Contactor

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A contactor is an electrical device used for controlling power flow. A contactor is activated by a control input which is a lower voltage / current than that which the contactor is switching. Contactors come in many forms with varying capacities and features.

Contactors range from having a breaking current of several amps and 110 volts to thousands of amps and many kilovolts. There are highly specialised contactors which are as large as a small car and those which are almost subminiature.

[edit] Construction

A contactor is composed of three different systems. The contact system is the current carrying part of the contactor. This includes Power contacts, Auxiliary contacts and contact springs. The electromagnet system is a driving unit of contact system. Force for closing of contacts is provided by this system. And the enclosure system, which is an external frame housing the contact and the electromagnet made of insulating materials like Bakelite, Nylon 6, and thermosetting plastics to constitute a protective structure, adequate insulation, protection to the components within, protection to personnel coming in contact & protection against dust and ingress of vermin.

Contactors are commonly fitted with overload protection to prevent damage to their loads. When an overload is detected the contactor is tripped removing power downstream from the contactor.

Some contactors are motor driven rather than relay driven and extremely high voltage contactors often have arc suppression systems fitted (such as an inert gas surrounding the contactor).

Magnetic Blowouts are sometimes used to increase the amount of current a contactor can successfully break. The field produced by the magnets in proximity to the contact forces the arc produced while breaking current to flow through the field which is curved and a greater distance than the straight path between the contacts. The magnetic blowouts in the pictured Albright contactor more than double the current it can break from 600 Amps to 1500 Amps.

Sometimes an Economizer circuit is also installed to reduce the power required to keep a contactor closed. A somewhat greater amount of power is required to initially close a contactor than is required to keep it closed thereafter. Such a circuit can save a substantial amount of power and allow the energized coil to stay cooler.

A basic contactor will have a coil input (which may be driven by either an AC or DC supply depending on the contactor design) and generally a minimum of two poles which are controlled.

[edit] Operating Principle

A contactor is similar to, but different from an electrical relay. Although commonly both use a magnetic coil to draw a contact closed, a contactor differs in that it is designed to "break" a high current load - although relays are designed for switching loads on or off as well they tend to be of much lower capacity and are usually designed for both Normally Closed and Normally Open applications. Apart from optional auxiliary low current contacts, a contactor normally only has Normally Open contacts fitted.

Electrical energy in the form of an electric current is converted into mechanical energy in the form of a magnetic field. When there is no current passing through the electromagnet, it is not magnetic, however when current is passing through it becomes magnetic and thus able to attract ferrous objects, in this case the moving core to the stationary core. While starting, coil draws more current than holding period as moving core has to overcome the initial inertia.

[edit] External Links

• http://www.repcoinc.com/e-contacts_fld/e-contacts.asp - resource for locating contactors by oem
• http://www.qsl.net/ki7cx/Coilrewind.htm - Instructions on rewinding contactor coils for different voltages.