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I like to drive a small (150W) single phase induction motor by an existing three phase inverter by removing the capacitor and just connecting the two windings to the inverter in an incomplete triangle circuit.

I've done that with very small (15W) motors before, which run well, despite a little bit more noisey at low frequencys. I guess this is because the single phase motor uses a 90 degree offset between the main and the capacitor generated phase, while the inverter provides a 120 degree phase offset.

So I like to know what problems (despite the noise) may occur by this abuse if driving very low torque and speed in respect to the motor capabilities.

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  • \$\begingroup\$ Are you interested in the answer for theoretical reasons, or are you just trying to drive the motor? If you just want to drive the motor, I think it might be easier to use one of the 3 phase legs from your inverter to drive the single-phase motor without modification. Obviously, check the voltage first. Some larger motors only use the capacitor for starting. They actually have a relay that cuts out the capacitor once the motor speed exceeds some threshold. \$\endgroup\$ – mkeith Dec 2 '14 at 8:04
  • \$\begingroup\$ I like to drive the motor for real. It is an induction motor, that is permanently fed by the capacitor. It has two windings I guess. Driving the motor with the capacitor is only possible if it is driven in it's natural frequency (50Hz). I like to drive extreme low frequencys, eg. 1 Hz. At this point, the capacitor would not provide a helper phase any more, but something totally out-of-phase for the second winding. So I decided to drive the two windings without a capacitor. \$\endgroup\$ – dronus Dec 11 '14 at 11:29
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I believe you can use a standard wiring diagram from something like a Commander SK which anticipates what you want to do. I can't remember the circuit off the top of my head. I use them for three phase motors.

You can change the phase angle by changing the capacitor. I believe the torque will be very low.

I had to recently turn a single phase motor fan into a variable fan and we used a single phase chopper like is used to slow down a (series wound) drill. It worked fine and rattled a lot at low speed. That's all. The fan has a very low start up load and at low speed the magnetic field just slips a lot - no problem.

It works by "trying" to be synchronous but the voltage is not high enough so it slips and therefore slows down. You are far better off using the correct frequency so it tries to keep up, but can't because of low power, than to try to be synchronous and go at a low frequency/speed using a motor that is inherently unable to do that.

The only issue you will have with the former case is you may want to manually switch out the starter windings if it is running below the cut out speed. It might not be necessary. Just see if it starts to overheat.

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Inverters have a pulse width modulated, high frequency output coming from a dc bus which requires that the motor be able to withstand a higher breakdown voltage across windings and a higher insulation class. On this account, check the insulation class of the motor or if it may be 'inverter duty rated'before applying an inverter to it. Otherwise, the motor windings will break down and fail early.

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