Drive isolation transformer

[thinker]

We specify a lot of VFD systems for dynamometer applications.All of them are based on PWM regen front end converters, and transformers are needed for voltage matching. Drive vendors recommend (or supply) drive isolation transformers. Those transformers were developed to handle additional losses caused by current harmonics associated with 6-pulse conversion technology. Since PWM input converters have practically sinusoidal current waveform (K-factor is just not applicable), we try to consider a cost reduction of the system by not paying extra for "drive isolation transformer" and rather use general purpose transformers. The only additional feature we would like to have in general purpose transformers, is electrostatic shield. It is interesting to know an opinion of engineering community: could a general purpose transformer be used for PWM converters instead of drive iso transformer?

 

[CJCPE]

You are probably correct. A general purpose transformer should be adequate for a drive with an active front end that practically eliminates harmonic content from the input current. If the drive has a short-term overload rating that might be utilized, that would need to be considered. I believe that traditional drive isolation transformers are often designed to supply drives that have a 150% one minute overload rating. You might ask your drive suppliers what else might need to be considered and why.

 

[dpc]

There's no reason a standard transformer can't be used, as long as you understand the tradeoffs. You seem to be aware of the differences. You always have the option of buying a slightly larger kVA rating in a standard transformer to partially deal with the harmonic heating issues.

As long as you are meeting your harmonic distortion requirements with a standard transformer and it is adequately sized, I don't see a big problem. Marketing and sales people have given "K-rated" transformers magical properties that they don't really possess.

But you should have your FAQ list ready when the end-users start asking questions.