In this topic, you study Speed Control of Induction Motor.
The speed of the induction motor is given by
The above expression clearly shows that the speed of the induction motor can be controlled by varying anyone Of the two basic factors, namely the synchronous speed Of the motor and its slip. The synchronous speed of the motor can be changed by changing the supply frequency or the number of poles. The slip may be changed by changing the supply voltage, introducing extra resistance in the rotor circuit, injecting emf in the rotor circuit or by operating the motor in tandem or cascade with another induction motor. Accordingly, we have the following methods for the speed control of induction motors.
- Frequency Control
- Pole Changing
- Voltage Control
- Rotor-Resistance Control
- Injection of an EMF in the Rotor Circuit
- Cascade, Concatenation or Tandem Operation
Injection of an EMF in the Rotor Circuit
Slip and therefore the running speed of an induction motor can be varied by injecting an emf (Ej) of slip frequency into the rotor circuit through slip-rings. If this injected emf is in phase opposition to the rotor induced emf (E2), the net emf in the rotor circuit reduces. Consequently, the rotor current and hence the torque developed by the motor reduces. This ultimately results into reduction in speed of the motor. Reverse is the case when the injected emf is in phase with the rotor induced emf Such an emf increases the rotor current, torque and hence the speed of the motor. Fig. 2.13 shows the speed-torque curves of an induction motor under the above two conditions. Injection of the desired emf of slip frequency into the rotor circuit can be carried out with the help of an auxiliary commutating machine or by providing the rotor itself with the commutator.Speed
Fig. 2.13: Speed-torque curves of an induction motor with injection of an emf in its rotor circuit.
This method of speed control is efficient and has better speed regulation. However, need of an arrangement to inject an emf in the rotor circuit makes it very expensive. Hence, it is used only for the motor; of very large rating.
Cascade, Concatenation or Tandem Operation
This method requires two induction motors mechanically coupled together. At least one Of these motors must have a wound rotor. This wound rotor induction motor is called the main motor, while the Other (which may be either Of slip-ring type or squirrel cage type) is called the auxiliary motor. The Stator Of the main motor is connected to the supply while its rotor slip-rings are connected to the Stator Of the auxiliary machine (Fig. 2.14 a). In case both the motors are Of slip-ring type, there is another way Of electrically connecting them together through their rotors for cascade operation as illustrated in Fig. 2.14 (b). Usually two motors are operated in cascade with their torques acting in the same direction.
This operation is called cumulative cascading. But they can also be run with their torques acting in opposition. The operation is then called differential cascading.
Fig. 2.14 : Cascade operation of two induction motors
If P1 and P2 are the number of poles of main and auxiliary motors respectively, f is the supply frequency, then the cascade operation of two motors can give the following four synchronous speeds :
(i) By Using Main Motor Alone :
revolutions per minute
(ii) By Using Auxiliary Motor Alone :
The auxiliary motor may be run separately from the mains. Under this condition, we have
revolutions per minute
(iii) By Using Both Motors in Cumulative Cascade :
revolutions per minute
(iv) By Using Both Motors in Differential Cascade :
revolutions per minute
Cascade operation is very rarely used in actual practice because it suffers from the following
Disadvantages:
(i) Necessity of two motors. This makes the method expensive.
(ii) Low power factor, efficiency and pull-out torque.
(iii) Complicated operation.
(iv) Limited number of speeds.