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Lack Of 1 Phase For Vsd - No Problem?


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Today I was trying a small 3ph 380V induction motor (I needed low rpm's) through a GE Vat20 - and it turned out that lacking 1 phase to the VSD the motor run normally. Surprisingly also, current drawn by all the motor's leads (taken by a clamp) when I restore the third phase to the VSD did not differ from what it was when the VSD received only 2 phases. What do you think, is it a (surprising for me) feature of all VSD's - to compensate in such a manner the lacking supply - or only small size drives are able to do so ? (In short - to make 3 ph out of 2ph)

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Hello Yuri


A VFD rectifies the incoming supply voltage converting it from AC to DC.

The DC is then chopped up and converted back to AC at the output of the inverter.


If you supply two phases to most modern VFDs, they are capable of running provided there is no protection to shut them down on input phase loss.


The output current is totally independent of the input and so there will be no difference between the output phases, but, the current on the remaining input phases will be higher than when three phases are present, putting more load on the rectifiers and the ripple voltage on the DC will be much higher putting more load on the capacitors.


You can run a three phase VFD off two phases (or a single phase supply of appropriate voltage) provided that you double the VFD size relative to the load. This is due to the extra stresses on the rectifier and capacitors.


Best regards,


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Hello and thanks Mark.


Another funny thing I saw once was a VFD (KEB) connected by some idiot wrongly (3 ph 380 V supply to its output, its input to a motor). It was bypassed by a stand-by scheme (which did not work) by a rotary multi-position hand switch -with most of power going to the motor through the switch (some 30 or 40 A) and 1A flowing through the drive connected in the above desribed idiotic way. Indication was absolutely normal! It shown not a little sign of protest against that "connection"!

Interested, I at once switched off the motor, disconnected power to at least reconnect the drive as it should be (and to see what would be?) - nothing had changed after that in its indication and everything: it shown the same 1A, speed, Hz, etc.

So, i do not maintain that I saw a vice-versely connected drive powering motor, but that 3ph 380Vv supply was on its otput - without any damage done to the drive!


Best regards.

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