These controls regulate DC motor speed. They govern adjustable acceleration and deceleration speeds, and they protect the motor from sudden electrical surges. Pulse width modulation (PWM) devices adjust voltage pulses to regulate speed. Silicon controlled rectifier (SCR) controls convert AC voltage to DC voltage and provide variable speed control. These speed controls are used with permanent magnet or shunt wound DC motors, and with DC gearmotors in constant or diminishing torque applications such as conveyors, packaging equipment, and centrifuges.