Dc Motor

1. DC MOTOR
ASHOK SINGH 12/IEE/009

2. CONTENTS
 INTRODUCTION
 WORKING PRINCIPLE OF DC MOTOR
 BACK EMF DC MOTOR
 TYPES OF DC MOTOR
 TORQUE OF DC MOTOR
 ARMATURE REACTION OF DC MOTOR
 INTERPOLES OF DC MOTOR
 CHARATERISTICS OF DC MOTOR SHUNT, SERIES AND
COMPOUND
 SPEED CONTROL OF DC MOTOR
 LOSSES IN DC MOTOR
 EFFICIENCY OF DC MOTOR

3. INTRODUCTION
o A dc motor in single words is a device that converts direct
current (electrical energy) into mechanical energy.

4. WORKING PRINCIPLE OF DC MOTOR
o Step 1:
Initially considering the armature is in its starting point or
reference position where the angle α = 0.
o ∴ τ = BIL w cos0 = BILw
Since α = 0, the term cos α = 1, or the maximum value, hence
torque at this position is maximum given by τ = BILw. This high
starting torque helps in overcoming the initial inertia of rest of the
armature and sets it into rotation.

6. o Step 2:
o Once the armature is set in motion, the angle α between the actual
position of the armature and its reference initial position goes on
increasing in the path of its rotation until it becomes 90° from its
initial position. Consequently the term cosα decreases and also the
value of torque.
The torque in this case is given by τ = BILwcosα which is less than
BIL w when α is greater than 0°.

8. o Step 3:
o In the path of the rotation of the armature a point is reached where
the actual position of the rotor is exactly perpendicular to its
initial position, i.e. α = 90°, and as a result the term cosα = 0.
The torque acting on the conductor at this position is given by.
τ = BILwcos90° = 0

10. BACK EMF OF DC MOTOR
o When the motor Armature continue to rotate due to motor action
the Armature conductor cut the magnetic flux emf is induced in
them is known as back emf .
o It Denoted by Eb
Eb ═ ΦZNP/60A

11. TYPES OF DC MOTOR
o SERIES WOUND DC MOTOR
o SHUNT WOUND DC MOTOR
o COMPOUND WOUND DC MOTOR

12. SERIES WOUND DC MOTOR
A
E
Ra
IL
V
+
_
Ia
AA
ISE
YY
+
_
M SUPPLY

13. SERIES WOUND DC MOTOR
o Armature current ═series field current═line current,i═I
Eb ═V - I(Ra+Rse)
o Power drawn P ═VI
o The crooss sectional area of of the wire used for field
coil has to be fairly large to carry the armature current
but owing to the higher current the number of turn of
wire in them need not be large.

14. SHUNT WOUND DC MOTOR
SUPPLYE
Ra
IL
V
FF
If
Ia
AA
+
_
F
M
A

15. SHUNT WOUND DC MOTOR
o In this type of excitation , armature and field windings are
connected across a constant source of supply.
o The field current If is drawn from the same source as that of
armature current.

16. COMPOUND WOUND DC MOTOR
o Compoud wound:- which has two field windings; one
connected inparallel with the armature and the other in series with
it.
A
E
Ra
IL
V
Ia
AA
ISE
YYY +
_
ISh
Z
ZZ
Rsh
SUPPLY
M

17. o There are two types of compound dc motor connections :-
(1) Short shunt connection :-When the shunt field winding is directly
connected across the armature terminals it is called short-shunt connection.
(2) Long shunt connectin :-When the shunt winding is
connected that it shunts the series combination of armature
and series field. it is called long-shunt connection.

19. TORQUE OF DC MOTOR
o Whenever armature carries current in
presence of flux, conductor
experiences force which gives rise to
the electromagnetic torque. In this
section we shall derive an expression
for the electromagnetic torque Te
developed in a d.c machine.
Obviously Te will be developed both
in motor and generator mode of
operation. It may be noted that the
direction of conductor currents
reverses as we move from one pole to
the other. This ensures unidirectional
torque to be produced. The derivation
of the torque expression is shown
below.

20. TORQUE OF DC MOTOR
Let, Ia = Armature current
Average flux density Bav = pφ/πDL
Then, Ia/a = Current flowing through each conductor.
Force on a single conductor = BavLIa/a
Torque on a single conductor = BavLDIa/2a
Total electromagnetic torque developed, Te =
zBavIaLD/2a
Putting the value of Bav,
we get Te = pz φIa/2πa

21. ARMATURE REACTION OF DC MOTOR
(a) Armature reaction is the
magnetic field produced by the
armature current.
(b)Armature reaction aids the main
flux in one half of thepole and
opposes the main flux in the
other half of the pole .
(c)However due to saturation of the
pole
faces the net effect of Armature
reaction is demagnetizing.

22. INTERPOLES OF DC MOTOR

23. INTERPOLES OF DC MOTOR
o These are fixed to the yoke in between the main poles of a dc
motor. These are usually tapered with sufficient sectional area at
the root to avoit megnetic saturation. The interpoles winding,
consisting of a few turn of thik wire, is connected in series with
the armature so that its magnetomotive force is proportional to
armature current.
o The effect of armature reaction in the armature zone can be
overcome by interpole placed in between the main pole. The
megnetic axis of interpole winding is the line with the quadrature
axis. Interpole winding is connected in series with armature so the
interpole mmf is able to neutralize the effect of armature mmf in
the interpolar zone at are level of load current not exceeding the
safe limit.

24. CHARACTERISTICS OF DC MOTOR
o There are three principal types of d.c. motors viz., shunt motors,
series motorsand compound motors. Both shunt and series types
have only one field windingwound on the core of each pole of the
motor. The compound type has twoseparate field windings wound
on the core of each pole. The performance of ad.c. motor can be
judged from its characteristic curves known as
motorcharacteristics, following are the three important
characteristics of a d.c. motor:-

25. (i) Torque and Armature current characteristic (Te/Ia):-
It is the curve between armature torque Ta and armature current
Ia of a d.c. motor. It is also known as electrical characteristic of
the motor.
(ii) Speed and armature current characteristic (N/Ia):-
It is the curve between speed N and armature current Ia of a d.c.
motor. It is veryimportant characteristic as it is often the deciding
factor in the selection of the motor for a particular application.
(iii) Speed and torque characteristic (N/Te):-
It is the curve between speed N and armature torque Te of a d.c.
motor. It is also known as mechanical characteristic.

26. CHARACTERISTICS OF SHUNT MOTOR
o the connections of a d.c. shunt motor. The field current Ish is
constant since the field winding is directly connected to the supply
voltage V which is assumed to be constant. Hence, the flux in a
shunt motor is approximately constant.

27. CHARACTERISTICS OF SERIES MOTOR
o The connections of a series motor. Note that current passing
through the field winding is the same as that in the armature. If the
mechanical load on the motor increases, the armature current also
increases. Hence, the flux in a series motor increases with the
increase in armature current and vice-versa.

28. CHARACTERISTICS OF COMPOUND MOTOR
o A compound motor has both series field and shunt field. The shunt
field isalways stronger than the series field. Compound motors are
of two types:-
(i) Cumulative-compound motors in which series field aids the shunt
field.
(ii) Differential-compound motors in which series field opposes the
shunt field.
o Differential compound motors are rarely used due to their poor
torque characteristics at heavy loads.

29. CHARACTERISTICS OF COMULATIVE COMPOUND MOTOR
o the connections of a cumulative-compound motor. Each pole
carries a series as well as shunt field winding; the series field
aiding the shunt field.

30. SPEED CONTROLOF DC MOTOR
o The d.c. motor is of considerable industrial importance. The
principal advantage of a d.c. motor is that its speed can be changed
over a wide range by a variety of simple methods. Such a fine
speed control is generally not possible with a.c. motors. In fact,
fine speed control is one of the reasons for the strong competitive
position of d.c. motors in the modem industrial applications.
o N=Vt – IaRa/ΦK

31. LOSSES IN DC MOTOR
o The losses occurring in a d.c. motor are the same as in a d.c.
generator :-
(i) copper losses
(ii) Iron losses or magnetic losses
(iii) mechanical losses As in a generator, these losses cause
(a) an increase of machine temperature
(b) reduction in the efficiency of the d.c. motor.

32. EFFICIENCY OF DC MOTOR
o Efficiency calculation of motor, first calculate the input power and
then subtract the losses to get the output mechanical power as
shown below .
o ∴ ηm = Poutm/Pinm
o ηm = Efficiency of dc motor
o Poutm= Output power of dc motor
o Pinm = Input power of dc motor

33. THANK YOU