The main functions of controlling a motor are starting, stopping, direction of rotation, speed regulation, starting current limitation, mechanical protection, electrical protection. https://www.mrosupply.com/v-belts/cogged-v-belts/bx-section/1251802_bx30_gates-rubber/ We will see details of some of them. In the direction of rotation 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.