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One operation of all motors, whether powered by ac or dc, calls for a controller. The simplest of controllers, called starters, merely connect and disconnect a motor to a power source. But controllers also protect the motor from overload and prevent excessive branch-circuit current. In some cases they protect the equipment they power as well as the operators. More sophisticated controllers, sometimes called drives, regulate motor torque, speed, or horsepower in response to remote commands.
Adjustable-speed ac drives offer advantages over dc drives because of the simplicity, high-speed capability, and low-maintenance requirements of squirrel-cage motors. Squirrel-cage motors can also adapt to adverse conditions,...