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 V , armature resistance R 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.
ARMATURE RESISTANCE 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.
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.
FIELD FLUX CONTROL
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.
