As seen from Eq. (3.3), the speed of an induction motor can be controlled by changing the number of poles, slip, and the supply frequency. The pole changing has already been described, and, if the motor has that capability, it only requires an appropriate switch. Changes of slip can be effected by varying the stator voltage, particularly in motors with soft mechanical characteristics. However, this method is inefficient, because rotor losses are proportional to the slip. Also, in most motors, it is ineffec-
tive because of the narrow range of controllable slip (from zero to the critical value). For wide-range speed control, adjusting the supply frequency constitutes the only practical solution. The frequency control must be accompanied by magnitude control of the stator voltage.
To produce adjustable-frequency, adjustable-magnitude,
three-phase voltage for induction motor drives, power electronic inverters are most commonly used. Inverters are dc to ac converters, so the regular 60-Hz (50-Hz in many countries) ac voltage must first be rectified to provide the dc supply for the inverter. Much less common are cycloconverters, which operate directly on the 60-Hz supply, but whose output frequency is inherently much lower than the input (supply) frequency. They are mostly employed in high-power synchronous motor drives. The soft-starters described in Section 3.2 are based on ac voltage controllers, which are ac to ac converters with adjustable rms value of the output voltage. The frequency is not changed in ac voltage controllers; that is, the output voltage has the same frequency as the supply voltage. Because of their marginal use in induction motor drives, cycloconverters and ac voltage controllers are not covered in this topic.
In the subsequent sections,
rectifiers and inverters employed in ASDs with induction motors are briefly described. Operating principles of inverters, knowledge of which is needed for in-depth understanding of control methods for induction motors, are particularly stressed. This necessarily sketchy information may be insufficient for readers with no background in power electronics, for whom studies of relevant literature are strongly recommended. See, for example, Introduction to Modern Power Electronics, 1998, by this author.