Single Phase Sourcing 480VAC VFD and Grounding
Single Phase Sourcing 480VAC VFD and Grounding
(OP)
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 nhttp://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
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 nhttp://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





RE: Single Phase Sourcing 480VAC VFD and Grounding
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Single Phase Sourcing 480VAC VFD and Grounding
RE: Single Phase Sourcing 480VAC VFD and Grounding
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Single Phase Sourcing 480VAC VFD and Grounding
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.
RE: Single Phase Sourcing 480VAC VFD and Grounding
The install, although not ideal, will work.
Thanks for the feedback everyone and I welcome more if anyone has anything else to add.
RE: Single Phase Sourcing 480VAC VFD and Grounding
RE: Single Phase Sourcing 480VAC VFD and Grounding
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
RE: Single Phase Sourcing 480VAC VFD and Grounding
RE: Single Phase Sourcing 480VAC VFD and Grounding
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