Pole amplitude modulation method (PAM) – In this method of speed control of the original sinusoidal mmf wave is modulated by another sinusoidal mmf wave having different number of poles. Let f1(θ) be the original mmf wave of whose speed is to be controlled. f2(θ) be the modulation mmf wave. P1 be the number of poles of whose speed is to be controlled. P2 be the number of poles of modulation wave. After modulation resultant mmf wave So we get, resultant mmf wave Therefore the resultant mmf wave will have two different number of poles Therefore by changing the number of poles we can easily change the speed of three phase induction motor. Adding rheostat in the stator circuit - In this method of speed control of rheostat is added in the stator circuit due to this gets dropped .In case of torque produced is given by T sV22. If we decrease supply torque ∝ will also decrease. But for supplying the same load , the torque must remains the same and it is only possible if we increase the slip and if the slip increase motor will run reduced speed. Speed Control from Rotor Side In this method of speed control of external are Adding external on rotor side – added on rotor side. The equation of torque for is The operates in low slip region .In low slip region term (sX)2 becomes very very small as compared to R2. So, it can be neglected . and also E2 is constant. So the equation of torque after simplification becomes, Now if we increase rotor , R2 torque decreases but to supply the same load torque must remains constant. So, we increase slip, which will further results in decrease in rotor speed. Thus by adding additional in rotor circuit we can decrease the speed of three phase induction motor. The main advantage of this method is that with addition of external resistance starting torque increases but this method of speed control of also suffers from some disadvantages : The speed above the normal value is not possible. Large speed change requires large value of and if such large value of resistance is added in the circuit it will cause large copper loss and hence reduction in efficiency. Presence of causes more losses.
Pole amplitude modulation method (PAM) – In this method of speed control of the original sinusoidal mmf wave is modulated by another sinusoidal mmf wave having different number of poles. Let f1(θ) be the original mmf wave of whose speed is to be controlled. f2(θ) be the modulation mmf wave. P1 be the number of poles of whose speed is to be controlled. P2 be the number of poles of modulation wave. After modulation resultant mmf wave So we get, resultant mmf wave Therefore the resultant mmf wave will have two different number of poles Therefore by changing the number of poles we can easily change the speed of three phase induction motor. Adding rheostat in the stator circuit - In this method of speed control of rheostat is added in the stator circuit due to this gets dropped .In case of torque produced is given by T ∝ sV22. If we decrease supply torque will also decrease. But for supplying the same load , the torque must remains the same and it is only possible if we increase the slip and if the slip increase motor will run reduced speed. Speed Control from Rotor Side Adding external on rotor side – In this method of speed control of external are added on rotor side. The equation of torque for is The operates in low slip region .In low slip region term (sX)2 becomes very very small as compared to R2. So, it can be neglected . and also E2 is constant. So the equation of torque after simplification becomes, Now if we increase rotor , R2 torque decreases but to supply the same load torque must remains constant. So, we increase slip, which will further results in decrease in rotor speed. Thus by adding additional in rotor circuit we can decrease the speed of three phase induction motor. The main advantage of this method is that with addition of external resistance starting torque increases but this method of speed control of also suffers from some disadvantages : The speed above the normal value is not possible. Large speed change requires large value of and if such large value of resistance is added in the circuit it will cause large copper loss and hence reduction in efficiency. Presence of causes more losses.
U6-L32_Speed-Control-of-Three-Phase-Induction-Motor.docx