Electrical Gurus: Motor run capacitor (induction)
 

Got a 0.45hp bench grinder thats stopped working (was running, then just lost torque, slowed to a halt and now just hums).

Its a induction motor rated at 2.4amps, 0.45hp, single phase, 2950rpm, cos 0.96

I reckon the run capactor has gone (it only has one capacitor; no switch start or start cap etc). So I figured to try out a new one.

BUT

On the can it says "8MF" Now what the hell is MF? (capital M )

Microfarads(uF)?
Millifards( mF)?
Megafarads (probably not as it'd be the size of a house :D )?

I'm sure there is a way to work out the run cap rating based on horsepower to confirm the correct size, but I'm too rusty to work it out. :o

Any help would be appreciated. :cool:

TBH,

factor in :-

Costs of

Searching for part

Finding & procuring part

dismantling item

fitting part

rebuilding

Testing

And then price up new grinder,

It may well be cheaper to buy a new grinder, given there low price these days...


It could also be one of the windings has gone open circuit, and no end of cap replacement would fix that puppy

Sounds like its stuck in the transition point from one winding to the next, hence the humming


Mart

http://uk.farnell.com/multicomp/8mf-...tud/dp/1692738

Looks like 8MF is a designator, not a rating. As you say, 8 megafarads would be a hefty item!

Capacitors rated in Farads

u. equals micro (millionth of a farad)

M equals milli (thousandth of a farad)

you can check if it is the Cap -- Just spin the grinding wheel (carefully), if it runs Ok = change Cap

8Mf is quite a common value (e.g. tumble dryers etc)

Not an issue it's a 30 year old Wolf; a modern one would cost £300 and last a tenth as long as its Chinese made rubbish. The capacitor is in the base; two screws. And it's induction so no brushes so the loss of torque is too much 'slip' between half-phases in the power supply. Bearings are good and windings don't appear to be shorted or open circuit. So worth throwing £5 at it on a new cap, which appears to be leaking.



Right so capital M is the same as lower case m?

That's my issue as my learning from A-levels was milli farads was mF spelt with a lower case m not upper case as I see on the casing of the cap. IYSWIM.

It was running when it died. If it was a start cap I'd agree. But this is a running cap. Currently it just turns very slowly.

Aren't tumble dryer motors typically brushed motors, not pure induction (brushless)?

Sorry to be pedantic, but if I'm ordering a cap to try, I just want to make sure it's the right one ( milli or micro)

"...Aren't tumble dryer motors typically brushed motors, not pure induction (brushless)?..."

No - they use start/run cap. when i go to a machine that is just 'buzzing', i just give the drum a push - if the machine then starts running, I know that the problem is the cap

I stand corrected :o I've just took apart the dryer to rob the cap and test it in the grinder tomorrow. It's rated 7.5uF so should be close enough ( assuming MF = uF ). :cool:

good luck ?!?!!!!

Result! :D

Cap was fubared.

Now I just have to contend with the verbal abuse about the tumble dryer not working ;):o

why do u need a tumble dryer ?? -- its summer and will not rain until .................................................. ...... tomorrow ;)

:lol1:

Thanks for the help :thumb: