<P> A basic contactor will have a coil input (which may be driven by either an AC or DC supply depending on the contactor design). The coil may be energized at the same voltage as a motor the contactor is controlling, or may be separately controlled with a lower coil voltage better suited to control by programmable controllers and lower - voltage pilot devices . Certain contactors have series coils connected in the motor circuit; these are used, for example, for automatic acceleration control, where the next stage of resistance is not cut out until the motor current has dropped . </P> <P> Unlike general - purpose relays, contactors are designed to be directly connected to high - current load devices . Relays tend to be of lower capacity and are usually designed for both normally closed and normally open applications . Devices switching more than 15 amperes or in circuits rated more than a few kilowatts are usually called contactors . Apart from optional auxiliary low - current contacts, contactors are almost exclusively fitted with normally open ("form A") contacts . Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents . </P> <P> When current passes through the electromagnet, a magnetic field is produced, which attracts the moving core of the contactor . The electromagnet coil draws more current initially, until its inductance increases when the metal core enters the coil . The moving contact is propelled by the moving core; the force developed by the electromagnet holds the moving and fixed contacts together . When the contactor coil is de-energized, gravity or a spring returns the electromagnet core to its initial position and opens the contacts . </P> <P> For contactors energized with alternating current, a small part of the core is surrounded with a shading coil, which slightly delays the magnetic flux in the core . The effect is to average out the alternating pull of the magnetic field and so prevent the core from buzzing at twice line frequency . </P>

What is the purpose of the auxiliary contacts on a motor starter