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Discussion Starter · #1 ·
Hey guys, hoping you guys can help me out. I went to look at my dad's friends ac unit, she told me that the breaker kept on tripping when she would turn the unit on, so I go to the panel, flip the breaker, hear a buzzing and it trips about 2 seconds after being energized. So I first think, bad breaker (older panel) so I remove the breaker, look at the bus, everything looks fine.

So I reinstall the breaker and remove the wires to the AC, flip the breaker, everything is fine, so I turn my focus to the unit itself, I check the start and run capacitors, both read what they should (37.5 & 57.5 micro farads) so I eliminate those, then i turn to the compressor motor, I read for resistance, and my numbers are all over the place, from 5000 ohms to zero than to OL.

So I came to the conclusion that the compressor motor is locked, which in turn, is drawing a large current thus tripping the breaker. So I would like to know If I'm on the right path here? Or completely out to lunch lol any suggestions would be greatly appreciated

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You can ohm out the compressor by C to S . C to R . Then R to S . C to R should be the lowest winding . C to S should be 2 to 3 times higher . R to S should be the sum of the first two readings . This will check the compressor windings but will not tell if its locked down . The lowest winding should be below 10 ohms so 5000 sounds like bad windings .
 

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I think you are on the right path. The wacky ohm readings could be trying to measure windings to ground or maybe you still had the cap hooked up. Check the readings that dthurmond mentioned but the final test if the windings are reading ok would be to amp the compressor or disconnect it completely.
 

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That's correct

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Didn't you state you removed the wires from the breaker, not the unit itself ?
I would check the wires to the relay in the unit. Probably a waste of time, because there really isn't any reason for the house wiring to fail , but you never know.
I would also check the 2 pole relay just to be sure.
Of course, if the AC unit is really old, it's a good possibility the compressor is toast.
I agree, it looks like you are reading the caps in the unit.
dthurmond, is on the right track. Just like checking out the windings on a submersible pump!
 

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I believe the breaker doesn't trip until there is a call for cooling so that basically checks the mains up to the compressor and fan contactors.
 

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Discussion Starter · #10 ·
I think you are on the right path. The wacky ohm readings could be trying to measure windings to ground or maybe you still had the cap hooked up. Check the readings that dthurmond mentioned but the final test if the windings are reading ok would be to amp the compressor or disconnect it completely.
I had removed the caps prior to testing the windings

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Hey guys, hoping you guys can help me out. I went to look at my dad's friends ac unit, she told me that the breaker kept on tripping when she would turn the unit on, so I go to the panel, flip the breaker, hear a buzzing and it trips about 2 seconds after being energized. So I first think, bad breaker (older panel) so I remove the breaker, look at the bus, everything looks fine.

So I reinstall the breaker and remove the wires to the AC, flip the breaker, everything is fine, so I turn my focus to the unit itself, I check the start and run capacitors, both read what they should (37.5 & 57.5 micro farads) so I eliminate those, then i turn to the compressor motor, I read for resistance, and my numbers are all over the place, from 5000 ohms to zero than to OL.

So I came to the conclusion that the compressor motor is locked, which in turn, is drawing a large current thus tripping the breaker. So I would like to know If I'm on the right path here? Or completely out to lunch lol any suggestions would be greatly appreciated

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If the cap checked good...I would try a new one anyway, then try a "hard start" kit. If all else fails and you decide to change out the compressor anyway try this; Place a two by four up against the compressor and give it a good wack when someone is calling for it to run via the t-stat. What do you have to lose?
 

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If it has a start capacitor I presume it has a start relay. If the points are bad on the relay, the start cap won't be in the system. Easiest way to test is to put your amp meter on the start capacitor wire and have someone momentarily turn the breaker on. Usually a locked rotor won't trip the breaker unless there's an issue with the windings as the thermal overload generally cuts out. Put your meter on the Megaohm setting and check the resistance between C R and S to the frame of the compressor(with the wires disconnected from the terminal fence). I presume you checked the fan motor for shorts? Kind of odd to have those sizes of caps together but anything is possible. If you can find the manufactures data for the compressor, it should tell you what the ohms should read between the windings.​
 

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Sometimes the compressor will actually be starting, but the start relay won't drop the start cap out and that will usually trip the main. You can always disconnect the hard start(they're usually not needed anyway) and see what happens
 

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I only do very basic AC troubleshooting.

Tripping?

On a clients equipment:

Pull the disconnect, turn on the breaker and (if it holds) test the circuit for proper voltage. Turn on AC and (if it holds long enough) read amperage.

Visually inspect breaker, bus, disconnect and terminations.

If I determine that the problem is in the unit, I have them call an AC guy.



On my own equipment I will chase down the component that is tripping the breaker by disconnecting the compressor and fan motor(s) one at a time.
 

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I had removed the caps prior to testing the windings

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Did you ohm the windings at the compressor terminals or through the wires going to the compressor? If at the terminals you have a bad compressor. If through the wires, check the terminals for burnt wire(s). If so repair wire(s). If not you have a bad compressor.
 

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Discussion Starter · #18 ·
I only do very basic AC troubleshooting.

Tripping?

On a clients equipment:

Pull the disconnect, turn on the breaker and (if it holds) test the circuit for proper voltage. Turn on AC and (if it holds long enough) read amperage.

Visually inspect breaker, bus, disconnect and terminations.

If I determine that the problem is in the unit, I have them call an AC guy.



On my own equipment I will chase down the component that is tripping the breaker by disconnecting the compressor and fan motor(s) one at a time.
Ya that's what I did, I looked at ask the electrical and eliminated that, then told her to get a AC guy down there to take a look

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