3 Phase to Single Phase Motor Conversion/Rewinding

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I've been giving some more thought to the 3-faze to single-faze motor conversion. It must be remembered the motor needs rewinding one way or another anyway.

Since with a static faze converter we're talking about a 20 to 33 % power reduction (depending where you look), I don't see that holding to the original 'optimized' motor design is doing much good.

So how to change it? Since nobody seems to have any ready-made formulas or guidelines, I've been forced to resort to thinking for myself. (Ugh!)

In 3-faze operation, voltage A to B = 230 VAC-RMS.

Voltage across each coil (A or B to the Y point) = 115 V / sine 120 degrees = 133 V

For single faze, again A to B = 230 V, but the Y point isn't properly driven by the third faze, so voltage is closer to 115 V across each coil instead of 133, 14% low.

So it makes sense to eliminate the Y point and use 14% fewer turns in the coils and have coils 'A' and 'B' together in parallel (magnetically/fysically in parallel; electrically in series) instead of at angles to each other but generating pretty much an overall parallel magnetic field anyway. I don't see that the rotor design should less optimum for this than having the static faze converter though I wouldn't swear to it.

66 original turns per faze * sine 120 degrees = 57 turns. Since I don't expect "faze C" to draw as much current I reduced that just a bit more to 55 turns, 9 per slot with the shortest coil having 10. So the now "main coil" (12 slot pairs of 18, combined "A" and "B") provides 5.66 HP of the original rated 7.5 - almost the total power or more than with the faze converter.

I have no idea what is optimum for "Faze C". I intend to wind it similar to how it was (ie in 6 slot pairs) with 55 turns - same wire as the others. I'll drive it by run capacitors - 100uF for starters - and see what the oscilloscope shows. Ideally it will be 90 degrees out of faze. With the full 240 V across it (in series with the capacitor), it should be driven pretty hard. But this time, there's no Y point and so there's only one place a cap(s) is needed. It only has to draw 7 amps and supply 1.84 of the HP at rated output load.

Thus, for the same 3450 RPM, we have 12 coils going one way for the main faze across the 240 volt line with no center point connection, and 6 coils in the remaining slots at 90 degrees is the capacitor driven coil.

The current in the main faze at rated output is then: 66 original turns / 55 new turns * 18 Amps (original rating) = 21.5 Amps (* 85.5%, the original rated efficiency, gives 5.66 HP), with the second capacitor driven faze accounting hopefully for the remaining 7 amps (gives 1.84 HP) of the 28.5 A total for the rated 7.5 HP.

As a fringe benefit, 6 of the coils now have 3" shorter overhangs for a total savings of 28' of doubled #15 wire, or 3/4 of a pound of copper. That leaves more breathing space - and cooling airflow - at the ends, which were pretty crammed before.

I've wound the coils, and I put in the first string of 6 today. I have a bit of room left in the slots - maybe one of the wires should have been #14 instead of two #15's. Oh well, I plan to put in an overheat tripout - it may (or may not) trip more than hoped for. I'm putting on aluminum heatsink fins on the exterior case.

I'm not sure about starting yet (would it be too much to hope that it might start with just the run capacitors?) There does appear to be a place for a centrifugal switch in the end bell.

I'll write with the results when I know them.

BTW the motor capacitor voltage ratings say "VAC", AC Volts, so they must take the peak voltages of the sine waves into account in the rating, unlike the DC caps I'm used to.

--Craig