VFD Isolation Transformers

[RAF7]

I have seen numerous applications of VFD's both with and without isolation transformers. I have put in many without them with no problems.

I have a new project where the client want's to use them. The easy way out is to just put them in, but are they REALLY needed?

What are your experiences related to this issue?


THANKS!

 

[Skogsgurra]

Why are you putting the transformer there? If voltage is OK and there is no RCD (aka GFI), then I see no reason to use a transformer.

Gunnar Englund

http://www.gke.org

 

[jraef]

In the past when a lot of AC drives were 6-step or current source design using GTO SCRs on the front end, it was necessary to use a "drive isolation transformer" to isolate the common mode noise created by switching the thyristors on and off. Modern VFDs and even most DC drives no longer use GTOs and that problem no longer exists. But the concept has carried through with some people, fostered I'm sure by transformer mfrs who are keen on having a continued market for their designs.

12 pulse (and 18/24 pulse) use transformers in front of them to shift the incoming phase angles as a way of mitigating harmonics, but that's not about isolation per se, it's about the phase angle.

http:/

http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[GordS]

Common mode problems increase with the newer drives due to electronic devices that switch a lot faster than the older style of drive (i=C dv/dt). In my opinion, if you are going to do an installation with a lot of electronic devices and drives, put in isolation transformers solely for common mode noise. In the last several years I have experienced problems twice due to this. In the first, we had 600V EHT on the same MCC as a drive. The 600V tracers were continually indicating ground fault. We moved the drives to a different transformer and the problem disappeared. Now I have a project where the designers put auto-transformers on the drive input. The electronic relays monitoring ground faults on the VFD feeders are continuously in alarm showing 50 - 150% of the supply transformer neutral resistor let thru current ( we like to set our ground fault detection at 50% of the let thru amps so we get lots of nuisance alarms)

 

[CJCPE]

VFD manufacturers rarely recommend drive isolation transformers and few specifiers require them. In the past, isolation transformers were sometimes used to protect the drive from line voltage transients and/or reduce the effects on the line from harmonics and line notching caused by the drive. In that case, the benefit is mostly due to the added impedance which is more economically provided by line reactors. In the NEMA Application Guide For AC Adjustable Speed Drive Systems, either line reactors or transformers are recommended for that type of protection. The NEMA Guide does state: "A ground common with electrical welding equipment or large current electrical equipment (5x rating of the control) should not be used. If either of these two conditions exist, use an isolation transformer sized for the installed control with a wye secondary neutral solidly grounded."

NEMA Application Guide for AC Adjustable Speed Drive Systems

http://www.nema.org/stds/acadjustable.cfm

(free download)

 

[Skogsgurra]

There are a few inverters that output a sine wave or near sine wave. Siemens, Danfoss, ABB and many more offer sine filters. Yaskawa has a multi-step output waveform that approximates a sinewave and NFO has a true sine wave.

Gunnar Englund

http://www.gke.org