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

Hi Andy,

Similarly for me it is about 40 years since I looked at single phase motors but dredging my memory there should be a main winding and a secondary winding with a different inductance/capacitance to create the rotating field to get the motor rotating. I suspect the main winding is the lower impedance (I am guessing you have just measured the dc resistance of the windings but I would still expect it to be the lower reading) and the higher impedance will be the auxiliary winding. 

Please remember this is in the educated guess category rather than the knowledge category. Best of luck with the testing.

Alasdair

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.

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

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

Alasdair

From the distant past .......... since it is centrifugal start, it's probably designed as a "split phase motor" with the centrifugal switch in circuit only to start the motor winding. This switch will be opened or "thrown out", when the motor is at run speed, then the motor continues running. Again if memory serves right. if the run winding is connected to the mains and the shaft twisted, the motor will run up to speed and continue to run correctly.

Jaymack    

True it only needs one winding to keep running once spinning- though you can start it the wrong way as well - in the days before health and safety being quite so prevalent I have started a few motors with sticky starter switches or failed starter caps when they were stalled and buzzing, by "pulling on the belt" but it is just as easy to get it spinning backwards which may not be desirable.

Mike. 

Just had a very pleasant Saturday afternoon getting the motor and reversing switch tested and running - wound it up on the variac and it ran beautifully with a satisfying gentle clunk as the centrifugal switch dropped out. Thanks all!

Whew! I had a brief thought that perhaps you may have found a motor where the start winding was actually the lower Z for some reason and were complaining that it had set fire to your bench or something. Very  glad to hear that it panned out OK and has no faults.

Now do you need to add DC braking ?- beware if you do, my first attempt at this stopped the lathe OK, but so suddenly  that the chuck carried on turning and unscrewed from the shaft and fell onto the lathe bed.  (a 24V DC supply of many amps capacity limited only by winding resistances  is clearly too much.)

M.

Hi Mike,

No, I think DC braking would be much too high tech for us  My friend's very happy with it as is.  I have a similar problem with my (otherwise very much loved) Hitachi cordless drill - it stops so suddenly it tends to release the drill bit from the chuck, which is a bit daft.

As ever, our single biggest problem yesterday was that even between us we still didn't have quite enough of the ideal size ring crimps, another Sod's law that how ever many crimps I have I always have one too few of the size I actually need...

Today's task is repairing my daughter's partner's valve amp, which is much more in my comfort zone! And if I get time starting to repair the legs of a harpsichord, which may be a bit more of a challenge...

Cheers,

Andy