Influence of VFD on short circuit current

[MikiBg]

Hi all!

IEC defines how to estimate a influence of motor to short circuit current, but it says nothing about how motor is supplied. What about motor driven by a VFD, is it need to be taken into account?
I'm asking because of often case I'm dealing with: 3 motors via VFD and one DOL, all four on same bus. So, it is a big difference should I calculate with four or just one motor for short circuit current contribution estimation.

I'll appreciate your answers.

 

[ScottyUK]

The VFD effectively separates the motor from the system so far as fault contribution is concerned. A regenerative drive might contribute a little to the fault but only up to rated current of the drive.



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If we learn from our mistakes I'm getting a great education!

 

[MikiBg]

Scotty,
I suppose by "regenerative drive" you think on VFD that allows regenerative braking, i.e. a flow of energy from motor back to grid via VFD? If so, is it a special sort of VFD that needs to be specified by order, since I guess that VFD's hardware must be reversible then, or is it standard function that you can start/choose on "ordinary" VFD?

 

[itsmoked]

Yes since it's sort of two VFDs back to back you need to hunt one down if you want that feature. They probably cost about %50 more than the basic ones.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Like the man says...

Regen capability is unusual on small drives, more common on big ones and it's inherent in things like current source LCI drives unless the controls are specifically configured to prevent regeneration. You won't find many of them in the sub-MW power range.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

Fully regenerative AC drives cost roughly almost twice what a non-regen drive does, and yes, they are not basic off-the-shelf products. ABB makes them as low as 7.5HP (5.5kW) by the way.

But as far as contribution to a fault, no, it would not. A motor contributes to a fault because when the upstream fault occurs, the motor field has not collapsed and the driven load can cause momentary regeneration. A Regenerative VFD must be commanded to regenerate, even if it is an automatic function based on the DC bus voltage. Nothing about an up stream fault will trigger the regen function..


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