air compressor problem (electrical)

[MMW]

My old air compressor does not start all the time. Always used to work. Electric, 240v. When you flip the switch to on it sometimes hums (won't turn over). If you turn it off & rotate the motor sometimes you can get it to start. I'm not sure if rotating the motor has anything to do with it or just the on/off action. Checked all wiring connections which are o/k. Maybe the capacitor? If so how do I check it & where would I get one. I am upgrading anyway but wanted to sell this one rather than scrap it.

[oldtimer]

first thing to do is to check your voltage coming in to the pressure switch next check the voltage leaving the switch at the wires going to the motor. if the voltage going to the motor is within spec then there is a very good cance that the rotary switch inside the motor is faulty. unless you are familiar with electric motors you should take the motor to a re-wind or motor repair shop. if it is the switch it may not be too expensive. if you have aproblem in the windings a new motor would probably be cheaper.
hope this helps
larry

[GT6Steve]

Capacitors dry out over time, sounds like yours has. Take it to the local motor repair shop and they'll sell you a new one for around ten bucks.

By spinning the motor to start it you're inducing the phase shift that the capacitor normally would.

Oldtimer's advice is sound, it could be the centrifugal switch but I'd suspect the cap first...

Steve the electrician...

[Mark Lindquist]

Mark Lindquist

Pump supply companys are a good resource for capacitors. You will need to know if their start or run caps as start are usually dry and run filled with fluid for continous operation. If This doesn't fix the problem and the centrifugal switch is ok then most likely the start winding has been comprimised.

[metal-doctor]

Had the same problem with our bansaw a few weeks ago. Turn the motor a bit & it would usually take off. Took motor apart & filed the points on the start switch & it hasn't missed a beat since. Check the simple things first before you assume the worst. Good luck with it & keep us posted on what you find.

[H80N]

Check the unloader...

Another possibility might be the compressor unloader.. if it sticks... the motor may not be able to start under load... so is possible that it is not electrical at all..
just a thought
Heiti

[monte55]

You got some good info here. Checking applied voltage is sometimes a misunderstood thing. You need to check voltage at the time the compressor is trying to start. You can get 240 volts through headphone wires but they can't carry the current to start a compressor. Most compressors use start capacitors as opposed to run caps. Start caps are cheap. Any HVAC supply will carry these. If the start switch is unavailable, it can be replaced with a solid state relay in conjunction with the existing capacitors

[hankj]

Didn't read through all of the posts, but did you consider a bad bearing in the pump or motor?

PS: Glad to see Heti is still around!

Hank

[MMW]

Thanks for all the replies. Going to pickup my new air compressor today so this will most likely get stuck in the corner for a little bit. I'll let ya'll know what I find. Thanks.