240VAC motors

[Cursitor Doom]

Hi all,

So I have this bench grinder (one of the ones with two wheels of differing coarseness on either end of a spindle, the motor being in the middle). Anyway, it packed up one day. Wouldn't spin by itself even with a prod; just made a humming noise. I suspected a duff capacitor which turned out to be correct. This grinder uses a 450V

2.5uF one and finding a spare online turned out to be simple enough, if not all that cheap at a fiver. Swapped out the duff cap for the new one and it worked fine again. Then today it packed up again, same failure mode as before. I whipped the bottom off and checked the new cap expecting it to have gone phut but it was fine, much to my surprise. So I'm stumped. I can't see any other caps in evidence, either. Any idea what's going on here?

[Roger Hayter]

This type of capacitor mostly fails with no external signs. They can be tested with the right equipment, but if not available the simplest thing is to try another new one. About the only other possibilities are the switch, a wire broken or fallen off or an internal break/short in the motor winding. The last one is probably a terminal event for a fairly cheap machine.

[newshound]

Thermal fuse within the windings playing up?

[Cursitor Doom]

I don't think there is one, Newsy. Not in this particular model anyway. And AFAIK they only tend to fail after the device has been running for a while.

[Cursitor Doom]

Tested it with one of these:

There's still a discernable humming coming from it when switched on, so we can rule out the switch and a break in the motor winding as possible causes at any rate.

[Roger Hayter]

Possibly still needs testing for insulation breakdown. Either a Megger or a

4-500V power supply plus 47k-100k resistor in series and a multimeter on 500 or more volt range, plus *eye protection* from possible drama.

[Fredxx]

I've read your posts but can't see anywhere where you've checked continuity of the 2 windings that should have a common connection.

You will still get a hum if only one functions, or where there is a short circuit turn.

Can you measure these? Also what is the voltage across the cap when switched on?

Does the motor get hot very quickly?

[Cursitor Doom]

I'm not at all familiar with the internals of those motors and didn't even know they have 2 windings.

Yes, I can see what you're getting at.

I can measure the voltage across the cap no problem and I *could* measure the windings continuity and resistance if I knew which wires in the rat's nest underneath go where. Sadly I have no info to assist, however.

I should imagine even a good motor that's on but not turning will get hot pretty quick. Motors really aren't my thing, obviously. These crappy Chinese pieces of junk (sourced from B&Q in this instance) aren't worth spending much investigative time on anyway.

[Brian Gaff (Sofa]

Yes the modern cap simply loses its capacitance over time and then there is no phase changed output on that bit of the motor and it sulks. Older ones tended to either fry, emit magic smoke or scare everyone with a bang, bark worse than bite in most cases.

I think, I'd possibly try to find one with a slightly higher working voltage this time as they have to survive on a much dirtier mains than they used to. Brian

[Brian Gaff (Sofa]

Yes and often that would these days be relatively rare, How old is it? Brian

[Chris Green]

Lots of not very expensive multimeters now have capacitance ranges and they will quickly show you if the capacitor is dead or not. A quick resistance check as well will show whether it's leaky.

[Chris Green]

If it's only leaky then it's still a capacitor and the motor should start OK I would have thought.

[Roger Hayter]

snip

Leakiness may be very voltage dependent.

[Roger Hayter]

If it is very leaky it may not achieve sufficient phase shift.

[Chris Green]

Why? It would have to be very, very leaky to have much effect surely.

[Chris Green]

May be. But I've not come across a capacitor that is leaky which doesn't show some measurable resistance using a reasonably good multimeter, it'll be higher than it shows with a large voltage but it will still be leaky.

[Roger Hayter]

Back of an envelope calculation suggests it would have to dissipate about 100W at a phase angle of 45deg, so you are probably right!

[Cursitor Doom]

About 5 years, but it's not done much work!

[Cursitor Doom]

I've checked it with a specific meter that tests for all that and ESR to boot. The old one was definitely blown; the new one is still fine (and it's rated at 450VAC).

[Fredxx]

A 2 split/two phase motor will generally have 2 windings. One is across the mains, and the other will be through your capacitor.

Unless it's fitted with a no-volts contactor the wiring should be simple.

A 150W motor will sit an hum for a minute without overheating the windings.

These motors are incredibly simple. Sometimes it's quicker to fix a fault than to survey the market and buy a new one.

If you see 0V across the capacitor then the auxiliary winding is O/C.

If you see 240V then the auxiliary winding probably has a shorted turn.

I think in normal working you would expect to see greater than mains voltage across the capacitor BICBW.