Until the advent of solid-state drives, induction motors were not used in many applications requiring speed control. The normal operating range of an induction motor is within less than 5 percent slip. At larger slip, the efficiency of the motor will drop significantly because the rotor copper losses are directly proportional to the slip of the motor ( P _RCL = sP _AG). The speed of an induction motor can be controlled by varying the synchronous speed or the slip for a given load. The synchronous speed can be varied by changing the electrical frequency or the number of poles. The slip can be changed by varying the rotor resistance or terminal voltage.

SPEED CONTROL BY CHANGING THE LINE FREQUENCY

The rate of rotation of the stator magnetic field depends on the electrical frequency. The no-load point on the torque-speed curve changes with the frequency (Fig. 7.1). The base speed is the synchronous speed at rated conditions.

Figure 7.1: Variable-frequency speed control in an induction motor: ( a) The family of torque-speed characteristic curves for speeds below base speed, assuming that the line voltage is derated linearly with frequency. ( b) The family of torque-speed characteristic curves for speeds above base speed, assuming that the line voltage is held constant.

Figure 7.1: ( Continued) ( c) The torque-speed characteristic curves for all frequencies.

The speed of the motor can be adjusted by using variable frequency control. A variable frequency induction motor drive can...