VFD on Open delta autotransformer fed from a WYE source

[JoeStillman]

I know that VFD's don't want to be fed from a 4-wire delta transformer. But if I hook up two autotransformers in open delta to buck a few volts off of the wye-fed circuit, the line to ground imbalance isn't as bad as on a 4-wire delta.

Will the VFD work ok? Anybody try this before?

 

[winnie]

Just how far are you planning to buck the voltage, and is there a big reason not to use three autotransformers in a wye or balanced closed delta configuration?

 

[Jraef]

The reason VFDs don't "like" delta power sources is because they are built EXPECTING a consistent voltage reference to ground on all 3 phases, as you would have in a Wye (Star) configuration. How you get to the delta is irrelevant in that it's all bad from the standpoint of the risks to the VFD components.

I'm not sure why you are doing this "buck a few volts"... the VFD could not care less so long as the line voltage was in tolerance. So for example of you have a 240V rated VFD and you need to feed a 200V rated motor, it will not know or care. You can still program the output voltage going to the motor to be 200V if that's what it needs, the VFD will do whatever you tell it. The only thing it CAN'T do is create voltage that isn't there; i.e. a 208V supply and having to feed a 230V motor.

 

[ModbusMan]

I know that VFD's don't want to be fed from a 4-wire delta transformer. But if I hook up two autotransformers in open delta to buck a few volts off of the wye-fed circuit, the line to ground imbalance isn't as bad as on a 4-wire delta.

Will the VFD work ok? Anybody try this before?

What does the IOM say? Back when I was working on Baldor and Toshiba drives (almost 25 years ago, mind you), both straight-up required wye sources. There's one site I remember doing a site-walk and feasibility check on installing a very large (500hp maybe?) regenerative drive where the floor ran almost entirely on 480v floating delta. We said "Thanks, no thanks" to that one.

I'm not sure why you are doing this "buck a few volts"... the VFD could not care less so long as the line voltage was in tolerance.

And this... again, what's the IOM say? With the IT gear I deal with today, we have the CBEMA/ITIC curve to guide us, but just about anything industrial should also have this included somewhere in their manuals.

 

[tortuga]

I have found a cost efficient way to do a VFD on a open delta is a 1-phase to 3-phase VFD, the vendor oversized the VFD.
Where phases A-B is a 240V stinger transformer,
A-C is 120/240 split phase lighting pot,
and B-C is open,

A 1-phase VFD will work well on the A-B set, even if B is 208 to ground and A is 120 to ground.
And it only loads kVA on that stinger xfromer.
A 1- phase VFD does not like the open B-C set (and it disproportionately loads both stinger and lighting pot)
Only a 3-phase VFD gets weirded out by a 240 delta, 1-phase works fine and you can put its load on either transformer.

 

[jim dungar]

The reason VFDs don't "like" delta power sources is because they are built EXPECTING a consistent voltage reference to ground on all 3 phases, as you would have in a Wye (Star) configuration.

And this would be even more of an issue with OP configuration.

The open delta would equally affect the (3) L-L voltages but it would totally mess up the L-G voltages one of which would remain at 120V while the other two would be somewhat lower.

 

[winnie]

This discussion has gone slightly awry. The OP has a wye supply, not an open delta. But they want to reduce the voltage for some reason, and want to use an 'open delta autotransformer'. This is quite a bit different than a typical 'open delta high leg supply'. The only similarity is that you use 2 transformers in an open delta configuration, but the grounding is closer (not exactly, just closer) to a typical wye grounded system.

 

[JoeStillman]

To provide a little more detail; installation is a church with 480Y/277V service from the POCO, several RTU's with VFD's. The VFD's have tripped on overvoltage on more than one occasion and blown out more than one VFD. The POCO analyzed their service and pronounced it within tariff (< +10%). Buit this is no help because the specs on the RTU's say max is +5% (504V).

The POCO won't dial down the taps because they say they don't have to. I want to propose bucking transformers and cut down the voltage by 24V or 5%. Maybe the solution is to push harder on the POCO - I don't know.

I didn't know bucking auto transformers come in 277V primary. Isn't that what I would need to make a balanced wye?

 

[tortuga]

Oh I see disregard my last post
I am not a VFD expert but I have seen ABB "400V class" drives and also "500V class" so it may be issue with selecting the right drive class;
I think the "400V class" is best suited for the less common range of 416/240 or old 460/266 nominal service voltages, with nominal utilization voltage range of 400/230 - 440/254.
The "500V class" would be best suited for standard US service voltages in the ANSI C84 voltage Range B 480/277 - 508/293 with nominal utilization voltages of 460-266 - 480/277.