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Which winding is which on a 1/3HP single phase motor?

Andy Millar

Hi,

Showing my ignorance while trying to help a friend wire up the motor on his Myford lathe.

As far as I can tell the motor is single phase with an auxiliary winding, I believe it's no capacitor but centrifugal switch. One winding measures around 15 ohms, the other 6 ohms. Anyone know, or educated guesses, as to which winding is which? I've seen contradictory advice online.

It doesn't particularly matter, but might help while I'm gently testing it (for a start to check it does have a centrifugal switch) before applying full throttle.

I was probably taught all about this, but that was 40 years ago. I do know a lot more now than I did this time yesterday!

Thanks,

Andy

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    Depends how they do the phase shift for the start - if there is no capacitance then it probably relies on the inductance of the starter winding makes it lagging.  It is very likely to be the longer thinner (higher resistance) winding for the starter.

    As it interrupts current in a significant inductor, the centrifugal switch needs some sort of arc suppression, (usually a series R-C lump either in parallel with the contacts or in parallel with the winding being interrupted ) if the contacts are not to get pitted quite quickly. I have seen an inductor start motor like this, of about half a HP, where without the suppressor the starter arc was so violent that the plasma flash shorted the centrifugal switch to the chassis briefly, and fired both an RCD and a 32A breaker on more starts than it didn't. )  0.1uF and 100 ohms worked wonders...

    If there is any chance at all of something nasty happening, start it with a 1kW heater or similar in series to limit the current. And make sure the motor is bolted down - attempts to test run  motors free on the bench often result in the thing tearing itself off the test wires and rolling to the floor.

    apologies if this is stating the obvious.

    Mike.

  • 0 in reply to mapj1

    Many thanks both, and particularly good point Mike re suppression - I found a model engineers site where someone was describing exactly the sort of arcing you describe with a centrifugal switch. And good point about the heater in series. 

    When my friend drops the motor round to me I'll start with an insulation test and then gently explore, it's a very solidly made Metro-Vick motor which suits the Myford perfectly in all respects, so would be nice to keep with it possible - if the insulation's still up to it.  I only had a very quick look at it last night while we were solving a separate problem.

    (Separate problem was helping my friend debug a gorgeous ride on model Land Rover with the electrics and drivetrain of a mobility scooter that he'd scratch built for his grandson. It's nice to do some proper engineering sometimes as opposed to the day job of meetings and staring at documents!)

    Cheers,

    Andy

  • 0 in reply to mapj1

    Many thanks for this explanation Mike. It aligns with what I understand is intended rather than my (ill-)educated guesswork.

    Alasdair

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  • 0 in reply to mapj1

    Many thanks for this explanation Mike. It aligns with what I understand is intended rather than my (ill-)educated guesswork.

    Alasdair

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