DC Contactors

A DC contactors is an electrically controlled switch used for switching an electrical power circuit, similar to a relay except with higher current ratings. It is controlled by a circuit which has a much lower power level than the switched circuit.

It come in many forms with varying capacities and features. Unlike a circuit breaker, it is not intended to interrupt a short circuit current. Range from those having a breaking current of several amperes to thousands of amperes and 24 V DC to many kilovolts. The physical size ranges from a device small enough to pick up with one hand, to large devices approximately a meter (yard) on a side.

Contactors are used to control electric motors, lighting, heating, capacitor banks, thermal evaporators, and other electrical loads.

Operating principle

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, it almost exclusively fitted with normally open ("form A") contacts. Unlike relays, it designed with features to control and suppress the arc produced when interrupting heavy motor currents.

When current passes through the electromagnet, a magnetic field is produced, which attracts the moving core of it. 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 coil is de-energized, gravity or a spring returns the electromagnet core to its initial position and opens the contacts.

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.

Because arcing and consequent damage occurs just as the contacts are opening or closing, they are designed to open and close very rapidly; there is often an internal tipping point mechanism to ensure rapid action.