VFD with delta high leg input

[malachi constant]

Hi all, I designed the remodel of a courthouse that has an existing delta high leg service. We are not touching the service and are adding a few three phase motors (air handlers etc). In the field someone on the mechanical side has raised the concern that the supplied VFDs don't work with a delta high leg - more specifically that they need to be upsized by like a factor of two to work. I do not believe this is correct, but as delta high legs aren't something I run into every day thought I should run it by you guys.

We are talking about 5 HP motors. The VFDs are ABB ACH550 models and are rated from 208V to 230V +/- 10%.

I read some chatter that if the high leg is generated by two pole mounted utility transformers it could cause trouble. We have a fairly new pad-mounted service here, not sure the particulars of what is inside the box but if there is a concern I can ask.

Any insight?

 

[ptonsparky]

Ask and confirm quickly. Some drives have a modification that can be done in field, others just say don't do it. RTFM.

 

[GoldDigger]

In any case, the VFD's problem with non-wye three phase input would be damage to surge protective components on the input, not a need to oversize.

If a normally three phase VFD is rated to instead be used to convert a single phase input to drive a three phase motor, then a factor of two oversizing may be appropriate, but the exact oversize factor should be gotten from the manufacturer.

 

[Jraef]

In any case, the VFD's problem with non-wye three phase input would be damage to surge protective components on the input, not a need to oversize.

If a normally three phase VFD is rated to instead be used to convert a single phase input to drive a three phase motor, then a factor of two oversizing may be appropriate, but the exact oversize factor should be gotten from the manufacturer.

Correct. It's not the "high leg" aspect, because that leg is only "high" with regard to ground reference. But you only FIND a high Leg system when it's a delta transformer secondary, and ALL drives have problems with delta secondaries because, as GD says, there are surge protective devices (MOVs) and common mode noise suppression systems inside of the drive that will be expecting to be used in a grounded wye system where those devices are all referenced to ground. In a 3Ph4W delta system, two of the legs will be referenced to ground, but not the third, and that can cause these devices to fail catastrophically, taking out other components around them.

As mentioned, SOME drives provide you with a simple way of disconnecting the internal ground references in the drives, some do not, but even those that do will likely have a notice telling you that it voids their UL listing if you remove it. Those that do not will have a "CYA" statement buried SOMEWHERE in their installation manual telling you to NOT install them on delta services, or more indirectly (which is what most Asian drives say) "This drive is suitable for installation on solidly grounded star (wye) services" and letting you decipher what that means in your situation. The issue there is, delta services ONLY exist here in North America, so with companies for whom we represent a small fraction of their world-wide sales, they don't really care. So yes, absolutely, RTFM!

The accepted practice if you can't use 208V because of the motors and need to maintain UL listing is usually to install a "drive isolation transformer" that is a 1:1 ratio, but has a delta primary and a solidly grounded wye secondary, sized to ONLY feed the drive(s). That secondary will be 230Y133, so the phase to ground voltage is useless for anything else, but the VFD will require it.

And, in anticipation of all of the probable "Done it that way for years with no problems" comments;
Yes, SOME people get lucky, but luck is a poor substitute for reliability.

 

[jumper]

The accepted practice if you can't use 208V because of the motors and need to maintain UL listing is usually to install a "drive isolation transformer" that is a 1:1 ratio, but has a delta primary and a solidly grounded wye secondary, sized to ONLY feed the drive(s). That secondary will be 230Y133, so the phase to ground voltage is useless for anything else, but the VFD will require it.

If the transformer is fed with 240V and it is a 1:1 ratio, why is the secondary 230V?

 

[malachi constant]

Great information.

Looking at the user's manual, pages "1-307" and "1-308" in the below link are with respect to ground connections on unsymmetrically grounded systems. I believe a delta high leg is represented in the upper right figure ("Grounded at the midpoint of a delta leg") on page 1-308. My reading of this is for an unsymmetrically grounded system the EM3 screw must be removed (which removes an internal ground connection used to reduce EM emission in symmetrically grounded systems).

https://library.e.abb.com/public/496eba9e5c5e4d9fa86e91009eab8f5e/3AUA0000004092_REVI.pdf

As near I can tell there are no other warnings, so we should be good to remove the EM3 screw and proceed with the install.

Out of curiosity, if one were to be super cautious and decide to avoid VFDs in a delta high leg application, what would you recommend we use for motor starters? I am young enough that pretty much all starters of significant size that the mechanical engineers have specified in my career are VFDs. And we put them in the Div 23 spec, so I am fairly ignorant on the world of motor starters.

Thanks all!

 

[Jraef]

If the transformer is fed with 240V and it is a 1:1 ratio, why is the secondary 230V?

I'm speaking nominal here... "220, 221, whatever it takes."

 

[Ingenieur]

I'm speaking nominal here... "220, 221, whatever it takes."

Mr Mom :lol:

 

[jumper]

I'm speaking nominal here... "220, 221, whatever it takes."

Okay, sorry, I was trying to figure how the configuration dropped 10V.

Thought maybe the high leg was somehow doing something weird to the set up.

 

[ActionDave]

Great information.

Looking at the user's manual, pages "1-307" and "1-308" in the below link are with respect to ground connections on unsymmetrically grounded systems. I believe a delta high leg is represented in the upper right figure ("Grounded at the midpoint of a delta leg") on page 1-308. My reading of this is for an unsymmetrically grounded system the EM3 screw must be removed (which removes an internal ground connection used to reduce EM emission in symmetrically grounded systems).

https://library.e.abb.com/public/496eba9e5c5e4d9fa86e91009eab8f5e/3AUA0000004092_REVI.pdf

As near I can tell there are no other warnings, so we should be good to remove the EM3 screw and proceed with the install.

Out of curiosity, if one were to be super cautious and decide to avoid VFDs in a delta high leg application, what would you recommend we use for motor starters? I am young enough that pretty much all starters of significant size that the mechanical engineers have specified in my career are VFDs. And we put them in the Div 23 spec, so I am fairly ignorant on the world of motor starters.

Thanks all!

You could use any starter you wanted. NEMA starters are built tougher than IEC starters.

 

[Jraef]

Great information.

Looking at the user's manual, pages "1-307" and "1-308" in the below link are with respect to ground connections on unsymmetrically grounded systems. I believe a delta high leg is represented in the upper right figure ("Grounded at the midpoint of a delta leg") on page 1-308. My reading of this is for an unsymmetrically grounded system the EM3 screw must be removed (which removes an internal ground connection used to reduce EM emission in symmetrically grounded systems).

https://library.e.abb.com/public/496eba9e5c5e4d9fa86e91009eab8f5e/3AUA0000004092_REVI.pdf

As near I can tell there are no other warnings, so we should be good to remove the EM3 screw and proceed with the install.

Out of curiosity, if one were to be super cautious and decide to avoid VFDs in a delta high leg application, what would you recommend we use for motor starters? I am young enough that pretty much all starters of significant size that the mechanical engineers have specified in my career are VFDs. And we put them in the Div 23 spec, so I am fairly ignorant on the world of motor starters.

Thanks all!

So it appears ABB has given you a viable option, although I know for a fact that other mfrs who give you that option ALSo warn you that by altering it, you void the UL listing. I don't know why ABB is immune from that, maybe they don't know?

Starters are irrelevant in this issue, it only applied to VFDs.