VFD & Fault Currents

[mike0w]

Through some reading, and other investigation, I was informed that if a VFD is on a motor the fault current back to the bus will be zero contribution from the motor. Is this accurate? Where could I find more information to support this and on short circuit studies. Thanks in advance.

 

[ron]

This is true since the VFD essential performs a double conversion. It is not true if there is a maintenance bypass, which means that it is an across the line starter, which would be the worst case for your study.

 

[msb10]

I used to believe exactly as Ron indicated; that is, that the only way a VFD can contribute to an up-stream fault is if it is bypassed. Unfortunately, I notice in the IEEE Red Book that, at least in the opinion of the editors, these drives can "under certain conditions, contribute current from the motors to a short circuit on the incoming ac electric power system." The only condition that I can think of (that might contribute current to the source) is one where the drive includes regenerative braking. I've never seen one of these in an HVAC application. There may well be other ways that I can't think of.

The Red Book goes on to recommend that that the "manufacturer should be consulted for information on whether adjustable speed ac induction or dc motors can contribute backfeed current ..." Good luck with that.

For my 2 cents, it's worth looking into with the manufacturer, particularly with a larger drive or with something exotic, like a DC drive on a rolling mill. But in most applications, I'm guessing the contribution will be zero.

If you're interested, and have a copy of the Red Book, it is paragraph 4.2.5.

Matt

 

[petersonra]

For my 2 cents, it's worth looking into with the manufacturer, particularly with a larger drive or with something exotic, like a DC drive on a rolling mill. But in most applications, I'm guessing the contribution will be zero.

If you're interested, and have a copy of the Red Book, it is paragraph 4.2.5.

Matt

A VFD is different from a DC drive. VFDs are not commonly regenerative due to the cost. Four quad DC drives are regenerative by their very nature.