is the armature current and Ish is the
shunt field current and is given by where V is the
supply voltage, Rsh is the shunt field
resistance
Ish = V
Rsh
DC Shunt Motors continues……. 56

Back emf developed, Eb = V – I Ra
Power drawn from the mains, P = VIL
= VIL – V Ish - I2
a Ra
= V( IL – Ish ) - I2
a Ra
= V Ia- I2
a Ra = Ia( V – Ia Ra )
= Eb Ia watt
57


5. Compound wound dc motors:
Compound wound motors are of two
types namely cumulative compound
motor wound and differential
compound wound motor.
58


Cumulative compound wound motor:
in this motor field winding are connected in such a
way that the direction of flow of current is same in
both the field windings as shown in fig. In the
motor of this type the flux due to
the series field winding
strengthens the
field due to the
shunt field
winding.
SERIES FIELD
_
DC SUPPLY
MAINS
+
IL Ish
Ish
Shuntfield
Ia
+
_
ARMATURE
Cumulative compound wound motor
59


Differential compound wound motor:
In this motor field winding are connected in such a
way that the direction of flow of current is opposite
to each other in both the field
windings as shown in
fig. In the motor of
this type the flux
due to the series
field winding
weakens the field
due to the
shunt field winding.
Differential compound wound motor
DC SUPPLY
MAINS
+
SERIES FIELD
Shuntfield
Ish
Ish
Ia
IL
IL
+
_
_
ARMATURE
60

SPEED EQUATION OF DC MOTOR
As already in the expression for back emf
developed in the armature of a dc motor is given
by the expressions
Eb = Ø Z N P volts ……(1)
60xA
and Eb = V – Ia Ra volts ……(2)
comparing expressions (1) and (2) , we get
Ø Z N P = V – Ia Ra
60xA
or N = V – Ia Ra x 60A
Ø Z P
Speed equation of dc motor continues……
61

Speed equation of dc motor continues……
or N α V – Ia Ra
Ø
Since Z, A and P are constant for a particular
machine.
Now in the above expression for speed, voltage V
is constant and the armature drop Ia Ra is
negligible compared to supply voltage V .
Therefore, the speed of the motor is inversely
proportional to the flux per pole i.e. the speed of
the motor increases with the fall in flux.
62

TORQUE
The measure of causing rotation of the wheel
or the turning or twisting moment of a force
about an axis is called the torque.
Torque is measured by the
product of force and the
radius at which this force
acts.
Torque continues…..
F
r
ROTATION
64

Torque continues…..
Consider a wheel of radius r meters acted by
circumferential force F newtons, as shown in the
fig. Below. Let this force cause the wheel to
rotate at n rps
Torque, T = F x r newton metres
Work done per revolution
= Force x distance moved
= F x 2 Π r joules
Work done per second
= F x 2 Π r x n
= F x r x 2 Π n
= T x 2 Π n joules 65

ARMATURE TORQUE OF A DC MOTOR
Let Ta be the torque developed in newton metres
by the motor armature running at n rps
Power developed = Work done per second
= Ta x 2 Π n watts …..(1)
Electrical equivalent of mechanical power
developed by the armature also
= Power input to armature
– armature resistance loss
= V Ia- I2
a Ra
= Ia( V – Ia Ra )
= Eb .Ia watt ….(2)
Armature torque continues…….66

Armature torque continues…….
Comparing expressions (1) and (2), we get
Ta x 2 Π n = Eb Ia = Ø Z N P x Ia
60xA
= Ø Z n x P x Ia
A
( since Ø Z N x P = Ø Z n x P
60 A A
( as n = N
60 67

or Ta = 1 x Ø Z x P x Ia N-m
2 Π A
= 0.159 Ø Z P x Ia N-m
A
since Z, P and A are constant for a particular
machine
therefore Ta α Ø Ia
i.e. torque developed by armature is proportional
to the product of flux per pole and armature
current.
68

In case of series wound motor Ø is proportional
to Ia ( before saturation) because current in
field winding and armature winding is same and
therefore Ta α Ia2
In case of shunt wound motor Ø is practically
constant and
therefore Ta α Ia
69

OPERATING CHARACTERISTICS OF DC
MOTORS
The performance and, therefore, suitability of a
dc motor is determined from its characteristics
known as performance characteristics. The
important characteristics of dc motors are:
1. Torque – Armature Current Characteristic. This
characteristic curve gives relation between
torque developed in armature, T and armature
current, Ia. This is also known as electrical
characteristic. 70

2. Speed - Armature Current Characteristic. This
characteristic curve gives relation between speed,
N and armature current, Ia. This is also known
as speed characteristic.
3. Speed - Torque Characteristic. This
characteristic curve gives relation between speed,
N and torque, T. This is also known as
mechanical characteristic
The important relations to be kept in mind while
discussing motor characteristics are:
(i) Ia = V – Eb (ii) N α Eb
Ra Ø
and (iii) T α Ia 71

CHARACTERISTICS OF DC SREIES
MOTORS
1. Magnetic characteristic.
In case of dc series motors the flux Ø varies with the
variation in armature current
as the field is in series with
armature. The flux first
increases following in linear
law with the increase in
load current, becomes
maximum at saturation
point and finally
becomes constant.
FLUXINWb
ARMATURE CURRENT
IN AMPS
Magnetic characteristic DC series motor
FLUXsaturation
point
72

2. Speed – Current Characteristic of DC
Series Motor.
From the expression for speed, N α V – Ia Ra
Ø
it is obvious that speed is directly proportional to
applied voltage and inversely proportional to flux
per pole. If the applied voltage remains constant,
speed is inversely proportional to flux. So if a
curve is drawn between reciprocal of the flux and
current I, the speed – current characteristic is
obtained, which is a rectangular hyperbola in
shape as shown in fig. in next slide.
Speed – Current characteristic DC series motor continues……73

Speed – Current characteristic DC series motor continues……
Since on no load the speed is dangerously high,
As obvious from speed current characteristic
curve, which will result in heavy centrifugal
Force which in turn will ]
damage the motor.
That is why, series
motors are never
started on No-load
Speed – Current characteristic DC series motor
ARMATURE CURRENT
IN AMPS
SPEEDINRPM
74

3. Torque – Current Characteristic of dc series
motor:
From the expression of mechanical torque T α Ø Ia ,
it is obvious that torque is directly proportional to the
product of flux per pole Ø and armature current Ia.
up to saturation point flux is
proportional to field current
and hence to the armature
current, because Ia = If .
Therefore on light load
mechanical torque T is
proportional to the square
of the armature current
i.e. Ta α Ia2
and
ARMATURE CURRENT
IN AMPS
TORQUEINN-m
Torque – Current Characteristic of dc series motor continues….
75

Torque – Current Characteristic of dc series motor continues….
and hence curve drawn between Torque and
armature current is a parabola as shown in the
fig. After saturation point flux is almost
independent of excitation current and so torque
is directly proportional to the armature current
i.e. T α Ia .
Hence the characteristic becomes a straight line.
76

4. Speed – Torque Characteristic of dc series
Motor:
Fig below shows the speed – torque characteristic of
a dc series motor. It is obvious from the fig below
that as the torque increases speed decreases.
Hence series motors are
best suited for the
services where the
motor is directly
coupled to the load
such as fans whose
speed falls with
increase in torque.
Fig. Speed - torque Characteristic of dc series motor
TORQUE IN N - m
SPEEDINRPM
77

CHARACTERISTICS OF DC SHUNT MOTORS
1. Speed – Current characteristic of dc Shunt motor:
If the voltage V is kept constant the field current will
remain constant hence flux will have maximum value
on no-load but will decrease
slightly due to armature
reaction as the load increases
but for most purpose the
flux is considered to be
constant, neglecting the
effect of armature reaction.
Speed – Current characteristic of dc Shunt motor continues……
ARMATURE CURRENT
IN AMPS
SPEEDINRPM
78

Speed – Current characteristic of dc Shunt motor continues……
From the expression of speed for a dc motor,
N α V – Ia Ra or Eb
Ø Ø
speed N is directly proportional to back emf, Eb or
( V – Ia Ra ) and inversely proportional. Since flux is
considered to be constant as mentioned above, so
with the increase in load current the speed slightly
falls due to increase in voltage drop in armature.
Since voltage drop in armature at full load is very
small as compared to applied voltage so drop in
speed from no-load to full load is very small and for
all practical purposes the shunt motor is taken as a
constant speed motor 79

2. Torque – Current Characteristic of a dc Shunt Motor :
From the expression for the torque of a dc motor,
T α Ø Ia , the torque is directly proportional to the
product of flux per pole and the armature
current. Since in case of
shunt motor the flux is
considered to be constant,
therefore, torque increases
with the increase in load
current following linear
law i.e. torque -armature
current characteristic is a
straight line passing
through origin as shown in fig. Speed - Current Characteristic of
dc shunt motor
ARMATURE CURRENT
IN AMPS
TORQUEINN-m
80

3. Speed - Torque Characteristic of a dc Shunt
Motor:
This characteristic curve can be drawn from the two
other curves i.e. speed – current curve
and torque – current curve
and is shown in the
fig.
Speed - Current Characteristic of
dc shunt motor
TORQUE IN N - m
SPEEDINRPM
81

CHARACTERISTICS OF DC COMPOUND
WOUND MOTORS
Speed Characteristic of a dc Cumulative Compound
Wound Motor:
The characteristics of a the cumulative compound
wound motor are the combination of shunt and series
characteristics. As the load is increased, flux due to
series field winding increases, and cause the torque
greater than it would have with shunt field winding
alone for a given machine and for a given current.
The increase in flux due to series field winding on
account of increase in load causes the speed to fall
more rapidly than it would have done in shunt motor.
The characteristics are shown in the fig given below82

Characteristic of a dc Cumulative Compound
Wound Motor:
Characteristics of Cumulative Compound Wound Motors
ARMATURE CURRENT
IN AMPS
TORQUEINN-m
CUMULATIVE
SHUNT
ARMATURE CURRENT
IN AMPS
SPEEDINRPM
CUMULATIVE
SHUNT
83

Characteristic of a dc Differential
Compound Wound Motor:
In differential compound wound motors, the series
field winding is connected in such away that the
series field opposes the shunt field while in
cumulative compound wound motor series field
helps the shunt field. Since the flux decreases with
the increase in load so the speed remains nearly
the constant as the load is increased and in some
cases the speed will increase even.
Characteristic of a dc Differential Compound Wound Motor
continues……
84

Characteristic of a dc Differential Compound
Wound Motor continues……..
The decrease in flux with the increase in load
causes the torque to be less than that of shunt
motor. The characteristics are similar to
those of a shunt motor. Since the shunt
motor develops a good torque and almost
constant speed, therefore, differential motor
is seldom used.
85

Characteristic of a dc Differential Compound
Wound Motor:
Characteristics of Differential Compound Wound Motors
ARMATURE CURRENT
IN AMPS
TORQUEINN-m
SHUNT
DIFFERENRIAL
ARMATURE CURRENT
IN AMPS
SPEEDINRPM
SHUNT
DIFFERENRIAL
86

SPEED CONTROL OF DC MOTOTRS
MEANING OF SPEED CONTROL:
Speed control means intentional change of the
drive speed to a value required for performing
the specific work process. The concept of speed
control or adjustment should not be taken to
include the natural change in speed which occurs
due to change in load on the drive shaft. Speed
can be controlled manually by operator or by
some automatic control device.
87

SPEED CONTROL BY MECHANICALS :
Speed can be adjusted mechanically by means of
stepped pulleys, sets of change gears, variable
speed friction clutch mechanism and other
mechanical devices. But the electrical speed
control has many economical as well as
engineering advantages over mechanical speed
control.
88

SPEED CONTROL BY ELECTRICAL
METHOD:
Expression of speed for a dc motor,
N = k V – Ia ( R + Ra )
Ø
The above expression reveals that the speed
can be controlled by adjusting any one of the
three factors appearing on the right hand side
of the expression:
Applied voltage to the armature terminals, V
(ii) External resistance in the armature circuit, R
(iii) Flux per pole, Ø
89

SPEED CONTROL OF DC SHUNT MOTORS
1.Field Control Method For DC Shunt Motor:
In this method speed variation is accomplished by
means of a variable resistance inserted in the series
with the shunt field. The power wasted in the
controlling resistance
is very small as the
field current
is very small.
Fig.
Field Control Method
For DC Shunt Motor
+
_ ARMATURE SHUNT
FIELD
FIELD RHEOSTATE
Ish
Ia +
_
V
IL
90

Field Control Method For DC Shunt Motor continues………
Since in this method of speed control the flux can
only be reduced (not increased) so the speed only
above the normal one can be obtained. The speed
is minimum at the maximum value of flux
and the speed is maximum
at the minimum value of
flux. The high speed limit
is also restricted due to
mechanical consideration
as the centrifugal forces
are set up at high speed
TORQUE
SPEED NORMAL FIELD
WEAK FIELD-1
WEAK FIELD-2
Fig. Speed – Torque Characteristics of Shunt Motor With Field Control
91

SPEED CONTROL OF DC MOTOTRS
2. Armature Control Method For DC
Shunt Motors :
This method consists of a variable resistance
connected in series with the armature as
shown in the fig in next slide. The speed at the
full load may be reduced any desired value
depending on the amount of resistance. With
this method the voltage across the armature is
lower than the line voltage.
Armature Control Method For DC Shunt Motors continues……92

Armature Control Method For DC Shunt Motors continues……
WITH RESISTANCE
CONTROL
RESISTANCE
SHUNT
FIELD
ARMATURE
+
_
+
_
IL
Ia
Ish
V
TORQUESPEED
NO RESISTANCE
Fig. Armature Control Method For DC Shunt Motors
93

SPEED CONTROL OF DC SERIES
MOTORS:
Speed of dc series motor may be obtained
through either armature or field control.
1. ARMATURE CONTROL METHODS FOR DC
SERIES MOTORS:
(i) Armature resistance
control For DC
Series Motors:
Fig.
Armature Resistance
Control For DC Series Motors
SERIES
FIELD
CONTROL
RESISTANCE
_
+
ARMATUREV
+
_
94

2. FIELD CONTROL METHOD FOR DC
SERIES MOTORS:
(i) Field diverter method of Speed Control For
dc Series Motors:
Fig. Field diverter method of Speed Control For dc Series Motors
ARMATURE
+
_
SERIES
FIELD
DIVERTOR
V
_
+
I
I div
Ise
96

(ii) Tapped Field Control Method For Speed Control
of DC Series Motors:
Fig. Tapped Field Control Method For
Speed Control of DC Series Motors
TAPPED SERIES
FIELD
ARMATURE
+
_
V
+
_
I
I
97

POWER LOSSES IN A DC MOTOR
The basic function of dc motor is to convert
electrical energy into mechanical energy.
Whole of the input energy is not converted
into useful output energy but a part of the
input energy is converted into heat and the
same is lost. The basic power equation for a
motor is
P input = Poutput + P losses
Power losses of dc motor continues……..
98

The power losses in a dc motor consist of
input power that is converted into heat.
These losses are divided into
(i) Copper losses or electrical losses
(ii) Iron or magnetic losses
(iii) Mechanical losses
Power losses of dc motor 99

1. Copper or Electrical Losses:
Copper or electrical losses include power
wasted in armature winding, series field,shunt
field interpole field brush contacts
Armature copper losses = I2
aRa
Shunt Field Copper losses = I2
sh Rsh
Series Field Copper Losses = I2
se Rse
100

2. Iron or Magnetic Losses:
These losses are also called core losses and include the
hysterisis and eddy current losses
(a) Hysterisis Losses P h = ŋ ( B max )
1.6
.f .V
watts
(b) Eddy Current losses P e = K e . B max. f 2 V
t2 watts
where ŋ = Steinmetz hysterisis coefficient,
V =Volume of core in cubic meters ,
f = Frequency of the magnetic cycles
per second
t = Thickness of core steel laminations
B max. = Maximum flux density 101

Value of Steinmetz hysterisis coefficient, ŋ for:
Good dynamo sheet steel = 502 J/m3 ,
Silicon steel = 191 J/m3,
Hard cast steel = 7040 J/m3,
Cast steel = 750 – 3000 J/m3,
102

3. Mechanical Losses in a DC Motor:
These losses consist of power loss due to friction
of bearings, air friction or windage and are
caused by the motion of the moving parts.
103

SUMMERY OF POWER LOSSES IN A DC
MOTOR
Useful Output
Total losses
Copper losses
Iron losses
Mechanical losses
Armature copper loss
Field copper losses
Hysterisis loss
Eddy current loss
Friction loss
Windage loss
Input
104

EFFICIENCY OF A DC MOTOR
The ratio of useful output to the total input is
called the efficiency of the machine is expressed
as
ŋ = Output = Input - total losses
Input Input
= Output
Output + total losses
105

APPLICATION OF DC MOTORS
1. Application of DC Series Motors:
As observed from the different characteristics for dc
series motor, these motors are suitable where high
starting torque is required such as , electric traction,
Hoists, trolleys, cranes, gears drives.
DC Series motors have dangerously high speed at
no-loads. So these motors are not suitable for the
services where the load may be entirely removed and
also these motors are not suitable for belts drives.
Applications of dc motors continues……..
112

2. Application of DC Shunt Motors:
shunt motors, these motors have almost constant speed
and due to this feature these motors are suitable where
constant speed is required in wide range of load such as,
lathe machines, milling machines, conveyors line shafts
etc. These are also useful where medium starting torque
is required such as boring machines, blowers, fans,
conveyors centrifugal pumps, shapers, spinning and
weaving machines, machine tools, printing presses DC
shunt motors are not suitable for use with flywheel
or with fluctuating loads services.
Applications of dc motors continues……..113

3. Application of DC cumulative Compound
Motors:
cumulative compound motors, these motors are used
in driving machines which are subjected to sudden
applications of heavy loads such as occur in rolling
mills, punching and shearing machines, lifts, mine-
hoist. These motors are also used where high starting
torque is required but series motors cannot be
employed conveniently such as cranes and elevators.
These motors are not suitable for applications
requiring adjustable speed for field control
Applications of dc motors continues…….
114

4. Application of DC differential Compound
Wound Motors:
In differential compound wound motors, since series
field opposes the shunt field, the resultant flux
decreases with the increase in load; thus the machine
runs at a higher speed than it would do as a shunt
motor. The decrease in flux with the increase in load
causes the torque to be less than that of a shunt
motor. So such motors are rarely used in practice as
the differential arrangement causes difficulties
during overloads and starting. 115

SPECIFICATIONS OF DC MOTORS
Specifications of dc motors are shown on the
specification plate of the motors. The meaning
of different specifications of Kirloskar make
dc motor as:
Specification of DC Motors continues….
116

KW / HP - Maximum power output of the
motor
VOLTS – Rated Armature Voltage
AMPS - Rated motor output current
DUTY – S1 for continuous operation
S4 for intermittent operation
XXX / XXX
Maximum Speed
Base Speed
RPM -
117

Frame:
SX / MX / LX (Core length in mm)
K X B DC XXX - X
100…250 ( Centre height in mm)
DC Motor
B for flange mounting
None for foot mounting
L for IC 01, 06, 17, 37
S for IC 0041
H for IC 0666
W for IC W37, A86
Make - Kirloskar
118

WDG – Connections Scheme of Winding of motor
EXC. V - Field Excitation Voltage
EXC. A – Field Excitation Current
INS. CL. – Insulation class used in winding
FWR – Field Winding Resistance
AMB – Ambient or room temperature
GD2 - Moment of inertia
Specification of DC Motors continues….119

IP – Type of Enclosure, IP21, IP22, IP23, IP44
(i) IP21: Screen protected
(ii) IP22: Screen protected drip proof
(iii) IP23: Screen protected splash proof
(iv) IP44: Totally enclosed duct ventilated
IC – Type of Cooling , IC06, IC17, IC37
(i) IC06: Motor mounted blower and free
outlet
(ii) IC17: Cooling air inlet via duct, free
outlet
(iii) IC37: Cooling air inlet and outlet via
ducts 120

BRG: CE – Bearing at commutator end
NCE – Bearing at non commutator end
Air Flow – Amount of Air required for the
cooling of Motor
121

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