Allen Bradley Inverter and Common Mode Capacitors question

What is the purpose of connecting the DC bus capacitor common to ground?

Why doesn't the DC bus capacitor common short out when connected to ground? How can it have zero potential with respect to ground?

Thanks for the help.

I have never heard of grounding the DC Bus in any form when tieing drives together in any form, whether with a common bus supply, or shared, or piggy back.

All of the documentation I know from AB suggests that you should in fact ensure that the positive and negative DC Rails are symmetrical with respect to ground to avoid serious noise issues.

If there IS zero potential to ground that would indicate to me either a fault condition, or a three phase supply that has no ground bonding or reference.

The screws are inserted or removed depending on the grounding scheme used on the power system that feeds the equipment in question.

Look here http://literature.rockwellautomation.com/idc/groups/literature/documents/in/750-in001_-en-p.pdf

On pdf page 74 and refer to table 13.

A model number would be helpful. You said in your post that this was a inverter. Is it a inverter for a common bus setup or is it a drive. A inverter and a VFD / VSD ( drive ) are 2 different things.

To further clarify most drives have a inverter section depending on the model but a VFD / VSD Drive is more than a inverter.

Then there are inverters that do just that invert power to the desired voltage and component.

I think you are mistaking the CE filter ground for a DC bus ground. Neither side of the DC bus can be grounded or the input fuses will release (or worse).

DickDV said:

I think you are mistaking the CE filter ground for a DC bus ground. Neither side of the DC bus can be grounded or the input fuses will release (or worse).

Click to expand...

I'm pretty sure your right DickDV. What I do know is that if you don't remove the jumpers for the CE filter ground on a Pflex and you have a resitevely grounded star point supply for a transformer, very bad things will happen.

I had several Pflex70 DC bus capacitors explode in such a situation when a ground fault appeared at the transformer. Because of the ground limiting resistor nothing tripped but you could watch the DC bus voltage rise on the drive HIM until the point the capacitors let go and spewed capacitor juice everywhere.

A very trying and expensive lesson indeed.

That's not just AB or Powerflex but pretty much everyone that puts a CE label on their drive. They have no tolerance for floating or unbalanced-to-ground power networks. That includes high resistance grounded wye's since they can also go unbalanced and keep running.

You generally can disconnect the CE network but if you don't, as you say, bad things happen.