Speed Control of Induction Motors
Speed Control of Induction Motors:
- A 3-phase induction motor is practically a constant-speed machine, more or less like a d.c. shunt motor. The speed regulation of an induction motor (having low resistance) is usually less than 5% at full-load. However, there is one difference of practical importance between the two.
- Whereas d.c. shunt motors can be made to run at any speed within wide limits, with good efficiency and speed regulation, merely by manipulating a simple field rheostat, the same is not possible with induction motors. In their case, speed reduction is accompanied by a corresponding loss of efficiency and good speed regulation. That is why it is much easier to build a good adjustable-speed d.c. shunt motor than an adjustable speed induction motor.
- Different methods by which speed control of induction motors is achieved, may be grouped under two main headings :
1. Control from stator side
(a) by changing the applied voltage
(b) by changing the applied frequency
(c) by changing the number of stator poles
2. Control from rotor side
(d) rotor rheostat control
(e) by operating two motors in concatenation or cascade
(f) by injecting an e.m.f. in the rotor circuit.
Fig: Speed-Torque characteristics of induction motor
- From the speed torque characteristic of the induction machine, we know that in the stable region of operation in the motoring mode, the curve is rather steep and goes from zero torque at synchronous speed to the stall torque at a value of slip s = ˆs. Normally ˆs may be such that stall torque is about three times that of the rated operating torque of the machine, and hence may be about 0.3 or less.
- This means that in the entire loading range of the machine, the speed change is quite small. The machine speed is quite stiff with respect to load changes. The entire speed variation is only in the range ns to (1 − ˆs)ns, ns being dependent on supply frequency and number of poles. It is known that the induction machine, when operating from mains is essentially a constant speed machine.
- Many industrial drives, typically for fan or pump applications, have typically constant speed requirements and hence the induction machine is ideally suited for these. However, the induction machine, especially the squirrel cage type, is quite rugged and has a simple construction. Therefore it is a good candidate for variable speed applications if it can be achieved.