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Contactor 101

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JWWebster:
Magnetic relays commonly known as definite purpose contactors are used in a variety of different applications throughout the HVAC and Appliance world. They can have anywhere  from 1 pole or up to 4 poles or more. The coils on them can be 24 volt 120 volt 240 volt or even 440 volt and better. Their main job is to power compressors and fan motors on various equipment. On a home unit the contactor is almost always a 24 volt 2 pole 30 amp one. Some smaller units have what is known as a pole and a half. or 1.5 poles. The coil pulls down only one side of the contactor, while the common leg has a straight bar across it. On home units smaller thermostat wires connect to the 24 volt coil. When the temp rises the thermostat calls for the contactor to yank down and power up the compressor and fan outside. if those small wires are chewed up by an animal or a weed eater then power to the coil cannot
do its job. Sometimes the coil simply dies like a light bulb. If the coil is open then nothing will happen when the thermostat calls for cooling. Sometimes the contacts on the contactor get burned and fall apart. The coil yanks down the contacts fine but sadly the points are fried. Sometimes a critter will make a home right under the contacts. When the weather turns warm the coil yanks down and you have crispy critter and no cooling either. Oh the inside will work but nothing will happen outside until the contactor is replaced. 240 volts will kill you. Make sure to kill the power to outside unit before servicing any part.

Icehouse:
Not trying to steal your show JW, but:

On most single-phase compressors with a single-pole contactor, there is a solid bar on one side of the contactor instead of having two contacts. One leg of power passes down the bar to the capacitor and to the run winding. It passes through the run winding of the compressor and from common back to the contactor. It lands on the load side of the normally open contact of the contactor. At off cycle, line voltage will be read from one side of the normally open contact to the other: Line 1 on the line side and Line 2 on the load side via the windings.
The crankcase heater will be attached to the contactor with one leg on the load side and one on the line side of the normally open contacts. At off cycle it will have line voltage applied to it and will heat the crankcase. Once the contactor is energized, both the line and load side of the normally open contacts become one. Since you cannot feed a load with one line of power, the crankcase heater will not work again until the contactor opens. This is a simple way to turn the crankcase heater off during the on cycle and to turn it on at the off cycle. Exercise caution when checking this type of setup because power is always present on the load side of the contactor.

JWWebster:
Nice post Bobby. I might get ya to be a contributing editor to my world class cheap ass Sublime appliance repair google site.

Icehouse:
 O0 O0 :) Thanks but between AJ,Jake,Scotty and on the "Educational Committee" for hvac-talk.com my bill is full.

shieldcracker:
Very nice AJ, did you really write this?  ::)

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