Steve said:

A diagram please?

Thank you

The exact design depends on the winding ratios in the motor.

Capacitor run motors often see more voltage across the

capacitor driven winding than the full line voltage applied

to the main winding, because of resonant boost effect. But

you have to run the motor unloaded, and with the removed

(starting the motor with a spin) and measure the voltage

across that winding, to know what the optimum relative

winding voltages are.

The relative phase of the main and capacitor windings is

generally 90 degrees (but that can be measured with the

unloaded test, also). So the trick to run them with a 3

phase drive is to extract the two required voltage ratios at

the required 90 degree phase shift from the 3 120 degree

shifted outputs from the drive. Here is a phasor diagram

(where the length of the windings represent voltage and the

orientation represents relative phase shift) of how to use

an auto transformer to get 2 phases from 3.

Cs represent capacitor driven winding, Ms represents main

winding, Ws represent autotransformer winding:

* @ phase A

* C

* C

* C

* C

* C

* phase B +MMMMMMMM\

* @ | >@ phase C

* \WWWWWWW+WWWWWWWW/

When driven from a variable speed drive, the capacitor is

not needed, since the drive provides the 90 degree phase

shift between the two windings.

There are lots of other possible ways to derive arbitrary

ratios voltages with a relative phase difference of 90

degrees from a 3 phase source. This one is just the easiest

for me to draw with characters. Doodle a little and see

what you can come up with.