Speed Control Of DC Series Motors

Speed Control Of DC Series Motors
Speed Control Of DC Series Motors

Speed Control Of DC Series Motors

Speed of dc motor is given by

    N  ∝ (V – IR)/Ⲑ. …………….. 1

Above relation shows that speed is dependent upon the supply voltage , armature resistance and the field flux Ⲑ.

There are some popular methods of speed control of dc series motor are:

1. Variation of armature resistance. This method is called armature resistance control.

2. Variation of field flux. This method is called field flux control.


In this case, a external resistance Re is connected in the armature circuit of the dc series motor. When the value of the external resistance is increased, it will cause increased voltage drop. Hence the effective voltage across the armature circuit will be reduced.


Armature-resistance Control
Armature-resistance Control

As seen from eq.1 , speed will also reduced. So the speed of the motor can be controlled by varying the resistance.  As the value of Re is increased then motor will run at lower speed.

This method suffers from some serious drawbacks:
1. A large amount of power is wasted in the external resistance.

2. Speed can only be reduced below the normal speed with this method. It cannot be used for increase of speed.


In this method , a variable resistance called diverter, Rd is connected in parallel across the series field winding. This resistance reduces the current flowing through the series field winding and it results in lesser field flux. Since the speed is inversely proportional to the field flux (as from eq1) , so the speed of the motor will increase.

Field diverters
Field diverters

If the value of diverter resistance is decreased then more current will flow through it instead of series field winding and lesser flux will produce which will increase the speed of the motor.
This can also be achieved by providing a tapped series field winding, which is most commonly used for traction purposes.

READ HERE  Transfer Function of Field Controlled DC Motor


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