The main functions of controlling a motor are starting, stopping, direction of rotation, speed regulation, starting current limitation, mechanical protection, electrical protection. We will see details of some of them. In the direction of rotation helical gears most of the motors except single-phase motors, such as the shaded pole and the repulsion, can be used in both directions of rotation depending on a suitable control.
In speed regulation, AC motors, other than universal motors, are constant speed machines. There is, however, the possibility of re-connecting the stator coils of an induction motor, so as to double the number of poles thus reducing the speed in half.
At start-up, an engine will only begin to rotate when the load moment to be stopped, when stationary, is less than its starting torque. At standstill, in certain applications there is a need for a rapid deceleration of the engine and the load. When disconnected from the power supply line, the motor uses a rotation reversing device with the motor still running. Stopping or shutting down the mains motor takes place through a relay preventing it from starting in the opposite direction. In the case of synchronous motors, dynamic braking is used.