Single Phase Sourcing 480VAC VFD and Grounding

[Kingstonrupp]

Hello,

I have come across a situation where available power on site is from a single phase 480VAC transformer with one leg grounded.

There are a couple VFD loads requiring 480VAC 3-phase power. Separate single phase VFDs were purchased and de-rated to account for the additional current requirements.

A question came back from the field as to whether or not the VFD would operated properly. The VFD is rated 80HP but is running a 40HP load. It is a basic 6-pule VFD with a diode rectifier bridge on the front end, DC bus with capacitor and two DC link reactors, IGBTs on the output. My rationale is that yes, the VFD will operate fine as long as the "loss of phase" monitoring is disabled. The VFD should not care if one of the supply conductors is grounded at the transformer or not as long as there is 480VAC between the two.

Further though, I started questioning what would happen if the DC bus within the VFD were to fault to ground. My thoughts are that if the + bus faults, my fusing should blow (good). If the - bus faults, I am not 100% sure what is n

http://www.eng-tips.com/ow

going to happen. I don't think the fusing would blow since this conductor is already tied to ground. The VFD may not operate properly anymore since the diodes are essentially bypassed on the - bus. Would my VFD fault on DC bus voltage issues?

I don't believe there are any safety issues with respect to electric shock since the VFD enclosure and the site ground are all the same potential.

Can anyone enlighten me on the possible predicaments with this setup. Has anyone used this type of setup before?

Thanks

 

[Skogsgurra]

DC+ to GND is identical to DC- to GND. What happens is that the "lower" part of the diode bridge delivers the short-circuit current and that blows the AC fuse just as fine as overcurrent in the "upper" part does.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Kingstonrupp]

If the VFD was wired with two ungrounded conductors as the supply, I would agree, but in this case, one of the supply conductors is the grounded conductor because the transformer is single phase 480vac with one leg grounded. The grounded conductor supplies L2 of the VFD while the ungrounded supplies L1

 

[Skogsgurra]

I will make you agree!


Consider the positive half-cycle: Current flows from L2 via diode "pointing up" to DC+ and GND.

Then consider the negative half-cycle: Current flows from GND and DC- via diode to the now negative L2 terminal.

Of course, it depends on what part of the DC bus faults to ground. You assumed the DC+ in your example and I assumed DC- in the second example. You may change your DC+ to DC- and my DC- to DC+ and still have similar (not identical) cases.

The reasoning uses conventional current flow, not electron flow.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

I'm not a fan of that connection at all, but not because of the concern with fault current and blowing fuses.

When the ungrounded conductor is connected to the DC- bus, the DC- is pulled to approximately negative 680VDC. Then, when the diodes switch and the grounded conductor is connected to the DC- bus, the DC- is pulled to ground. A similar thing happens to the DC+ terminal except it switches between positive voltage and ground. So, you're having this effect where the bus voltage is swinging around by 680V 120 times a second. Then, the 3-phase motor output voltage also follows this swinging voltage.

 

[Kingstonrupp]

Odd setup I agree. Unfortunately it's what we were given to work with. I see what you mean about the + and - DC bus and ground referencing, but as long as the system is not intentionally grounded anywhere beyond the transformer, there should not be any alternate return paths and therefor no objectional current over bond and ground conductors during normal operation. The VFD has been derated to account for additional current and the supplier has verified the DC capacitor can operate on single phase supply to deal with the added voltage swings. And now that I have been convinced fuses will blow if we do get an unintentional ground fault, I think
The install, although not ideal, will work.

Thanks for the feedback everyone and I welcome more if anyone has anything else to add.

 

[MatthewDB]

The other item to watch for is the cooling fan on the VFD. Many VFDs above ~25 HP switch to three phase cooling fans. To run them on single phase (or DC) they require a small VFD to power the cooling fan connected to a buried connector.

 

[itsmoked]

Many VFDs above ~25 HP switch to three phase cooling fans

Really!? That amazes me. I could easily see a VFD motor's auxiliary fan doing that but heatsink fans...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

I've seen this in older drives, but not down at 25HP level. I can't imagine drives in the >200HP ever running on single phase.

 

[jraef]

Think about what circumstance could make the DC bus go to ground. It would have to take place INSIDE of the VFD, and would by design necessitate a catastrophic failure of some sort ALREADY. Once that has already happened, your drive is toast, so waiting for the line fuse to clear is inconsequential at that point.

3 phase motors used on VFD cooling fans is still done, but not at that power level. Some drives use an AC power supply for control power, so you will not be able to use those even if you disable the phase loss protection, you must use the type that derive their control power off of the DC bus and at that power level, many do, just double check.

I'm not sure about LionelHutz's concern here, I would think that a corner grounded delta feed to a VFD would have the same issue it that were true, and that is done all the time. There are issues related to the MOV protection schemes ahead of the diode bridge being referenced to ground that must be addressed on grounded delta systems, but they have to do more with the delta supply, not the grounding I don't think. Still, it can't hurt to ask that question of your VFD supplier.

There was a small drive mfr in the Midwest, where 480V single phase SWER supplies are more common for rural farming areas, who sold a VFD specifically for doing this. They didn't make the drive, I believe it was brand labeled from Yaskawa or Hitachi, someone like that, but they did supposedly modify it to work on this. I don't know what they did exactly, and they went out of business (I think).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington