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Help with electric motor fault diagnosis? :¬(

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Deadeye

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I took the motor off to clean and re-paint it. Probably a mistake in retrospect :(

I also replaced the wiring (plug to switch; switch to motor).

On re-mounting it trips my circuit breaker when switched on. The arbor starts to turn and then the trip goes.
I didn't take the housing off, but suspect some dust/scrapings went in from sanding the outside (no paint; I covered the ends for that).
The fact that there's some movement gives me a glimmer of hope, although that's perhaps just further foolishness.
The motor is a 1/4HP 240V single phase that runs c. 1500 rev/min.
Suggestions? Where to start? Have I buggered it?
 

Pete Maddex

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It could be the run cap, its usually bolted to the outside of the motor.
Whip it out and look for signs of melting, bulging and smelling.
Replacements are avalable on eBay.

Pete

Pete
 

Deadeye

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Hmm. Nothing bolted to the outside and no obvious capacitor inside either. I'll take some photos
 

Myfordman

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When you say trips the breaker do you mean the RCD (earthleakage) or the MCB (over current)?

There is a world of difference.
 

Myfordman

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B0l0cks. I just typed a long reply and lost it grrr!

That is a combined RCD and MCB - horrid things!

Insulate the motor body and clamp to a bench. take off the earth wire and apply power. DO NOT TOUCH THE MOTOR BODY!
If it still trips then you have an over current problem
if it runs fine, then you have an earth leakage problem.
THIS WAS ONLY A TEST AND MUST NEVER BE USED AS A SOLUTION. Replace the earth wire.

Over current could be debris preventing the centrifugal switch from working either mechanically or electrically. It does not indicate a short as the trip would be instant that you turned it on.

Earth leakage could also be debris but between the terminals and the motor body. Would really need proper measurement equipment to diagnose.

Somethign to bear in mind - no motor or machine works any better for a coat of paint
 

Deadeye

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Thanks Myfordman.
I have a basic ohm meter that I can run between the terminals to see coil resistance...but don't really know whichpairs or how to interpret the results.
I will go and try the test you describe. If I don't report back, I failed at the insulate and don't touch instructions...
 

Deadeye

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OK! Survived that. Earth reattached (or rather all wires removed)
Still trips... so overcurrent? It still just gets the pulley in motion and then cuts.
Where do I look now? I have access to a vaccum cleaner, an air-blower and a ohm-meter.
 

Myfordman

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Normally it would need to get up to say 1000rpm for the switch to open.
If the breaker is tripping after literally a fraction of a turn, then it sounds like a short circuit.
Just to be clear, you have not been inside the motor? Are you 100% confident that the wires and brass straps are back exactly as they were before?
 

Deadeye

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I've only removed the little panel to get at the electrics in the photo. I haven't touched the brown and black wires coming from inside - just changed the external feed like for like (took photos).
 

Deadeye

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Incidentally - I'm truly appreciative of your help here. I feel like a mug for causing the issue! I guess that's how beginners learn.
 

shed9

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Are your connections solid and tight?

Are you wiring this with an undisturbed path to the mains, i.e. your not going through an adapter or mains extension, etc?
 

Myfordman

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Well done for taking photos. I still do even though I'm supposed to know what I'm doing after years of tinkering with motors.
It's quite a low power motor so I'd guess at either winding being 25-50 ohms, so with the centrifugal switch closed as it should be for starting, the two windings will be in parallel and no capacitor used, the dc resistance should be in the region of 12 to 25 ohms.
Certainly no lower.
 

Deadeye

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Hi
Yes connections seem sound - I've bypassed the switch for this, so straight to a plug; I'm using the plug switch to test. It's next to the main consumer board. In the cellar.
Resistance is 32-35 ohms between any combination that's not directly linked
 

Myfordman

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There is something very strange here. With that resistance across the mains the current is around 8 amps and should not be capable of tripping a 20amp breaker.
Measure the resistances at the plug pins. L-N , L-E and N-E
 

porker

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I know it's not always helpful to throw in random suggestions especially as your already getting advice from the resident expert but I had a similar problem that turned out to be the RCBO had failed so might be worth swapping this with another in your consumer unit.
 
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