A contactor is a type of electrical equipment that allows the control of multiple electric circuits from a single switch. A contactor consists of a set of contacts, which are designed to survive multiple arcings that are opened and closed by an electromagnet.

:For example, let's assume that you are building a new school and would like to light the parking lot so as to keep the young 'uns from using the school parking lot for parking. The lot is pretty big, requiring forty light poles to do the job, each drawing 2 amps. Since the lights are going to be on all night, which is more than the two contiuous hours that makes a load rated at continuous duty, you can put only eight poles on a single circuit. Therefore, to light the lot, you need five circuits.

From your Square D catalog, you notice a six pole contactor that will do the job. Contactors are rated by power in amperes, current in volts and poles, which describes the number of circuits (or phases the contactor can control. From the electrical panel bring all five lighting circuits into the contactor and land them on the line side. You then pull the same five circutis from the load side of the contactor, and pull it out to your light poles.

To control the circuits tap one of the lgithing circuits and you will run a wire to some type of switch, be it a standard light switch, a timer, a photocell, or any combination thereof. Return the wire from the controlling mechanism(s) to one pole of an electromagnet. Ground the other pole. Once this is done, whenever the the controlling switch is closed, the magnet will be energized, and it will pull the contacts closed, energizing all five lighting circuits. Viola! Your parking lot is lit. Chastity is preserved.

Suppose that you want the switch to turn off the lights, say if another light is lit. Contactors come with reversible contacts. Usually the factory ships them in the normally open position. That means the circuit is off whenever the magnet is not engaged. But the contacts can be reversed to a normally closed position, which means that whenever your controlling switch is closed the lights go out.

Some contactors are mechanically locking, which means that a control signal must be sent for them to change state. Contactors are used as part of other electrical equipment, and in motor starters. For example, a three pole contactor is necessary to control a three phase motor. At the same time, a contactor is not a motor starter, which has other features that may include both thermal and overload protection.. A relay is really a single pole contactor.

Contactors are a very useful piece of electrical control equipment. They can be found in almost every commercial building, and in most three phase equipment. Without them, it would be very difficult to control any application requiring more than as single circuit.

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