Motor Controls and Power Electronics

Motor speed and torque control systems for most adjustable speed drives in the past thirty years have been dominated by SCR and TRIAC converters, but are being supplanted rapidly by converters based on IGBTs. Typical SCR converters were connected to dc motors, which provided a variable armature current (as well as field current) necessary to operate the motor across a speed range. Power devices such as high performance transistors in the mid seventies introduced chopper drives to accomplish the same task for lower horsepower machines.

The next step in motor control was the inverter, to allow ac machines to replace dc motors. Early forms of inverters existed in stepper motor drives, popular in applications such as printers, robots, and other precise speed applications such as disk drives. With the invention of the IGBT, three-phase inverter for high power motor applications became practical. At present, motor inverters based on IGBTs are quickly taking the place of all other motor drive circuits. Units up to 200 kW (more than 250 HP) are available commercially. At even higher power levels, recent developments in gate-controlled thyristors are supporting performance into the multi-megawatt range.

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