Contactor

Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents.

When the contactor coil is de-energized, gravity or a spring returns the electromagnet core to its initial position and opens the contacts.

The effect is to average out the alternating pull of the magnetic field and so prevent the core from buzzing at twice line frequency.

Rapid closing can, however, lead to increase contact bounce which causes additional unwanted open-close cycles.

[5] The heat developed by the resulting electrical arc is very high, ultimately causing the metal on the contact to migrate with the current.

The extremely high temperature of the arc (tens of thousands of degrees Celsius) cracks the surrounding gas molecules creating ozone, carbon monoxide, and other compounds.

The arc energy slowly destroys the contact metal, causing some material to escape into the air as fine particulate matter.

High voltage DC contactors (greater than 600 V) still rely on air within specially designed arc-chutes to break the arc energy.

North American and European ratings for contactors follow different philosophies, with North American general purpose machine tool contactors generally emphasizing simplicity of application while definite purpose and European rating philosophy emphasizes design for the intended life cycle of the application.

The horsepower ratings are based on voltage and on typical induction motor characteristics and duty cycle as stated in NEMA standard ICS2.

Exceptional duty cycles or specialized motor types may require a different NEMA starter size than the nominal rating.

Manufacturer's literature is used to guide selection for non-motor loads, for example, incandescent lighting or power factor correction capacitors.

Vacuum contactors are also widely used at medium voltages from 1000 to 5000 volts, effectively displacing oil-filled circuit breakers in many applications.

Vacuum contactors are therefore very efficient at disrupting the energy of an electric arc and are used when relatively fast switching is required, as the maximum break time is determined by the periodicity of the AC waveform.

Contactors usually have provision for installation of additional contact blocks, rated for pilot duty, used in motor control circuits.