backfeeding transformer grounding

[lennycharge]

Installing a temporary 480 3 phase line for a lathe.

Transformer supplied to me is 280 to 480 delta wye. Lathe has a VFD built in, neither I or anyone around me are familiar with VFD, but I have read not to hook a VFD up to delta. Is this so ground faults are only 277v if hooked up wye?
I have contacted the VFD manufacturer and they say this VFD will be happy to run on 480 delta, but not to corner ground as this will cause noise issues, just to run the machine ground back to the panel.

They would like to run like this: Breaker in the 208v panel> transformer> safety switch> flexible cord to the lathe with no breaker on the 480v side.

Should I leave X0 unhooked, and run a ground back to the 208 side from the machine ground/chassis of transformer or just leave the 480v side ungrounded and run a ground fault detector?

Maybe im overthinking all this because of the VFD.

Building will eventually have a wye step up transformer and a 480v panel installed, but they just need to get rolling for a few months with this lathe.

Opinions on ocpd on primary side, 125% or 250% of transformer.

 

[GoldDigger]

The NEC has some very specific requirements for an ungrounded supply. Make sure you meet them.
Also, do not connect the 208Y/120 side X0 to either neutral or EGC.

Sent from my XT1585 using Tapatalk

 

[kwired]

Installing a temporary 480 3 phase line for a lathe.

Transformer supplied to me is 280 to 480 delta wye. Lathe has a VFD built in, neither I or anyone around me are familiar with VFD, but I have read not to hook a VFD up to delta. Is this so ground faults are only 277v if hooked up wye?
I have contacted the VFD manufacturer and they say this VFD will be happy to run on 480 delta, but not to corner ground as this will cause noise issues, just to run the machine ground back to the panel.

They would like to run like this: Breaker in the 208v panel> transformer> safety switch> flexible cord to the lathe with no breaker on the 480v side.

Should I leave X0 unhooked, and run a ground back to the 208 side from the machine ground/chassis of transformer or just leave the 480v side ungrounded and run a ground fault detector?

Maybe im overthinking all this because of the VFD.

Building will eventually have a wye step up transformer and a 480v panel installed, but they just need to get rolling for a few months with this lathe.

Opinions on ocpd on primary side, 125% or 250% of transformer.

You have conflicting information in there. Is the transformer secondary 480 volt 3 wire delta or is is 480/277 4 wire wye?

If the latter ground the neutral point of the wye, if it is delta, you have two choices, ground a phase or leave ungrounded - but will need ground fault detection if you do so.

A big reason a lot of 480 volt drives are not recommended to be used on delta systems is they are designed with surge protection that is designed for 277 volts to ground. They clamp voltage transients over 300ish volts to ground Having 480 volts to ground means they are constantly clamping current and will burn out.

 

[lennycharge]

You have conflicting information in there. Is the transformer secondary 480 volt 3 wire delta or is is 480/277 4 wire wye?

If the latter ground the neutral point of the wye, if it is delta, you have two choices, ground a phase or leave ungrounded - but will need ground fault detection if you do so.

A big reason a lot of 480 volt drives are not recommended to be used on delta systems is they are designed with surge protection that is designed for 277 volts to ground. They clamp voltage transients over 300ish volts to ground Having 480 volts to ground means they are constantly clamping current and will burn out.

Transformer is a 208 primary 480 secondary delta wye but its being being back fed,primary will be 208v and ill be left with 480v 3 wire delta, the VFD manufacturer said not to corner ground due to noise.

Ill leave the machine ground unhooked and install a ground fault detector on the 480 side. Unless I can talk them into finding a transformer that will make 480 wye.

 

[electrofelon]

Transformer is a 208 primary 480 secondary delta wye but its being being back fed,primary will be 208v and ill be left with 480v 3 wire delta, the VFD manufacturer said not to corner ground due to noise.

Ill leave the machine ground unhooked and install a ground fault detector on the 480 side. Unless I can talk them into finding a transformer that will make 480 wye.

Note that even with an ungrounded system, you still need to bond the frames of your machine, equipment and raceways together and to earth. The 480 transformer side will look pretty much the same as "normal" with an EGC and everything, there just wont be a system bonding jumper from the EGC to the grounded conductor.

 

[kwired]

Transformer is a 208 primary 480 secondary delta wye but its being being back fed,primary will be 208v and ill be left with 480v 3 wire delta, the VFD manufacturer said not to corner ground due to noise.

Ill leave the machine ground unhooked and install a ground fault detector on the 480 side. Unless I can talk them into finding a transformer that will make 480 wye.

Sounds like you are trying to say the transformer is supposed to ordinarily be 480 volt delta primary x 208/120 wye secondary but you are reverse feeding the low voltage side with 208 volts.

What is confusing is you say 208 primary 480 secondary then immediately follow that with delta wye - which ordinarily would be taken to mean 208 delta primary x 480 volt wye secondary, but then other things you mention seem to indicate 480 side is the delta side.

One other suggestion I have is if the wye side is the input side, leave the X0 terminal "floating" (do not connect it to the neutral of the supply)

 

[lennycharge]

Sounds like you are trying to say the transformer is supposed to ordinarily be 480 volt delta primary x 208/120 wye secondary but you are reverse feeding the low voltage side with 208 volts.

What is confusing is you say 208 primary 480 secondary then immediately follow that with delta wye - which ordinarily would be taken to mean 208 delta primary x 480 volt wye secondary, but then other things you mention seem to indicate 480 side is the delta side.

One other suggestion I have is if the wye side is the input side, leave the X0 terminal "floating" (do not connect it to the neutral of the supply)

Yes, sorry, I should have been more clear, I was trying to do too many things while typing. I was planning on leaving X0 unhooked. Still trying to get them to get the correct transformer, this all makes me nervous with the VFD.

 

[ActionDave]

.... Still trying to get them to get the correct transformer, this all makes me nervous with the VFD.

I would do more than try, I would insist as much as I could. A step up transformer with a Wye secondary is not hard to obtain, nor are they expensive, and the drive will be happier, and you won't have worry if you replace the drive and the new one can't run on a delta system.

 

[Jraef]

Did this information on the VFD being OK with a 480 delta source come from the VFD MFG in writing? Or did you get this info from some distributor or reseller anxious to get the sale? I know of NO VFDs that are totally OK with this unless modified, and of those that allow it by modification will typically tell you that in modifying it to allow delta, you are sacrificing the surge protection of the rectifier and in most cases, voiding the UL listing. 99.9% of hungry resellers don't know (or care) about these details. If it isn't coming from the mfr in writing, i.e. shown as an option in their installation manual, it's likely not true and the reseller is passing along heresay information or they are outright telling a lie to avoid losing the order. I know of no exceptions. If you have one and have it in writing from the mfr, please post the brand name and model, I'd like to learn.

As stated, just go the extra yard here and use a transformer with a Wye on the 480V side, ground the Wye point and give the VFD what it requires. Doing so is better even if the mfr says it's ok, doing otherwise probably puts your customer's investment at risk, then when the VFD fails and they send it in, they will get a report back saying that it was connected in a way that voided the warranty.

Plus as also stated if you can't do a corner ground, the NEC dictates that you must add ground fault protection to that delta secondary, which will likely add more cost than just getting the correct transformer.

 

[lennycharge]

Did this information on the VFD being OK with a 480 delta source come from the VFD MFG in writing? Or did you get this info from some distributor or reseller anxious to get the sale? I know of NO VFDs that are totally OK with this unless modified, and of those that allow it by modification will typically tell you that in modifying it to allow delta, you are sacrificing the surge protection of the rectifier and in most cases, voiding the UL listing. 99.9% of hungry resellers don't know (or care) about these details. If it isn't coming from the mfr in writing, i.e. shown as an option in their installation manual, it's likely not true and the reseller is passing along heresay information or they are outright telling a lie to avoid losing the order. I know of no exceptions. If you have one and have it in writing from the mfr, please post the brand name and model, I'd like to learn.

As stated, just go the extra yard here and use a transformer with a Wye on the 480V side, ground the Wye point and give the VFD what it requires. Doing so is better even if the mfr says it's ok, doing otherwise probably puts your customer's investment at risk, then when the VFD fails and they send it in, they will get a report back saying that it was connected in a way that voided the warranty.

Plus as also stated if you can't do a corner ground, the NEC dictates that you must add ground fault protection to that delta secondary, which will likely add more cost than just getting the correct transformer.

Yes, this was is in an email directly from yaskawa support. This came from an application engineer and said it will be all good on 480v delta. He said that corner grounding was not going to work due to noise issues. This is a G7A45P5 model Yaskawa in a South Bend lathe.

 

[electrofelon]

Yes, this was is in an email directly from yaskawa support. This came from an application engineer and said it will be all good on 480v delta. He said that corner grounding was not going to work due to noise issues. This is a G7A45P5 model Yaskawa in a South Bend lathe.

I am not an expert in this area, but It sounds fishy to say something is ok on ungrounded but not ok on corner grounded, because you can't rely on an ungrounded system to float in the "center". I have measured ungrounded systems that were very close to "center" but others where the voltage to ground was all over the place.

 

[GoldDigger]

I am not an expert in this area, but It sounds fishy to say something is ok on ungrounded but not ok on corner grounded, because you can't rely on an ungrounded system to float in the "center". I have measured ungrounded systems that were very close to "center" but others where the voltage to ground was all over the place.

FWIW, which is not necessarily very much, the surge protection elements (varistors, whatever) should be able to conduct a small enough current to balance normal capacitive current voltage divider effects without damage. If there is a reason (high unbalanced leakage, for example) why the ungrounded delta which is feeding only the VFD is floating stiffly off center, then it is a fault that needs to be corrected.
However there are certain types of less than bolted fault (restriking arc faults for example) that can offset the delta up to twice the nominal line to line voltage with respect to ground. If such a fault occurs it is likely that the surge protection components and maybe the rest of the VFD will not survive.
Even a bolted line to ground fault can raise the other line to ground potentials to the point where the protective components may immediately be toast while OCPD will not open until a second fault occurs.
Ground leakage devices would have to shutdown the circuit rather than simply raise a warning in this case.
Overall not a good idea IMHO. The engineer is telling you that absent a fault the drive will be fine. He is not telling you that it will survive a fault.

 

[lennycharge]

Hey, everyone. I really appreciate your expertise and insight here. Just had to tell ya, wealth of information and i appreciate all the info. I am so unknowledgeable with VFDs that I couldnt really think too straight about any of it. You all could have been huge @$$holes rather than helpful, I appreciate it.

The proper transformer will be installed.

 

[ActionDave]

Hey, everyone. I really appreciate your expertise and insight here. Just had to tell ya, wealth of information and i appreciate all the info. I am so unknowledgeable with VFDs that I couldnt really think too straight about any of it. You all could have been huge @$$holes rather than helpful, I appreciate it.

I have no idea how that happened.:angel:

The proper transformer will be installed.

Good call.