How contactor works?


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Old 07-28-10, 10:08 AM
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How contactor works?

I found this great tutorial on how a contactor work.

My question is, if the contactor is brand new and for some reason, the magnet does not pulls it in, what could be the problem assuming both sides do get a 24v?
 
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Old 07-28-10, 10:12 AM
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Either it's faulty, requires a higher voltage..or the 24V is not a complete circuit.

How are you measuring the voltage?
 
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Old 07-28-10, 10:20 AM
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Originally Posted by Gunguy45
Either it's faulty, requires a higher voltage..or the 24V is not a complete circuit.

How are you measuring the voltage?
The red wire goes to the positive probe and white wire goes to the negative probe on my multimeter. It measures a little bit over 25v. The strange thing is, the contactor does work for about 10 or so hours and now it does not pull in anymore.
 
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Old 07-28-10, 10:39 AM
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To test the wire when it's connecting to the contactor, can I just use the probe to poke the metal part where the wire touches the contactor or do I have to remove the wire off of the contact to test it?
 
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Old 07-28-10, 11:12 AM
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A contactor is simply a switch. It turns the voltage to the compressor and fan on and off. When low voltage is applied to the low voltage leads, it pulls the high voltage contacts together. When the low voltage is off, the high voltage contacts spring apart, shutting off the compressor. It is basically a specialized relay.

You can read the low voltage with the low voltage wires attached to the contactor. If you find voltage and the compressor stops while your thermostat is still calling for cold, disconnect one low voltage wire from the contactor and then check across the low voltage terminals of the contactor with your meter set to measure ohms. If you get no reading, the coil in the contactor is open. Replace the contactor.
 
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Old 07-28-10, 11:27 AM
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Originally Posted by goldstar
A contactor is simply a switch. It turns the voltage to the compressor and fan on and off. When low voltage is applied to the low voltage leads, it pulls the high voltage contacts together. When the low voltage is off, the high voltage contacts spring apart, shutting off the compressor. It is basically a specialized relay.

You can read the low voltage with the low voltage wires attached to the contactor. If you find voltage and the compressor stops while your thermostat is still calling for cold, disconnect one low voltage wire from the contactor and then check across the low voltage terminals of the contactor with your meter set to measure ohms. If you get no reading, the coil in the contactor is open. Replace the contactor.
How do I know which wire is the low voltage?
This is a similar contactor that I have.
 
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Old 07-28-10, 12:04 PM
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Originally Posted by goldstar
...disconnect one low voltage wire from the contactor and then check across the low voltage terminals of the contactor with your meter set to measure ohms.
Measuring ohms are the same as resistance, correct? So the greater the ohms ore resistance, the less continuity, correct?
 
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Old 07-28-10, 04:40 PM
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Some contactors have a 120 volt coil instead of 24. Make sure the new one is a 24V coil designed for HVAC applications. The specs should be on the contactor somewhere. If it indeed is a 24V contactor and it still wont pull in it could be a weak 24V transformer.
 
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Old 07-28-10, 06:04 PM
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I did not notice low voltage wires in your pic. They would be around 18 - 22 gauge - like door bell or transformer wiring.

Yes, ohms is the measure of resistance. In this case, you will either see some resistance, usually a low number, if the coil in the contactor is good. If you get infinite resistance the coil is open, as in a broken winding.
 
 

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