The simplest controller is a switch that connects the motor to the power supply, such as in small household appliances or power tools. The switch can be operated manually or automatically started or stopped using relays or contactors connected to sensors. The switch can have different selectable positions, allowing the motor to connect to the power supply in different ways, such as reducing the starting voltage, reversing operation, or selecting different speeds. Small controllers may omit overload and overcurrent protection devices, relying on external overcurrent protection circuitry to prevent excessive current. Some small motors have built-in overload protection that automatically opens the circuit to prevent overload. Larger motors will have overload relays or temperature-sensing relays, as well as fuses or circuit breakers added to the controller to prevent overcurrent. Automatic motor controllers may also include limit switches or other devices to protect the machinery.
More complex motor controllers can precisely control the speed and torque of the motor and may also be part of a closed-loop control system for controlling the mechanical position.
