DC

1. Direct Current Motors
When a current carrying conductor is placed in a
magnetic field, it experiences a mechanical force
whose direction is given by Fleming’s left hand rule.
Constructionally, D.C. Generator and D.C. motor have no
basic difference
Principle of Operation

2. Constructional Features of DC Motors
• Commutator along with the armature on the rotor
• Salient-poles on the stator
• Field windings – Two exciting field windings, the shunt and
series windings
A 4-Pole DC Motor

3. Significant Features of DC Motors
• DC motors convert electrical energy into mechanical energy
• Constant mechanical power output or constant torque
• Types – Shunt motors, series motors and compound motors
• Rapid acceleration or deceleration
• Extensively used as a positioning device because its speed as
well as torque can be controlled precisely over a wide range
• 1W to 10,000 hp
• Applications – in automobiles, robots, VCRs, movie camera,
electric vehicles, in steel and aluminum rolling mills, electric
trains, overhead cranes, control devices, etc.
• Biggest advantage over other motors – Torque-speed
characteristics of dc motors can be varied over a wide range
while retaining high efficiency

4. Operation Principle
 As soon as the switch is closed, a large amount of current
will flow through the armature conductors.
 The current carrying armature conductors are in a magnetic
field produced by the current in stator field winding.
 The armature conductors will experience a mechanical force
or torque which will cause the rotor of the dc motor to spin.
A 4-Pole DC Motor

5. Back EMF
+
-
I
Shunt Field V
Ia
Ish
Rsh
Ra
Armature
When the motor armature rotates, the conductors cut the flux. So,
according to laws of electromagnetic induction, EMF is induced
in them whose direction, according to Fleming’s Right Hand Rule is
in opposition to the applied voltage. So, it is referred to as
Back EMF Eb.
So, V has to drive Ia against Eb.

6. +
-
V
Ia
Eb
Drops
Brush
R
I
V
E a
a
b 


volt
60
φNP
Eb 
a
a
b R
I
V
E 

Neglecting Brush Drops (since small)

7. Voltage Equation of a Motor
+
-
I
Shunt Field V
Ia
Ish
Rsh
Ra
Armature
Eb
a
2
a
a
a
a
a
b
R
I
R
I
R
I
V
E






a
b
a
b
I
E
VI
E
V
VIa = Electrical Input to the armature
EbIa = Electrical Equivalent of Mechanical Power
developed in Armature
Ia
2Ra=Armature Copper Loss

8. Armature Torque (Ta)
If Ta be the torque developed by the armature of a motor
running at N rps
Power developed = Watt
πN
Ta 2

Electrical Power converted into Mechanical Power in the Armature
= EbIa Watt
a
b
a I
E
πN
T 

 2 volt
φNP
E
Again, b K

a
a NPI
K
πN
T 


 2
a
a
a
I
T
or
m
N
P
T
or
m
N
P







,
φKI
159
.
0
,
Kφφ
2
1
T
a
a
a

9. Series Motor
+
-
Ia=Ise=I
Series Field
V
2
a
a
a
a
a
I
T
I
I
T







10. +
-
I
Shunt Field V
Ia
Ish
Rsh
Ra
Armature
Shunt Motor
a
a
a
a
I
T
I
T



constant
almost
is



11. Shaft Torque (Tsh)
Whole of Armature torque Ta is not available for useful work.
A percentage of it is required for supplying iron and friction losses
In the motor.
Tsh is the shaft or useful torque.
Watt
N
T
output sh 
2


Ta-Tsh is the lost torque

12. Speed of a D.C. Motor
 





b
b
b
E
N
E
k
N
K
E
N
or
rpm
K
N
A
P
ZN















,
R
I
V
R
I
V
60
R
I
V
E
a
a
a
a
a
a
b

13. Speed Control of a D.C. Motor







 




a
a
b
R
I
V
k
N
E
N
So Speed can be varied by varying
1. Flux/Pole – field Control
2. Armature Drop – Armature Control

14. Speed Control of Shunt Motor
1. Field Control
+
-
I
Shunt Field
V
Ia
Ish
Field
Rheostat

b
E
N 
1
1
1

b
E
N 
2
2
2

b
E
N 
2
1
2
1
2
1


b
b
E
E
N
N


15. 2. Armature Control +
-
I
Shunt Field
V
Ia
Ish
Rheostat

a
a R
I
V
N



a
a R
I
V
N 1
1



Total
a R
I
V
N 2
2


1
2
1
2
b
b
E
E
N
N
