It is seen that speed of the motor is inversely proportional to flux. Thus by decreasing flux speed can be increased and vice versa.
To control the flux, a rheostat is added in series with the field winding, as shown in the circuit diagram. Adding more resistance in series with field winding will increase the speed, as it will decrease the flux. Field current is relatively small and hence I2R loss is small, hence this method is quiet efficient. Though speed can be increased by reducing flux with this method, it puts a limit to maximum speed as weakening of flux beyond the limit will adversely affect the commutation.
Armature Control Method
Speed of the motor is directly proportional to the back emf Eb and Eb = V- IaRa. That is when supply voltage V and armature resistance Ra are kept constant, speed is directly proportional to armature current Ia. Thus if we add resistance in series with armature, Ia decreases and hence speed decreases.
Greater the resistance in series with armature, greater the decrease in speed.
Voltage Control Method
• Multiple voltage control:
In this method the, shunt filed is connected to a fixed exciting voltage, and armature is supplied with different voltages. Voltage across armature is changed with the help of a suitable switchgear. The speed is approximately proportional to the voltage across the armature.
• Ward-Leonard System:
This system is used where very sensitive speed control of motor is required (e.g. electric excavators, elevators etc.) The arrangement of this system is as required in the figure beside.
M2 is the motor whose speed control is required. M1 may be any AC motor or DC motor with constant speed. G is the generator directly coupled to M1. In this method the output from the generator G is fed to the armature of the motor M2 whose speed is to be controlled. The output voltage of the generator G can be varied from zero to its maximum value, and hence the armature voltage of the motor M2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method.
Voltage Control Method
• Multiple voltage control:
In this method the, shunt filed is connected to a fixed exciting voltage, and armature is supplied with different voltages. Voltage across armature is changed with the help of a suitable switchgear. The speed is approximately proportional to the voltage across the armature.
• Ward-Leonard System:
This system is used where very sensitive speed control of motor is required (e.g. electric excavators, elevators etc.) The arrangement of this system is as required in the figure beside.
M2 is the motor whose speed control is required. M1 may be any AC motor or DC motor with constant speed. G is the generator directly coupled to M1. In this method the output from the generator G is fed to the armature of the motor M2 whose speed is to be controlled. The output voltage of the generator G can be varied from zero to its maximum value, and hence the armature voltage of the motor M2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method.
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