Regenerative VFD Fault Contribution Duration
Regenerative VFD Fault Contribution Duration
(OP)
We're trying to fine tune some short circuit calcs involving a large number of crane VFDs. These are all regenerative drives with dynamic braking so I know (or think I know) that there will be some contribution from these into a fault on the line side of the drive.
We've been assuming that the duration of the contribution would be similar to a motor running across the line, but now I'm wondering if the dc bus capacitors in the drive would keep the motor excited longer during a fault condition.
Any data on fault contribution magnitude and duration for a four-quadrant VFD?
Thanks,
Dave
We've been assuming that the duration of the contribution would be similar to a motor running across the line, but now I'm wondering if the dc bus capacitors in the drive would keep the motor excited longer during a fault condition.
Any data on fault contribution magnitude and duration for a four-quadrant VFD?
Thanks,
Dave





RE: Regenerative VFD Fault Contribution Duration
I presume that we are talking about active front end type regenerative drives rather than drives with a braking resistor.
The regenerative drive is able to continue to provide regeneration as the vars required for the stator magnetizing can be supplied by the capacitors and load. Some drives will adjust the rate of deceleration to ensure that sufficient energy is pulled out of the load to keep the bus voltage up when the input voltage is removed.
I would expect that the peak energy from the drives will be much less than direct connection, but the contribution would be for a longer period.
I do not have any figures or calculations, but I would suggest that you could asume that the maximum energy would be limited to around 150% of the motor rating (by the inverter operation) but the period of enegy flow could be dependent on the load inertia.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Regenerative VFD Fault Contribution Duration
This isn't really covered in the standard short circuit calculation procedures in ANSI and IEEE, at least that I'm aware of.
Thanks again.
Dave
RE: Regenerative VFD Fault Contribution Duration
RE: Regenerative VFD Fault Contribution Duration
RE: Regenerative VFD Fault Contribution Duration