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II UNIT
D.C MOTORS
UNIT-II
D.C. MOTORS : Principle of operation of DC Motor, Types of
Motors, Back EMF Equation, Characteristics of DC motor, Torque
Equation, DC Motor Starter (Three Point starter), Efficiency
Calculation, Swinburne's Test and speed control of armature and
field.
DC Machines
DC Motor DC Generator
DC motor
Force in a Conductor
Diagram of a Simple DC Motor
Eb 
and it is indicated by Eb
ZNP
60 A
Net voltage across armature circuit =
If Ra is the armature resistance , then
Eb 
V  Eb
V  Eb
Ia 
Ra
ZNP
60 A
This back e.m.f is also called as counter e.m.f and indicated by
Eb
Significance of Back E.M.F
The presence of back e.m.f makes the dc motor a self regulating machine
i.e. it makes the motor to draw as much armature current as is sufficient to develop the
torque required by the load .
V  Eb
Ia 
Ra
Terminal Voltage Equation
VT  Eb  I A RA
VT = voltage at motor terminals
E = Back EMF induced in armature (V)
IA = armature current (A)
RA
RA = armature resistance
+
+
Eb
VT
-
-
Power Equation
VT  Eb  I A RA
VI a  Eb I a  I a Ra
2
VI a
is the electrical input to the motor
2
I a Ra represents the copper losses in the motor
Eb I a
is the electrical
equivalent
mechanical power developed in armature
of
Electric Motor
TYPES OF D.C MOTORS
(i) Shunt wound motors : in which the field winding is connected in
parallel with the armature.
(ii) Series –wound Motor : in which the field winding is connected in series
with the armature
(iii) Compound Wound Motor : which has two field windings ,one connected
in parallel with the armature and the other in series with it
Torque of a dc motor
• The turning or twisting moment of a force
about an axis.
• It is measured as the product of force and
radius at which it acts.
• T=F*r N-m
• Work done by force= F*2*3.14*r N-m
• Power developed=F*2*3.14*r *N
• P=T*W watts
(since T=F*r)
• P=(2*3.14)NT/60 watts
• P=NT/9.55 watts
Armature Torque
• If Ta is the torque developed by armature
and N is speed in rps
•
•
•
•
P=Ta*2*3.14*N watts
Electrical power: Eb*Ia
Eb*Ia =Ta*(2*3.14*N)
Ta=(Eb*Ia)/(2*3.14*N)
• Eb=ZNP/A
• Ta= (ZP/A)*(Ia/(2*3.14))
• Ta=0.159* ZIa(P/A) N-m
• Ta is proportional to (Ia)
SPEED CONTROL OF DC MOTORS
1. Vary the voltage supplied to the armature
circuit while holding the field constant.
2. Vary the field current while holding the
armature supply voltage constant.
3. Insert resistance in series with the armature
circuit.