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Power 3phase Motor With Single Phase And Cap


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this is a cct to start a three phase motor with a single phase supply and using a capacitor...

to charge the cap with 220V AC at 50 HZ, which is my supply, I have done the following calculations...


C = 1 / 2 * PI * F * XC

VC = 220 V

XC = 220 / 2 * PI * F

C = 4.55 mF


is all this correct? I just want to know how to calculate the capacitor value actually...


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Hello Switch_639


I am not in favour of this technique, therefore I am unable to provide you with the answers that you want.

The three phase motor needs to be supplied with three phases at 120 degree offset. This techniques does not doe this, but does provide a phase shifted voltage on the third phase to ensure that the motor does start in one direction.

The performance of the motor is severely compromised by this circuit and you would need to seriously derate the motor to prevent stalling and damage.

If you wish to operate a three phase motor from a single phase supply, I would recommend a single phase input three phase output VFD.


Best regards,


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I often have those monophase/ fase 3 phase motors come in and have always wondered how they can be efficient with uneven phase angles. They are normally burnt from overloading. When detailed all 3 phases are identical in turns and csa just like an ordinary 3PH motor, only have a 16mic 400V cap. I think they sacrifice effeciency with the ease of producing them as single and three phase. A good temporary trick if your stuck in the middle of nowhere with a single phase supply a three phase motor and a very unhappy farmer!!, but I wouldnt recommend them.
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