SPEED CONTROL BY CHANGING THE ROTOR RESISTANCE
The shape of the torque-speed curve of wound rotor induction motors can be changed by inserting extra resistances into the rotor circuit, as shown in Fig. 7.3. However, inserting additional resistances into the rotor circuit will reduce the efficiency of the motor significantly. This method is usually used for short periods.
SOLID-STATE INDUCTION MOTOR DRIVES
The solid-state variable frequency induction motor drive is the preferred method for speed control. A typical drive is shown in Fig. 7.4. The drive is very flexible. Its input can be single-phase or three-phase; 50 or 60 Hz; and any voltage in the range of 208 to 230 V. The output is a three-phase voltage whose frequency can vary in the range of 0 to 120 Hz and whose voltage can vary in the range of 0 to the rated voltage of the motor. The control of the output voltage and frequency is achieved by using the pulse-width modulation (PWM) technique.
The output frequency and output voltage can be controlled independently. Figure 7.5 illustrates how the drive controls the output frequency while the root-mean-square (rms) voltage is maintained at a constant level. Figure 7.6 illustrates how the drive controls the rms voltage while maintaining the frequency at a constant value.