Speed Control of Three-phase Induction Motors:
Until the advent of modern solid-state drives induction motors generally were not good machines for applications attaining considerable speed control. The normal operating range of a typical induction motor is confined to less than 5% slip and the speed variation over that range is more or less directly proportional to the load on the shaft of the motor. Even if the slip could be larger, the efficiency of the motor would become very poor.
Actually there are two techniques by which the speed of an induction motor may be controlled. One of is to vary the synchronous speed, which is the speed of the stator and rotor magnetic fields, as the rotor speed always remains near Ns. The other technique is to vary the slip of the motor for a given load.
The synchronous speed of the machine is
Ns = 120 f / P
The synchronous speed can be varied by
(a) Changing the electrical supply frequency.
(b) Changing the number of poles on the machine.
Slip control can be accomplished by varying either the rotor resistance or the terminal voltage of the motor.
There are two main approaches to change the number of poles in an induction motor.
(a) The method of consequent poles.
(b) Multiple stator windings.
The primary method is an old method for speed control developed in the year of 1897. The number of poles in the stator windings of an induction motor may easily be changed by a factor 2: 1 with only simple changes in coil connections. The rotor in such motor is of the squirrel cage design. The traditional approach to overcome the restriction of first approach was to employ multiple stator windings with different number of poles and to energise only one set at a time.
By utilizing variable frequency control, this is possible to adjust the speed of the motor either above or below the base speed. This may control the speed of an induction motor over a range from as little as 5 % of base speed up to about twice base speeds.
In wound-rotor induction motors, this is possible to change the shape of the torque-speed curve by inserting extra resistances into rotor circuit of the machine. Though, inserting extra resistances into the rotor circuit of an induction motor seriously decrease the efficiency of the machine. Such a method of speed control is normally utilized only for short periods due to this efficiency problem.