Grounding Regen Resistors Enclosure

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david.mullins

david.mullins

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Santa Barbara, CA, USA
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Electrical Engineer
We are building a large motor control system. It has 8 motors in an enclosure and they can run in regen mode for extended periods of time. The motors are servo controlled by listed AC Brushless motor controllers. They have outputs for regen current when the motors, acting as generators, drive the controller internal supply bus above about 650V and can go as high as 1000VDC. We are mounting the regen resistors (load resistors) in a well ventilated metal enclosure mounted on top of the metal controller enclosure, which is about 7 feet tall. There will be two leads for each resistor, + and -, neither of which is grounded. The resistors are in metal tubes and sealed with potting compound. In order to minimize cabling, we will wire from the lower controller enclosure to the top regen resistor enclosure with paired cables, hopefully 2 or 4 pair per cable.

In this configuration, can I run a single ground wire from the lower enclosure to the upper one or do I have to have a grounding conductor in each cable?

P.S. I know the statement of the problem can be the biggest source of contention on these forums so I have tried to carefully word it. Please ask me to clarify anything you have questions about!
 
Realistically, the mounting hardware is going to affectively bond the resistor cages to the enclosure, as another poster mentioned. I don't think that I would use cables though. It will get fairly hot in there and most cable insulation is not intended for that kind of warmth.

My personal opinion is that by running a separate ground wire, or more than one, you are bringing into play the question of whether the ground wire needs to be run in the cable or not as required by article 300 I think. If there is no ground wire run then it's not an issue.
 
david.mullins said:
We prefer not to. It is semi portable equipment and gets hoisted by cranes into place and bolted together. We are generally on a limited time budget so cables and MIL marine grade connectors are required.
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Then I would think a single, large, external, EGC with its own MIL connector would be a good match.
 
petersonra said:
I don't think that I would use cables though. It will get fairly hot in there and most cable insulation is not intended for that kind of warmth.
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The vendor of the resistors makes them with a connection box which houses thermostats and terminal blocks. Most marine cable is rated for 90°C or greater and should be ok in that box.

petersonra said:
My personal opinion is that by running a separate ground wire, or more than one, you are bringing into play the question of whether the ground wire needs to be run in the cable or not as required by article 300 I think. If there is no ground wire run then it's not an issue.
Click to expand...
Not all cables are required to have an EGC, such as type TC, 336.2.

As a side note, 1000V (not 600/1000 dual rated) cable is not easy to find. We actually have more options at 2000V or 3000V. Then, when we add in LSZH and sun/oil/water rated...
 
LarryFine said:
Then I would think a single, large, external, EGC with its own MIL connector would be a good match.
Click to expand...

I wonder if it would be wise to include a control circuit looping through such a connector that would prevent the drives from powering up unless this connector is attached. That would be for safety reasons to make sure that the EGC connection is present.
 
synchro said:
I wonder if it would be wise to include a control circuit looping through such a connector that would prevent the drives from powering up unless this connector is attached. That would be for safety reasons to make sure that the EGC connection is present.
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I would care more to make sure the connector is in place
 
david.mullins said:
We are building a large motor control system. It has 8 motors in an enclosure and they can run in regen mode for extended periods of time. The motors are servo controlled by listed AC Brushless motor controllers. They have outputs for regen current when the motors, acting as generators, drive the controller internal supply bus above about 650V and can go as high as 1000VDC. We are mounting the regen resistors (load resistors) in a well ventilated metal enclosure mounted on top of the metal controller enclosure, which is about 7 feet tall. There will be two leads for each resistor, + and -, neither of which is grounded. The resistors are in metal tubes and sealed with potting compound. In order to minimize cabling, we will wire from the lower controller enclosure to the top regen resistor enclosure with paired cables, hopefully 2 or 4 pair per cable.

In this configuration, can I run a single ground wire from the lower enclosure to the upper one or do I have to have a grounding conductor in each cable?

P.S. I know the statement of the problem can be the biggest source of contention on these forums so I have tried to carefully word it. Please ask me to clarify anything you have questions about!
Click to expand...
Sounds as if there is a remote unit that is temporary or portable, it would seem to me if there is EGC between the motors and the resistor box (metal?) that is bonded as required, a singular EGC sized to largest load would be permissible. It would to me also seem there should be in place a mechanism (not just policy or instructions) that would require the EGC be "first connect - last disconnect" so that it would be in place as long as there is current carrying conductors connected, or each independent cable would have to have an EGC integrated to guarentee grounding of equipment. Also the metal tubes that you are using as safety covering in the resistors it would seem to me if they are not really part of the current carrying conductors (isolated independant enclosure) they should have bonding jumper to the EGC or enclosure that would allow for any short withing the resistor connections can be cleared by the OC/SC breaker.
When you say mounted on top of the controller enclosure, are you indicating physically attached to the controller enclosure or simply above it somewhere? If physically connected the means to connect should be such that bonding actually happens when the two enclosures are connected, then no other wire connection should be needed. If the enclosures are not physically connected, then a wiring method considerations mentioned above are in play.

To the temperature issue raised by some, they do have conductors rated at higher than the 90deg C of THHN if needed. (Not cheap though.)
 
Fred B said:
Sounds as if there is a remote unit ... then wiring method considerations mentioned above are in play.
Click to expand...

Thank you for the best overall answer, so far!

It is all transportable equipment. Trucks, cranes, harbor, weld it to the deck of the ship. Literally. The resistor assembly bolts to the top of the control enclosure but I don't trust that connection due to weathering and corrosion. Line diagram is 480VAC 3ph to the drives, 640VDC switched PWM power from drive output to the motor, regen power from motor to drive, switched PWM regen power from drive to resistors. So, we will ground the resistor box to the drive enclosure.

Sounds like the biggest consideration is guaranteeing the ground is in place before power is applied. Cabling with included ground satisfies that but finding >1000V, 1, 2, or 4 pair with G, 10AWG, LSZH, sun/oil/water marine grade cable is like looking for di-lithium crystals on the moon.

We can also fall under "engineering controls" as this will never be operated without at least 3 SME engineers operating it and supervising all work. Hence, the question, can the ground be wired separately?
 
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