Isolation Transformers in VFDs

[NickParker]

1. Why three winding transformers are used in VFDs typically? why not two winding transformer?
2. Why the isolation transformers used in VFDs are not grounded (usually delta-delta)? Would there be any substantial ground fault current for the phase to ground fault at motor? Does the fault current will return to source or VFD?
3. Why these transformers are called as "Isolation transformers"?

 

[jraef]

I have never seen a delta-delta drive isolation transformer, and it makes no sense. The primary purpose of a drive isolation transformer is for when you HAVE a delta service and you want to use VFDs, which are typically NOT designed for delta supplies. So the drive isolation transformer is a 1:1 ratio, but is delta primary and wye secondary, with the wye center point (X0) grounded, because that is how most VFDs are built, especially those designed and made for Europe and Asia where they don't have delta systems.

Way back in the day when DC drives were still common, you often had a transformer ahead of the DC drive because you needed a voltage change on the input, i.e. 480V supply, 240V DC drive. In the old SCR based DC drives, delta or wye was irrelevant, the SCRs were robust enough to take it. What you may have seen was an old DC drive isolation transformer ahead of a VFD that had replaced the DC drive. If that were the case, someone didn't read the fine print...

AC VFDs are all designed based on the L-G voltage reference being 58% of the L-L voltage. SOME of them, designed BY and FOR the North American market where we still have delta system, will allow connection to a delta source, but require alteration to remove all of the internal ground reference points inside, for things like MOVs and Common Mode Capacitors. If you don't remove that ground reference, any ground fault on the line side attempts to flow through those components and they vaporize, usually causing collateral damage.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ScottyUK]

Typically a 3-winding transformer for a drive has a delta primary and both a star and delta secondary to provide 30° of phase displacement between secondaries to allow use of a 12-pulse rectifier. This arrangement is typical on medium / large drives. Very large drives may have more complex transformers with zigzag (interstar) windings with smaller phase shifts to allow higher pulse-count rectifiers to be used.

 

[dpc]

1. As Scotty says, generally three-winding transformer is used to take advantage of the phase shift between a delta and wye winding to reduce supply side harmonics - 12-pulae drive.

2. Agree with Jeff - if two-winding transformer, the drive side is generally grounded wye. For three-winding xfmr, one winding will be delta and the other grounded wye.

3. The buffering impedance of the transformer reduces the harmonics seen on the supply side. It is an "isolation" transformer in the sense that there is galvanic isolation if it is a true two- (or three-) winding transformer. But in terms of harmonic "isolation", a series reactor of equivalent impedance can be just as effective as a two-winding transformer.