Handbook of Electrical Engineering

VARIABLE SPEED ELECTRICAL DRIVERS 389

Figure 14.1 Variable applied voltage with a normal design of an induction motor.

the same as that for full voltage operation, but scaled up or down by the ratio (V/Vr)^2 whereV is
the applied voltage for the new speed andVris the rated voltage, see Figure 14.1.

It may be seen that if this method is used to control the speed of a standard, almost constant-
speed type of induction motor, then the actual range of speed control obtained will, in fact, be small
before stalling occurs. The situation could be improved by using a motor with a high rotor resistance
as shown in Figure 14.2. The rotor resistance at full-load has been increased by a factor of 10 in
order to demonstrate the effect on the torque-speed characteristic.

However, this method is not used for medium and large industrial drives, because of the
practical difficulty in designing a high resistance rotor. If a slip-ring wound rotor design is used then
an external high resistance can be added, but this method is seldom acceptable in the oil industry
because of restrictions imposed by hazardous area classification.

The voltage applied to the stator can be varied in two ways:

• In steps using a transformer that has various taps on its secondary winding. This gives a course
  control and is used for ‘open loop’ control, i.e. no feedback regulation is used.


• Continuously by using some form of thyristor controller which will allow feedback action in the
  form of ‘closed loop’ control to be used to accurately regulate the speed. However, if such a
  scheme is used then it is the customary practice to adjust the applied frequency so as to maintain
  a constant air-gap flux, see 14.3.2 and 14.6.

14.2.2 Pole-changing of the stator winding

If an induction motor has more than two poles, e.g. four or eight, then it can be arranged to operate at
two different synchronous speeds, one being half of the other. This technique is one of several which