On Fri, 27 May 2005 12:28:37 -0400, JohnM wrote:

Eric R Snow wrote:
On Thu, 26 May 2005 17:46:01 -0500, Jon Elson
wrote:



Eric R Snow wrote:


I have this grinder that runs at 1725 rpm so it should have 4 poles.
Shouldn't I be able to tell which wires are for which poles by
checking the resistance? And If that works, can it be re-connected as
a two pole machine and still work at 115 volts? This is a capacitor
start/run machine with two identical windings and it uses two
capacitors to provide the phase shift. I don't know if the caps are
connected in parallel but it seems they should be as the motor only
runs in one direction. No starting windings in this motor, no
centrifigul switches.



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

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


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