Dual Power units on a VFD

[rockman7892]

We have a new 480V VFD being installed and looking at the drive I have some question in regards to why some of the components are installed. I have installed a schematic of this drive.

My first question relates to the fact that this drive has 2 power units. What is the purpose of a drive having 2 power units? Is it to double the power of the drive? Can the drive still run with only one of these power units at a reduced power? Each power unit appears to be rated at 700HP, so would this mean that the drives total power is 1400HP?

The other part of the drive I was curious about was the RC filter assembly that is shown on the drive. What is the purpose of this filter assembly? Is this installed for surge or harmonic purposes?

Lastly on the output of each power unit the cables appear to go through a doughnut CT looking device indicating "1 Turn" on the schematic. This device is not a CT becuase it does not have any secondary wires from it so I'm trying to figure out what its purpose is? Is this some sort of core reactor used to mitigate common mode ground currents or such?

Thanks for the help

 

[electricuwe]

Before posting documents on the internet you should read read it. Bottom right it clearly says:

This document is property of Rockwell Automation, Inc. or its subsidaries .. and may not be disclosed,...., except as authorized..

It does not look like it contains really confidential information, but nevertheless you should respect this.

 

[rockman7892]

The more I look at them I am really curious what the CT looking devices are on the ouput for this drive? I cant seem to determine what they are. Anyone have a clue?

 

[LionelHutz]

It can be considered as one big VFD. I would suspect those are reactors of some sort to balance the currents between units.

 

[jraef]

"...2 power units": That looks to be a "12-pulse front end drive" meaning the input to the drive is from two rectifiers, each (typically) providing a phase shift so that much of the harmonics is mitigated without additional filtering. What they have done here must be something proprietary that I am unfamiliar with, because I don't see the typical phase shifting Delta-Wye transformers found in standard 12-pulse designs, so you would have to ask A-B what their method is. But no, that does not double the capacity, each of those makes up one drive as LionelHutz said.

"...RC Filer": Yes, that looks as though they have one overall RC filter for RFI/EMI mitigation. Not uncommon in large VFDs, it's just that usually it is buried in the front-end design. The reason you see it here looking separate is probably because they have 2 separated front-end rectifiers, but they really only need one RC filter for the entire packaged unit.


"...doughnut CT looking device": Toroidal chokes most likely. More expensive than core-and-coil type, but much smaller. They are probably for EMI mitigation of the VFD outputs.



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[niallnz]

This is simply a high power output drive that uses two paralleled inverter modules. I don't think this is a 12 pulse drive, if it was there would be a transformer shown for each power module.

The individual current rating of the output modules may well be 733A, but when they are paralleled Rockwell will de-rate them. But also this is a heavy duty application on a kiln motor so the whole drive will be sized on the heavy duty current rating, which is less than the normal duty current rating.

The RC filter assembly looks to me to be a harmonic filter.

I think the CT's on the output are common mode chokes to prevent/minimise EDM of the motor bearings.

Cheers niallnz

 

[sibeen]

Is it possible that the 1 turn CT on the outputs is just a ferrite toroid used for high frequency noise reduction.