VFD Transformer sizing

4 replies [Last post]
sankaran03's picture
Joined: 2014-08-14


We propose to install two 12-pulse, AC VFD drives for two 690V, 3Ph AC motors of rating 500KW each. (Total load = 1,000KW). Both the VFDs are fed by a common 3 winding phase shifting transformer. The harmonics generation may be less because of 12 pulse configuration.


However, what is the margin to be given on the capacity of the transformer in order to offset the adverse effect of the harmonics?

MasseyTRuth's picture
Joined: 2014-08-18

The isolation transformer needs to be sized for continuous input amps of the VFD. The isolation transformer has current overload built in. There is no need to oversize the isolation transformer for the VFD overload. The isolation transformer is rated in KVA and not Amps. The 1st step is to convert amps to KVA.

sankaran03's picture
Joined: 2014-08-14

The major concern is the heat generated by the harmonics in the transformer winding by the VFD. I agree with you that the transformer has to be sized for the maximum current drawn by the VFD. But the current measured by the Ammeter is RMS value of the fundamental frequency,either 50HZ/60HZ.
That's why I wanted to know whether the transformer is to be oversized to offset the effects heat generation. If so,how much?

cja8's picture
Joined: 2012-09-13

You may want to consider a K-rated transformer if you are concerned with harmonics.

sofiakleeman's picture
Joined: 2014-09-02

If you want to change transformer size then you have to order for your comfortable size.

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