Single phase motors

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Some things are difficult to get to grips with unless your an expert in that field, this is one case where difficult reins for me and I am sure it does for many others. Having virtually rebuilt my shaper with no problems at all I found the motor to be a right bugger. It had been wired incorrectly and as such I had to determine what why and how. Hopefully this may help others who suffer the same fate. The following information represents most of my weekend trying to sort this out, and even with the help of my good friend John, who looses me far too easily on electrics.

This represents the typical older motor with one capacitor sat on the outside, a capacitor is usually a cylindrical object and normally has two wires coming from it. Inside it is a brushless rotor, what they call a squirrel cage rotor, that is there is buried within it a copper cage which is joined at each end and runs through the rotor, there are no electrical connection to this from anywhere. The rotor carries a centrifugal switch which open circuits as the motor speeds up.

So onto the "what" What you should have is four tails (wires) coming from the stator, each pair are from a set of windings, these motors have two sets of windings, one being the start winding and the other the run winding. Now to identifying which is which. From all I have read the start winding is always of finer wire, which may not help as you cannot always see which tails are attached to which winding. But, apparently the start winding is "always" the higher resistence of the two, so if you have a meter measure them, whilst your at it it would be a good idea to check each against the casing with a boosted signal from your meter, if it does this. Because if you get any kind of a reading to earth its duff.

Now take a look at the drawing, (the numbering is mine and not typical of anything) The run winding is connected to the permanent supply on terminals 1 and 2, the start winding is connected parallel to it on terminals 4 and 5, but is in series with the capacitor. So with the motor switched off the centrifugal switch is closed and power "would" come in on T1 through the switch to T3 then via the capacitor to T5 through the start winding and back to T4 accross the strap and back to neutral. Once the motor is running and they quote about 75% of speed the centrifugal switch "should" open and power is terminated to both the capacitor and the start windings.

There is a few things to add, direction of drive is not guaranteed with AC current as it changes polarity, that's where the 50 cycles per second come in. So direction is acheived via the start windings, if it goes the wrong way swap over the tails to 4 and 5.

Well that's how it should work, of course things dont always go that easy, mine didnt. I found that the centrifugal switch was not closed when stationary, so when started it went which ever way it felt like and only then with a struggle. Looking at the switch on mine there was no excessive wear the shoes that rub on the rotor were in good order, so I measured how far out it was against the casing and adjusted it with a little bending, this must be done with a great deal of care, it is a delicate mechanism and must work correctly. I now had a switch I could prove was made at start, but now needed to prove it opened after run up, not so easy this one. You cant measure it running with a meter on volts as you get generated current fed back, you also cant measure accross it with a resistance meter or you will blow that up. In the end I elected to drive the motor from another source whilst all connections where off, this proved the switch to be working both running up and running down.

Well thats all folks, if you know better or have anything to add, please do get in touch.