Re: High Voltage Input Inverters

[Lee Hart <leeahart@earthlink.net>]

Jim Phillips wrote:
> is the second winding on a capacitor-run motor REALLY just an
> identical second phase?

It depends :-)

Yes, it is mechanically a true 2-phase motor. The capacitor winding is
displaced 90 degrees rotationally from the main winding.

If the motor is built to be run in either direction, then the two
windings are identical. The two windings and capacitor are all connected
in series (in a ring). The common connection between windings goes to AC
neutral. To run clockwise, switch AC hot to the first winding (and the
capacitor powers the second winding). To run counterclockwise, switch AC
hot to the second winding (and the capacitor powers the first winding).

If the motor is built to run in one direction (like a compressor motor),
then it is more complicated. The motor's torque-speed characteristics
are determined by the capacitor value and how many turns are on the
capacitor winding. In general, the two windings will have different
numbers of turns, and run at different currents.

This only complicates our life a little in driving such a motor with a
2-phase inverter. You can still use a 6-transistor H-bridge, wired just
like a 3-phase H-bridge, but with only 2 motor windings instead of 3.
The transistors need to be driven to produce a 2-phase signal on the 2
windings. By independently controlling their duty cycles, you can apply
different voltages to each of the two windings to mimic their
performance with single-phase AC. 
-- 
Lee A. Hart                     Ring the bells that you can ring
4209 France Ave. N.             Forget the perfect offering
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leeahart@earthlink.net                  Leonard Cohen