Page 199 - Electric Drives and Electromechanical Systems

194 Electric Drives and Electromechanical Systems

FIG. 7.5 Operation of a two-pole, three-phase induction motor with a variable voltage, fixed frequency supply.
1
The supply frequency in this case is 60 Hz, giving a synchronous speed of 1800 rev min . In the Speed-torque
curve, the peak torque occurs at the same speed, irrespective of the supply voltage.
only suitable for small motors with a high value of the breakdown slip; even so, the
motor losses are large, and forced cooling will be required even at high speeds.
The more commonly used method of speed control is to supply the motor with a
variable-frequency supply, using either a voltage- or a current-fed inverter. Since
current-fed inverters are used for drives in excess of l50 kW, they will not be discussed
further. A block diagram of a voltage-fed inverter drive is shown in Fig. 7.6. The speed-
loop error is used to control the frequency of a conventional three-phase inverter. As the
supply frequency decreases, the motor’s air gap will saturate; this results in excessive
stator currents. To prevent this problem, the supply voltage is also controlled, with the
ratio between the supply frequency and the voltage held constant.