AC contacter

[KingHVAC]

I have a package unit where the AC is being called for, but the compressor contactor wasn't pulling in. There was no voltage to pull in the contactor (24 v), but the thermostat was calling for it. I had voltage at the contactor and went to bump it to see if the compressor would run. When I pushed it in, it stayed pulled in and ran, and suddenly I had voltage (24v) to pull in the contactor!

Now I assume there must be an issue with one of the switches that the 24v goes through before it gets to the contactor to pull it in, but, I'm curious as to how the contactor stays pulled in and the 24v shows up upon pushing the contactor in? When I turned the power off and back on, the original situation returned. When I bumped the contactor again, it stayed running and had the 24v again.

I just want understand how the contactor stays pulled in and the 24v shows up?

 

[berkshire]

Look for an additional set of contacts on the contactor with 24 volts wired through.
B.E.

You are judged not by what you know, but by what you can do.

 

[urgross]

It would be typical to have an outside air damper end switch and/or proof-of-flow included in the control sequence. Have you checked the ladder diagram or any schematic provided with the unit?

 

[ScottyUK]

Google 'three wire motor starter' and look in images. There are two parallel paths - one for the start command and one for the latching contact which passes through a contactor auxiliary. My guess is the start command path has a high resistance or open point in it, but when you manually pushed in the contactor it latched through the other path.