Speed of DC Motor is given by the relation
N ∝ ( V - Ia*Ra) / Ⲫ ......eq1
So, we can see that Speed of the motor mainly depends upon the supply voltage, armature resistance and field flux. In practice, variation of any these factors can be used for speed control of DC motor.
Hence mainly these methods are used for controlling the speed.
1. Variation of Armature Resistance, called Armature Resistance Control Method.
2. Variation of Field Flux, called Field Flux Method.
Now the current flowing through shunt winding will be = V / (Ra + R)
N ∝ ( V - Ia*Ra) / Ⲫ ......eq1
So, we can see that Speed of the motor mainly depends upon the supply voltage, armature resistance and field flux. In practice, variation of any these factors can be used for speed control of DC motor.
Hence mainly these methods are used for controlling the speed.
1. Variation of Armature Resistance, called Armature Resistance Control Method.
2. Variation of Field Flux, called Field Flux Method.
ARMATURE RESISTANCE CONTROL
In this method, a variable resistor is inserted in the series with the armature circuit. Figure below shows the connection diagram for a Shunt Motor.
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| ARMATURE RESISTANCE CONTROL FOR DC SHUNT MOTOR |
Note that, in this method field flux is not affected by the variation of resistance because field winding is connected directly across the supply.
When the value of the variable resistor is increased, effective voltage across the armature is reduced and as from eq1 speed of the motor also decreases.
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.
3. Speed reduction not only depends upon the variable resistance, it is also affected by the motor load .
When the value of the variable resistor is increased, effective voltage across the armature is reduced and as from eq1 speed of the motor also decreases.
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.
3. Speed reduction not only depends upon the variable resistance, it is also affected by the motor load .
FIELD CONTROL METHOD
In this method, speed is varied by inserting a variable resistor in series with the shunt field winding. Connection diagram of field control method for DC Shunt motor is shown in fig. below.
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| FIELD CONTROL METHOD OF DC SHUNT MOTOR |
Now the current flowing through shunt winding will be = V / (Ra + R)
Due to connection of resistance R, field current reduces and hence flux produced will be lower. And as from eq1, reduction in value of flux will increase the speed of the motor. Therefore, motor runs at a speed higher than the normal speed.
Since the current in the field winding is low, so the power wasted in the variable resistance is comparatively lower.
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