Air Compressor Problem - Motor Stalls

After reading all the posts, several things stand out. The most significant: There is no excessive current draw when the motor stalls. By all the laws of physics, that is flat out impossible. There are two explanations (well, three, but I'm assuming you aren't making this up). The first is slippage at the belt, motor pulley, or rotor. Given the knowledge you have exhibited, I say that can't be the problem; it's pretty hard to miss the fact that the motor is spinning but the belt is slipping.

That leads to only one conclusion: The actual voltage to the windings is dropping. Possible causes are a bad thernal overload protector, bad run capacitor (some motors hide the starting capacitor inside, while the run capacitor is in a cupola on the side), or a corroded terminal so only one of the paired windings is used.

All of these possible causes result in a lot of heat generated at one small point EXCEPT the single winding and the bad run capacitor. The single winding explanation is unlikely because it would not deteriorate gradually. That leaves the bad run capacitor.

PlainBill

There is one more possible cause: Some or all of the windings on the rotor that are supposed to be shorted are open. An induction motor works because windings on the rotor are shorted to induce the field needed to turn the rotor.

David

David Inscribed thus:

Thats a possibility, I've seen open rotor bars on induction motors. Sometimes they arc at the break and leave burn marks.

I've never seen that, but it is definitely a possibility and would also explain the symtoms.

PlainBill

PlainBill: I once saw a fractional HP induction motor on a fan had problems starting in a certain position. After moving the rotor a bit, it would run. There was one open winding on the rotor. The open was visible on the side of the rotor and applying a significant amount of solder cured the problem. There can be a lot of current in these shorted turns on a loaded motor.

David

With all the talk, I would have liked to hear the final answer to the story. Is the compressor working yet. Its been a long time.

greg

Um...can I phone a friend? :-)

Ok, here's the status. I have disassembled the motor. I have found the following:

• The windings appear to all be in good shape. I don't see anything obviously open or burned.

• The bearing surfaces are all in good shape.

• The contacts of the centrifugal switch are badly carbonized. I will clean those up before reassembly.

• I have ordered a replacement start capacitor. I don't have a capacitor tester, but for , why not just replace it, eh?

• The thermal overload tests OK (dead short,) but I can't easily test it under load.

• The rotor showed no evidence of hitting the stator and appeared undamaged.

I will clean all the connections before reassembly, although all looked fairly good already. I'll also measure to make sure I have good continuity through all the windings. Hopefully, that should get things going again. Thanks to everyone for their help and suggestions.

I'll post another update when I get it all back together.

Thanks!

Sounds good, please give more updates as you go along. Maybe it was the start switch contacts that you mention.

It's taken longer than I hoped, but I have finally put my air compressor motor back together and it's working again!

To review, over time, the compressor had developed a problem of the motor stalling before it reached full pressure.

Bearing in mind everyone's suggestions, I took the motor apart and found that the bearings looked good, there was no sign of rotor/stator contact, and the internal wiring and connections looked OK. The only obvious problem was somewhat burned contacts on the centrifugal switch.

Unfortunately, as I was taking it apart, I broke the voltage selector switch, and that's what's taken me so long to get around to fixing.

Finally, with some JB weld and spare circuit board material, I managed to glue the switch pieces back together.

I then cleaned and reseated all the internal connectors. I measured all the windings for continuity or shorts (they looked good.) I installed a new start cap, lubricated, and reassembled the motor.

It ran, but wouldn't start. After disassembling it again, and some careful bending of the centrifugal switch, it was starting and running smoothly.

Then, the moment of truth, I put the pulley and belt back on and fired it up. As I carefully watched the pressure gauge, I kept adjusting the regulator upwards. I ran it up to 140psi, and there was no sign of stalling or even slowing down! Success!

I then adjusted the regulator back down to 120psi and buttoned everything back up.

So, thanks to one and all for your help and suggestions. I'm back in business!

Am I missing something?

Yeah, you can yank the pulley to get the motor turning, then it runs. But it won't start turning on it's own.

Jeff

Just replaced a "run capacitor on my 5 H.P. Champion compressor that was having a similar issue...All is well now.

Yes!

This will very most likely be the problem. The motor is an AC induction motor, no brushes. But the run capacitor is critical to proper torque. And, if you are already in there and the happens to be a start-cap, do that one too.

Peter Wieck Melrose Park, PA