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Old 11-22-2008, 04:01 PM   #1
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Air Compressor Motor Hums, Trips Breaker


Hey all:

I am new to the forum so first let me thank you for looking at this thread and sharing your knowledge.

Photo 1: The cracked 6" fan blade with 5/8" shaft diameter
Photo 2: Photo of Air Compressor Motor Specs
Photo 3: Gray Motor Start Capacitor, 300uf, 125V AC
Photo 3: Black Motor Run Capacitor, 30uF, 250VAC


I have a PUMA Air Compressor that I got 2nd hand for free. I am posting pics of the specs, etc. The motor is wired for 120v and draws about 17 amps. When I attempt to start the motor, it "hums" and then trips a 20A breaker after 2 or 3 seconds. I can manually "trip" the motor into starting by using a non-conductive tool or rod like a pencil to pressing on the contact point under the fan blade.

I have searched a few threads and believe I may have a bad starting capacitor but would like your inputs as well.

Would a bad motor start capacitor exhibit these symptoms?

Also, I broke off the fan blade (PICTURE #1) when I was trying to tighten the set screw that keeps it on the motor shaft. Do you know where I can find a new one? It is 6" in diameter and has a 5/8" output shaft diameter. Thanks!
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Old 11-24-2008, 09:42 AM   #2
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Air Compressor Motor Hums, Trips Breaker


bump to the top

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Old 11-24-2008, 10:01 AM   #3
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Air Compressor Motor Hums, Trips Breaker


I don't know about motor problem but you can probably throw a hose clamp around the fan hub and tighten it down lightly.
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Old 11-24-2008, 10:54 AM   #4
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Air Compressor Motor Hums, Trips Breaker


Every time I've ever seen a start capacitor blown, it leaked watery juicy-wax all over itself. Anything like that going on? If the capacitor is bad nothing will happen when you turn the machine on.

The motor may be drawing 20 amps or more at startup or when starting under load. That would explain the breaker tripping.
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Old 11-24-2008, 12:48 PM   #5
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Air Compressor Motor Hums, Trips Breaker


Quote:
Originally Posted by poindexternasty View Post
When I attempt to start the motor, it "hums" and then trips a 20A breaker after 2 or 3 seconds.
Would a bad motor start capacitor exhibit these symptoms?
If you have a VOM, some of these,
http://images.google.com/imgres?imgu...%3D2%26hl%3Den
a 1MΩ resistor and a 9v battery you can check the cap.

If you can get the trip curves for your 20A breaker I think you can check that too, but for a non-spinning motor I think the breaker is acting like it should.
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Old 11-24-2008, 09:03 PM   #6
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Air Compressor Motor Hums, Trips Breaker


There are several things that can go wrong with a single-phase motor like this.

First, a bit of theory, sort of. This motor is called a single-phase capacitor start capacitor run induction motor.

Single phase, because it operates on single phase AC power, as opposed to 3 phase AC power, or DC power.

Capacitor start. This is where it gets complicated. There are two separate windings inside this motor. A start winding, and a run winding. All motors turn because there is a rotating magnetic field generated in the stator (the stationary part that has the windings) which causes the rotor (the part that turns), to rotate.

In a single phase motor, this rotating magnetic field turns both ways. It causes the rotor to turn in whatever direction it's already turning. If the rotor is not turning, it doesn't know which way to begin turning, so it just sits there. This is why a start winding is needed. This start winding needs to be slightly skewed from the run winding, so as to cause the magnetic field to turn only one direction. This can be accomplished by mechanically skewing the location of the start winding as it is installed in the stator, or by using a capacitor to electrically skew the winding.

Once the motor is at about 1/2 speed, the start winding must be disconnected, or it will fight against the run winding. This can be accomplished in one of several different ways, in this motor there is a centrifugal switch installed in the end bell opposite the shaft extension. It has weights and springs that move an electrical contact on or off.

Now, for the real stuff! If the breaker trips instantly, and a few other breakers do the same, the windings are almost certainly burned up. They can be re-wound at a motor shop, but on a small one like this it's usually not worth the cost.

If the motor hums, and the breaker trips after a few seconds, (this sounds like yours) and the shaft will turn freely by hand, usually the problem is that the start winding is not energized. Usually, there are two reasons for this. Either the start capacitor is bad, or the start switch is bad. My experience runs about 50/50. If you can spin the shaft by hand, then energize the motor and it starts, you've proved that the start winding is not engaged.

Fixing it might not be so easy. If you can take the end bell off (the non-shaft end), you can see the centrifugal switch inside. If you can see the contacts, you can see if they're closed or not, or better yet, use an ohmmeter to measure resistance across them. It should read less than 1 ohm. If this looks good, and you have no easy way to test the capacitor, I'd replace it.

Capacitors are about $10 or so. The run capacitor is probably OK. If it's not, the motor will start, but not develop much power.

When replacing a capacitor, the uF rating must be the same, and the voltage rating must be the same or higher, but not lower.

Rob
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Old 11-28-2008, 04:28 PM   #7
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Air Compressor Motor Hums, Trips Breaker


Thank you very much to the contributors to this thread.

I replaced the start capacitor and the motor still hums. It has not yet tripped the breaker because I shut it off after a second or two. So here's the UPDATE: The switch that kicks the motor on, the one close to the shaft, is failing to make contact with the other contact point when the motor is switched on. Attached are some photos of the switch in question. The switch contact points are located in the yellow box.



I can take a non-conductor and nudge the switch to close the circuit and then the motor will run just fine. So it seams that I need to replace/repair this switch. Is it possible to repair or must it be replaced?
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Old 11-29-2008, 11:23 PM   #8
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Air Compressor Motor Hums, Trips Breaker


A lot of times a shot of WD-40 will do the trick.

It can be quite difficult to get parts for start switches unless the motor is still in production. Even then, it still can be tough.

Rob
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Old 11-30-2008, 03:30 PM   #9
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Air Compressor Motor Hums, Trips Breaker


What force causes the switch to close the circuit? Is it centrifugal motion or magnetism?

Can I clean this switch to fix it?
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Old 12-01-2008, 02:24 PM   #10
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Air Compressor Motor Hums, Trips Breaker


bump to the top,

Can I clean a centrifugal start switch? Repair?
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Old 12-02-2008, 10:20 PM   #11
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Air Compressor Motor Hums, Trips Breaker


When a start switch is at rest, the springs hold the contacts together. When it is rotating, the centrifugal force of the weights overpowers the springs and causes the contacts to open. The springs are not terribly strong, you should be able to push the weights outward with your fingers. They sort of 'snap' in and out.

The switch assembly certainly can be cleaned, and must operate freely. It looks like yours might be a bit rusty, maybe even stuck. If you clean it up, and lube it a bit, it'll likely work again.

Rob

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