Induction motor question

May 01, 2010 5 Replies
P

I have a 350W induction motor which tends to blow up when starting. There is a large flash and bang from about where the centrifugal switch is and the fuses blow.



If the fuses are replaced it just starts and runs as normal until the next time it decides to blow up. Which is fine except the bang is disconcerting and off-putting, it's expensive in fuses, and fuses are not always available to hand.



I'm thinking perhaps the start capacitor is shorted? Would that make sense?



Does the actual value of the stat capacitor matter much?



At present it's marked 150 uF 250 VAC. I don't have one that value but I might be able to find one of a different, probably larger, value.


-- Peter Fairbrother


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R

Seems to me you have a problem with the centrifugal switch sometimes shorting to ground. I would strip the motor and take a look. If the capacitor was shorted why would you get a bang and flash, then ok on the next run?

I had a similar problem once on a grain elevator on a farm (about 30 years ago) but that one intermitently took out the ELCB. I called in the Brook Crompton man who diagnosed the problem. I gained "The little Red Book" (Installation and Maintenance of Electric Motors) which I still have

Richard

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H

Peter Fairbrother expressed precisely :

Once a capacitor goes bad, they tend to stay bad. So your problem would not then be intermitant.

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P

Thanks, that was it. I epoxied some of the film you get on ready meals which go in the oven over the relevant parts, and it seems fixed.

-- Peter Fairbrother

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M

You are a very sad person

Regards Mark Rand RTFM

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P

Yes - I need a 1mm thick 6mm (or 1/4") diameter sharp carbide slotting saw for copper, and I can't find one :(

Also I need some bolts to keep the top of my press closed - it's 95mm dia and 150 mm high, and it needs 78 tons of force to keep it closed. :(

But I won't cry ...

-- Peter Fairbrother

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