Compressor won't start.

[mike101650]

My Sears 120v compressor started tripping the 20 amp breaker recently. It has worked OK for years. It trips immediately when turned on. It will occasionally start. I removed the cover to get at the motor and capacitors and tried it before I opened the motor housing and it started. Turned it off and back on and problem is back. Once more later, it started again. I have no idea what I did to make it work. This has me scratching my head.

In checking the motor winding. There are three leads connected to what I will assume is a thermal device. They all read .1 to .2 ohm between each other and OL to motor case.

There are also 2 leads that connect to the capacitors. They also read OL to motor case.

I gather this stuff is pretty normal and OK.

The centrifugal switch that is normally operated (contact closed) and goes to open at speed works OK. I think it cuts out the start cap.

I have switched the 120v to an adjacent circuit to eliminate a defective breaker. I also have a second compressor and swapped the switches so switch assembly is OK except I am not sure what the tube going to the switch does. I think it is an over pressure protection and wouldn't be involved at this stage of operation.

Now for the question. I have an EXTECH 22-816 meter that has a CAP position on the dial. The book has limited info but does say to connect it to a capacitor and in a few seconds a reading will occur. There are two caps. Run cap is 45 uf. When I connect it I get a 45uf reading. The start cap is 400-480 uf. When I connect it, after a few seconds, I get an OL read. I have another cap that says 90uf and it reading is correct. The start cap tests OK (charges and discharges) with an OHM meter. I am wondering if I actually have a bad capacitor or have exceed the capability of my meter. I hate to buy stuff I don't need.

Any thoughts about this.

Thanks in advance for any help.

[kc8adu]

Re: Compressor won't start.

post pics.
mine did that and both caps were shot and the start contacts fouled where someone over oiled it and some got to the contacts.was cheap and easy to fix.

[T30M31]

Re: Compressor won't start.

The user's guide for the EXTECH 22-816 multimeter states that 200uF is the maximum capacitance it will measure so the 400-480uF capacitor is over ranging the meter.

[delaware74b]

Re: Compressor won't start.

Mike, I would check the condition of the start switch contacts. I think the start switch may not be fully closing or the contacts are pitted or burned. They can be gently cleaned (a new $1 bill works in a pinch). I don't think the start cap is the issue with your compressor.

What type of air compressor is it? (direct or belt drive, oil bath or oil-free compressor) Pics or full model number (XXX.XXXXXXXX) will help us to help you.

[T30M31]

Re: Compressor won't start.

You might check the unloader valve to make sure it is working correctly. If it isn't working the compressor will be starting against an increased load and will draw more current when starting. You can try releasing all of the air pressure from the compressor tank before starting the compressor. This should allow it to start with a minimum load. If it starts let it run until the pressure switch shuts it off. At that point you should here a short burst of air being released which is the unloader valve bleeding off the pressure in the discharge line.