The contactor is the switch that turns the motor on and off; on its own it provides no protection. Correct selection needs three pieces of information: the motor's rated current at 400 V, the operating pattern and the control voltage.

  • Utilisation category: for squirrel-cage motors that start normally and are switched off while running, the contactor's AC-3 rating applies. If the motor is frequently switched off while starting, plug-braked or inched, the contactor is sized by its AC-4 rating, noticeably larger.
  • Current: the 400 V AC-3 current on the contactor must not be lower than the motor's rated current. For example, an 11 kW motor drawing 20.4 A needs an AC-3 contactor carrying at least this current.
  • Coil voltage: the coil must match the control circuit voltage, such as 24 V DC or 230 V AC.
  • Auxiliary contacts: allow for the number of contacts needed for self-holding, signalling and interlocking.

A star-delta starter has three contactors: the main and delta contactors carry winding current (about 0.58 times line current), the star contactor a lower current. In a forward-reverse circuit, the two contactors are interlocked electrically and mechanically. Short-circuit protection (fuse or motor protection circuit breaker) goes before the contactor, overload protection after it. For the panel bill of materials, see our article on motor protection devices.