Contactor for motor control and protection

A contactor is an electrically operated switch whose contacts are closed by an electromagnet (coil) and held closed as long as the coil is energised. It is designed for frequent making and breaking of normal load currents, so it is the standard device for starting, stopping and protecting motors when combined with an overload relay and fuses (a DOL starter).

Scheme: DOL starter using a contactor

 Power circuit                Control circuit
 L1 L2 L3                     L ---[F]---+
  |  |  |                                |
 [F][F][F]  fuses (SC)            [STOP NC]
  |  |  |                                |
 [K][K][K]  main contacts      +--[START NO]--+
  |  |  |                     |              |
 [OL OL OL] thermal relay      +----[K aux]---+
  |  |  |                                |
  (  M  )  3-ph motor              [OL NC contact]
                                         |
                                   (K coil)---N

Working

1. Starting: pressing the START button energises the contactor coil K. The armature is pulled in and the three main contacts close, so the motor gets full supply.
2. Holding (latching): an auxiliary NO contact of K, connected across START, closes at the same time. When the START button is released, the coil stays energised through this contact.
3. Stopping: pressing STOP (NC) breaks the coil circuit; the coil de-energises, the spring opens the main contacts and the motor stops.
4. Overload protection: the thermal overload relay (bimetal strips in each line) heats up if the motor current stays above its setting. Its NC contact in the coil circuit opens, the contactor drops out and the motor is disconnected. It is set at about 100–110% of motor full-load current with a time delay so that starting current does not trip it.
5. Short-circuit protection: a contactor cannot break large fault currents, so back-up HRC fuses or an MCCB ahead of it clear short circuits.
6. Under-voltage (no-volt) protection: if the supply fails or falls to about 70–80% of rated value, the coil cannot hold the armature and the contactor opens. When the supply returns, the motor does not restart by itself because the hold-in contact is open; the operator must press START again. This prevents accidents and a large simultaneous starting load.
7. Single phasing: a differential type overload relay or phase-failure relay opens the coil circuit if one phase is lost.

Other control functions

• Remote control: extra START/STOP buttons can be wired in parallel (START) and in series (STOP) from any location.
• Interlocking: in a reversing starter, two contactors are electrically interlocked by NC auxiliary contacts so that both cannot close together.
• Star-delta and auto-transformer starters use two or three contactors with a timer.

Thus the contactor gives frequent switching, remote and automatic control, and with its overload relay and fuses, complete protection of the motor.