Drive transformer protection

[thinker]

We use VFDs from various vendors, and noticed that some of them (but not all) supply drive isolation transformers equipped with winding thermal switches which are incorporated into drive protection system. It is clear that this is a good idea. However, the question is whether this is just a good design practice, or there is specific requirement in standards or regulations (NEMA or UL) which would stipulate this protective feature as mandatory, so we could force our drive vendors to have it installed as a default?

 

[DickDV]

The answer, as usual, is----it depends!

Very large drive/motor systems tend to have input isolation transformers to limit harmonics passing back into the power source.

Drive/motor systems operating in plants with floating delta, high resistance grounded wye, or corner grounded power networks will need an isolation transformer to re-establish a grounded wye source. This will permit the ground fault protection in the drive to work properly.

Occasionally, drive isolation transformers are used to correct source voltage problems, either too high or too low, thru the use of +/- taps.

Other than the above, it is usually unneccessarily expensive to specify isolation transformers. Harmonic suppression can easily be handled thru the use of reactors or filters.

DC drives, on the other hand, tend to work better with a transformer upstream and are often specified that way. A transformer would always be required with a floating or high resistance source.

 

[thinker]

DickDV, thanks for reply. But I did not ask whether to use drive isolation transformer or not to use. My question was about winding thermostat usage in drive protection system..

 

[dpc]

This may be a UL requirement above a certain size kVA rating for a dry-type transformer.

For smaller transformers (<500 kVA), we never specified temp sensors in the transformer. I would tend to leave that decision to the drive supplier. Since the captive transformer will be seeing a very well defined maximum load, I would think the transformer should be designed in such a way that there is small risk of overheating (assuming ambient temperature is OK). On the other hand, I don't see much wrong with having a temperature switch, other than having one more component that can fail and shut down the system.

You still have to meet NEC requirements for transformer overcurrent protection.