# Single phase motor not working with a VFD/Inverter



## cts_casemod (Aug 23, 2012)

Hello again to all,

Okay so I am having a bit of a problem controlling a single phase AC compressor motor.

I had this problem with the Fuji first, now I am using my new inverter and the same happens. 

A single phase capacitor start motor, runs fine on 230VAC from the wall, but when powered by the VFD at 230V 50HZ either it doesnt start or starts weakly and dies. I see no reason for this to happen, I have other single phase motors running the same way and none has issues, they start just fine with a lower frequency (fridge compressor, Fans, etc.

I tried carrier frequencies from 2.5KHz to 20KHz and also tried to increase the start capacitor size, aceleration time, torque boost, nothing works!

Not really the right forum but thought someone might have an idea


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## Siwastaja (Aug 1, 2012)

Capacitor start or capacitor run is just a poor kludge, the only reason to use it is that it's simple. It's probably not going to work well outside the intended frequency or load. If you are doing a VFD anyway, make it a two-phase VFD and use the motor as a two-phase motor, which it is in reality. You will have MUCH better efficiency and torque because the second phase is exactly 90 degrees apart all the time which is impossible with a capacitor. Just note that the second phase is "weaker" so you need lower voltage for that, especially in capacitor-start motors.

I did this using a center-tapped transformer so that I could use only two half-bridges. From a single DC link, 2-phase needs two H bridges so it's more complex than 3-phase.


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