finding the poles in 4 pole single phase motor?

Possibly. A few permanent split-phase capacitor motors are designed for two- or three-speed operation. Is there any useful information on the data plate?

Chris

The answer is yes and no. Yes, you could identify the poles as wound with an ohmmeter if you could get to them - but no, you probably can't reconnect it because the individual pole windings are not brought out to a convenient point such as the junction box. In most cases to do this would be tantamount to complete rewind - a truly daunting task for the uninitiated.

What differentiates one pole from another is which way the wire is wound around the coil. You would have to find the place where the winding switches direction, and break the wire at that point and reconnect. It would require a number of connection changes. As a 4-pole winding, the polarity changes 4 times going around the full stator. As a 2-pole winding, it would have to change only twice around the circle. Both the main winding and the phase shifted winding would have to be rewired this way. When done, you would end up with twice as many turns per pole as you have now. That would make the stator iron pretty happy with the magnetic situation, but as the rotor field would be moving twice as fast through twice as many turns per pole, I'm not sure how that would work out. (Something tells me it will all balance out, as the rotor field will equilibrate at 1/2 the strength, and you'd get half the torque at twice the speed.)

Jon

This is true if the motor is designed as a single speed, four pole machine, but I think Eric is referring to a machine which is designed to run at two speeds. If he has a motor like this, it should be possible to change the speed without even opening the case.

Chris

No. It's a Baldor motor. Two voltage capability. But nothing about different speeds. ERS

Jon-I didn't think about the torque drop. Of course it would have to drop. Otherwise the motor would change from a 1/2 hp to 1 hp. Maybe I should just leave it alone. Eric

I'm pretty sure you get full torque at (about) double speed. Same stator windings = same flux in the same rotor. The rotor is moving through half the number of lines of flux twice as fast. Based on the thought of a motor being a transformer with a rotating secondary, the rotor currents should be twice nominal with no load, and the rotor voltage twice nominal at full (double the nameplate HP) load. If that last part is backwards, someone please correct me.

John

Eric, you can't change the speed of that grinder. Sorry. - GWE

Because of the way the windings are distributed in the stator slots (multiple coils spanning a 90 deg segment) there is no way of reconnecting the coils to produce an efficient new distribution spanning the 180 deg segment needed for a 2 pole winding.

A complete rewind is required using fewer turns of thicker wire and, if operated at the same flux density will produce the same torque at twice the speed - i.e. twice the HP. Various second order effects chip away at this power increase but an 80% power increase is entirely possible.

Jim

The dual voltage capability will be the reason for the two identical main windings. They will be connected in parallel for 115 V operation or in series for 230 V operation. They aren't intended for speed control. I think the only way you can vary the speed of this motor is to change the supply frequency.

Hope this helps,

Chris

And not very much, with a single phase cap start motor -- unless you are willing to switch the start capacitor value each time you change the frequency.

Enjoy, DoN.