Speed Control of 3-Phase Induction Motors
Speed Control of 3-Phase Induction Motors:
The speed of an induction motor is given by:
The speed N of an induction motor can be varied by changing
(i) supply frequency f
(ii) number of poles P on the stator and
(iii) slip s.
The change of frequency is generally not possible because the commercial supplies have constant frequency. Therefore, the practical methods of speed control are either to change the number of stator poles or the motor slip.
1. Squirrel cage motors
The speed of a squirrel cage motor is changed by changing the number of stator poles. Only two or four speeds are possible by this method. Two-speed motor has one stator winding that may be switched through suitable control equipment to provide two speeds, one of which is half of the other. For instance, the winding may be connected for either 4 or 8 poles, giving synchronous speeds of 1500 and 750 r.p.m. Four-speed motors are equipped with two separate stator windings each of which provides two speeds. The disadvantages of this method are:
(i) It is not possible to obtain gradual continuous speed control.
(ii) Because of the complications in the design and switching of the interconnections of the stator winding, this method can provide a maximum of four different synchronous speeds for any one motor.
2. Wound rotor motors
The speed of wound rotor motors is changed by changing the motor slip. This can be achieved by;
(i) varying the stator line voltage
(ii) varying the resistance of the rotor circuit
(iii) inserting and varying a foreign voltage in the rotor circuit