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Chapter 4: Power electronic converters for induction motor drives

In this chapter, we review power electronic converters used in ASDs with induction motors. Various types of rectifiers providing the dc supply voltage for inverters feeding the motors are presented, and we describe voltage source inverters, including three-level and soft-switching inverters, and current source inverters. Control methods for inverters, with a stress on the use of voltage space vectors, are illustrated. Finally, we outline undesirable side effects of the switching operation of power converters. | 4 POWER ELECTRONIC CONVERTERS FOR INDUCTION MOTOR DRIVES In this chapter we review power electronic converters used in ASDs with induction motors. Various types of rectifiers providing the de supply voltage for inverters feeding the motors are presented and we describe voltage source inverters including three-level and soft-switching inverters and current source inverters. Control methods for inverters with a sttess on the use of voltage space vectors are illustrated. Finally we outline undesirable side effects of the switching operation of power converters. 4.1 CONTROL OF STATOR VOLTAGE As seen from Eq. 3.3 the speed of an induction motor can be conttolled 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- 55 56 CONTROL OF INDUCTION MOTORS 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 de to ac converters so the regular 60-Hz 50-Hz in many countries ac voltage must first be rectified to provide the de 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 .