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3-phase drive operation from 2-phase supply.


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There are a few issues that must be looked at:

1. The current passing through the rectifier (diodes or SCR's) will be increased by root 3, in comparison to the three phase bridge.

2. You will have an increase in DC ripple voltage.

3. The average DC bus voltage will be reduced significantly.



1. De-rate the drive by 50% ie. if you have a 10 amp motor, use a 20 amp VSD (you really only need 17.3 amps, but better to be safe than sorry)

2. Add either a DC link choke, or extra bus capacitors. Extra capacitance is more suitable as this will also increase the bus voltage, where the choke will have a voltage drop. Some manufacturers sell an extra capacitor that connects to the external bus terminals.

3. Check to see if the VSD has a phase loss fault - if this can not be defeated, forget all of the above!!


I have done this many times.

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Can anyone advise (in detail) the implications of running a 3-phase variable speed drive from a 2 -phase supply source.

Yes the rectifiers will carry motr current, the ripple voltage will be much higher and the ripple current into the capacitors will also be much higher. This will reduce the life expectancy of the rectifier and the capacitors.

The frequency of the ripple voltage with a three phase input is 300 Hz and with two phase, is 100Hz (assuming 50Hz supply) For the same time constant, you would need three times the capacitance when only two phases used.

Best regards,

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  • 6 years later...

It is probably reasonable to assume that the question is really about connecting a three phase drive to a single phase supply and the replys are valid.  Connecting a drive to a real two phase supply might lead to some issues.  A pair of two phase leads would be in quadrature and probably provide unsatisfactory voltage.  If the two phase supply is two phases with an enlarged neutral, one diode pair would see 1.4 the phase current, but might work fine, if the neutral is not grounded.  If it is a four wire source, only one phase could be utilized without adding additional rectification external to the drive. probably not a good idea for a first try.


It is probably a single phase service.



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If you measure the voltage across two phases (without neutral) you are effectively measuring a single phase supply at a higher voltage.

Commonly, if there are two hot legs, (both above neutral) this is referred to as two phase control.

Technically, it is single phase control but displaced from neutral.


480 volt two phase (single phase doubled) is not uncommon in some rural areas in NZ and Aus and we use three phase input drives oversized for this application.


Best regards,


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