by Answer:
Three can be three ways to control the speed of squirrel cage induction motor.
- Voltage changes
- Frequency changes
- Stator poles
- Voltage changes
First method is by means of changing the input voltage applied for excitation of stator winding. Although it looks simple to change the value of voltage, however very large change in voltage is required to slightly change the speed of the motor and such voltage changes also effects the flux, so this method is used very rarely.
- Frequency changes
Second method can be changing the frequency of applied voltage signal for excitation. As relation between speed and frequency is given by:
Ns= (120*f)/P
Thus, speed is directly proportional to the frequency of applied voltage for excitation of stator. However, this method has very limited range and cannot be used for large range of speed changes.
- Stator poles
From relation Ns= (120*f)/P, it is clear that speed of stator Ns is inversely proportional to the number of poles of winding. So increasing number of poles will reduce the speed of motor. While decreasing poles will results in increasing speed.
Thus, number of poles can be increased from two to any number in order to decrease the speed of motor. For this purpose, multiple windings can be used each having different number of poles. Then selection between them is made, according to speed requirements.
