The start/stop control of a constant speed electric motor is one of the most common PLC applications. In this application, a single digital output from the PLC is used to turn the motor on and off. The most common method of motor control uses a magnetic motor starter. A magnetic motor starter is an electromagnetically operated switch, which provides a safe method to channel the flow of electricity to the electric motor.

Many electric motors use 3-phase electricity and therefore use a 3-phase magnetic motor starter. A 3-phase magnetic motor starter has three sets of contacts, each controlling power flow to one of the three motor windings. The motor windings create the magnetic field that causes the motor to move. Magnetic motor starters contain a solenoid that provides the interface between the PLC and the starter contacts. When the PLC output energizes the solenoid, the three contacts close and allow power to flow to the motor.