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 Post subject: Back feed from DC drives
PostPosted: Mon Feb 07, 2011 9:18 pm 

Joined: Fri Oct 30, 2009 9:20 am
Posts: 47
Location: Texas
I understand that VFDs without any bypass do not allow back feed to the electrical system in the event of a fault, but what about DC drives? Are you esentially trying to run diodes in the reverse direction so everything is blocked? It would seem that any back feed from the motor would be heavily dependent on the rate of decay of the field current. I was also wondering if DC drives have any filtering capacitors on the field that would sustain that current longer than decay of the armature during a fault. We have four 150HP motors being operated on Eurotherm DC drives. The drives are fed from a 480V MCC. I appreciate any advice you may have. I don't know much about DC drives.


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PostPosted: Tue Feb 08, 2011 12:26 pm 

Joined: Wed Nov 12, 2008 10:26 am
Posts: 46
Location: CA
DC Regen Drives

The only way a 480vac to DC drive could contribute to an AC fault is if it is a 4 Quadrant regenerative drive. Regen drives take DC and synchronously convert the DC power back into AC power source.

Since converter drives usually base their operation on AC voltage zero crossings, it is doubtful that during an arcing fault that the drive would be able to correctly identify the 60Hz zero crossings among the other line noise from the arcing fault. The manufacturer should be able to help you with this.

You are right to comment about the field supply. However many DC motors use permanent magnet (PM) fields and thus would generate voltage regardless of field supplies and capacitors. From your description it is not clear whether these drives would continue to operate or would be shutting down as the AF occurs.

It could be modeled as a decaying synchronous motor or generator.


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PostPosted: Tue Feb 08, 2011 5:01 pm 

Joined: Wed Jun 24, 2009 8:47 am
Posts: 25
Location: Canfield, OH
Zero Seq is correct that 4 quadrant drives can send fault current back onto the line. It is very common in the paper or metals industry to have a drive operating in regen mode in order to provide web tension. Winders and bridles in particular may be in regen mode as a coil progresses since the idea is to control web tension and not necessarily constant torque or current. You may want to model these drives as a generator to see what happens to fault current if they operate in regen mode as part of normal operation.


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PostPosted: Wed Feb 09, 2011 7:24 pm 

Joined: Fri Oct 30, 2009 9:20 am
Posts: 47
Location: Texas
Thank you both for your help. We do have regenative drives. They are serving motors driving extruders, and I have now been informed that an extruder is a really dumb application for a regenative drive. Our drives will probably never be in regen because the speed is set to a fixed value, and KW of the motor is controlled by adding water to the process thereby making the extrusion take less energy.


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PostPosted: Wed Mar 02, 2011 12:35 pm 

Joined: Wed Nov 12, 2008 10:26 am
Posts: 46
Location: CA
Regen drives are often selected over non-regen drives because they have quicker speed control. If the load suddenly drops on a non-regen drive the motor slows down only due to mechanical load. In a regen drive if the load suddenly drops the motor is fed reverse current to bring the speed down much quicker.


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