480 corner grounded delta service and new cnc machines

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Mitchell Setzer

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VP operations
I have seen a couple post from this forum about corner grounded delta. They have answered some of my question but not really give an answer or solution to the problem I am having.

Story:
We are a small prototyping and engineering firm that ust to be a general machine shop with welding, cnc and fabrication throughout our plant. We have found a great niche and have been growing for a couple years. The building is quit old and after calling and testing the incoming service this is what we have. After the past week of research have found out that Incoming from POCO is 480 3 Phase Delta. In the electrical room it is switched to 480 corner grounded delta where one leg is carrying the neutral and a load. This has never really been an issue in the past since most of the equipment was welders, pumps, motors, and manual lathes that really never had VFDs or servo amplifiers within the machine.

Over the past couple of months we have started purchasing new CNC lathes and mills. Brand new Haas VF5ss is being installed. Install tech from Haas is testing incoming before turning machine on. Phase to Phase is all great 480 479 478. Then leg to ground 480 479 0. This is coming from the corner grounded delta that is feeding the buss. Not sure if it is B or C phase that is corner grounded. This normally would not be to big of an issue however the Haas machine VFDs and servo amplifiers probably require the 277 for surge overload and if a storm hits or we get a fault in on leg it will have the potential to send 480 surge instead of the 277. Any ideas on how to solve this or who to call that might have knowledge of possible solutions. Every electrician I call tell me I am dealing with a unicorn and that they cannot help. Machine is running fine and we cut power during hot part of the day in case POCO starts increasing voltages or if potential for storms are present. I maybe freaking out for nothing but I dont want to fry my machine. This is a huge investment for my small company. Any advice would be greatly helpful.
 
Simple: You need a 480v delta to 480/277 wye transformer. Bond/ground the newly-derived system's neutral as usual.

This is a relatively common issue and solution; not a unicorn at all. If you need more help, I'm in VA. Hire me. ;)
 
Mitchell Setzer said:
Incoming from POCO is 480 3 Phase Delta. In the electrical room it is switched to 480 corner grounded delta
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That's a bit confusing. Is the incoming ungrounded delta or what? Could be corner-gounded delta from the PoCo. Kinda depends on what's common in that area.

If you're getting more and more modern machines, I'd investigate the options of having the PoCo switch you to a wye-connection (which probably means at least some new disconnects/panels, or installing a whole delta->wye system to feed anticipated new loads. There is a bit of "pay me now or pay me later" in all this.
 
Hello all thanks for all the replies and that is awesome information. To answer the one question it is Delta ungrounded coming in but in the electrical room at the disconnects it creates the corner ground within the building. Right after I had posted this I had gotten a call from a buddy who had worked in the furniture plants in my area and had seen the same setup a couple times for companies that have running dedicated pumps on certain systems that would cause catastrophic failures if one of the legs ground faulted.

Our plan is close to 60,000 ft² and has multiple bust so switching the whole supply over is going to be a major headache and very expensive. After discussing with my friend it looks like your suggestion is pretty much spot on I need to isolate the 480 on the bus duct that I am using for my new machines to go from Delta to wye. This should reduce the noises well. The final question is of the three machines 2 are 480 and one is 240 all 3 phase. So I guess it's get a larger KVA transformer to go from 480 Delta to 480 wye. Then feed two 480 cncs with secondary and feed a stepdown transformer for the 240. If my friend does not feel comfortable with the installation I'll get in touch soon. Thank you all
 
Mitchell Setzer said:
To answer the one question it is Delta ungrounded coming in but in the electrical room at the disconnects it creates the corner ground within the building.
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That's normal.
Mitchell Setzer said:
The final question is of the three machines 2 are 480 and one is 240 all 3 phase. So I guess it's get a larger KVA transformer to go from 480 Delta to 480 wye. Then feed two 480 cncs with secondary and feed a stepdown transformer for the 240.
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Your second transformer should not be supplied by the first one; it should have its own 480v primary.

It should not be difficult to find a 480v delta-to-240v Y unit. (I forgot the L-N voltage; 136?)
 
zbang said:
That's a bit confusing. Is the incoming ungrounded delta or what? Could be corner-gounded delta from the PoCo. Kinda depends on what's common in that area.

If you're getting more and more modern machines, I'd investigate the options of having the PoCo switch you to a wye-connection (which probably means at least some new disconnects/panels, or installing a whole delta->wye system to feed anticipated new loads. There is a bit of "pay me now or pay me later" in all thi
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LarryFine said:
That's normal.

Your second transformer should not be supplied by the first one; it should have its own 480v primary.

It should not be difficult to find a 480v delta-to-240v Y unit. (I forgot the L-N voltage; 136?)
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Yes that is what I need it to be on L to N 136 L to N on all three legs for the 240 volt. That machine is currently on a delta to delta trans so it is having problems. So pretty much I need a delta to wye for the 480 and a data to wye step down for the 240. With properly rates KVA to supply machines. 70 amp 240 3 phase is 29.064 KVA. Is a 30 KVA enough for that single machine or should I go with 45 KVA?
 
Mitchell Setzer said:
Yes that is what I need it to be on L to N 136 L to N on all three legs for the 240 volt. That machine is currently on a delta to delta trans so it is having problems. So pretty much I need a delta to wye for the 480 and a data to wye step down for the 240. With properly rates KVA to supply machines. 70 amp 240 3 phase is 29.064 KVA. Is a 30 KVA enough for that single machine or should I go with 45 KVA?
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Also I assume that the fact that the building is corner grounded delta has no effect on the delta to wye transformers. They do not make a corner grounded delta to wye transformers 😂. This is really just to give the potential to ground equal across legs and to isolate the supply. Should take it from l to n 480 480 0 to 277 277 277 correct?
 
Mitchell Setzer said:
Also I assume that the fact that the building is corner grounded delta has no effect on the delta to wye transformers. They do not make a corner grounded delta to wye transformers 😂. This is really just to give the potential to ground equal across legs and to isolate the supply. Should take it from l to n 480 480 0 to 277 277 277 correct?
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Correct.

The corner grounding was a way to fix voltage(s) to ground.

It's providing isolation so you can ground a different point.

It's a wye to derive a centered-voltage reference point.
 
LarryFine said:
Correct.

The corner grounding was a way to fix voltage(s) to ground.

It's providing isolation so you can ground a different point.

It's a wye to derive a centered-voltage reference point.
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Just a fyi I am not a electrician. I am an engineer but mechanical. I have learned more about 3 phase in the past 3 days than I have in the past 15 years. I wear to many hats being the owner. Thanks again everyone!
 
I’ve done this completely differently. The problem is that you had ungrounded delta instead of a wye feed. So one way to ground it is corner grounding. It’s simple and cheap. You just have to use say 2 fuses on the incoming disconnect. The grounded phase is not fused because otherwise you lose your ground.

But you still don’t have a neutral. There is nothing wrong with using a lot of VFDs on a corner grounded system. You just have to remove a jumper or switch something to disable MOVs to neutral if they exist. Most VFDs have instructions. But on machine tools often you don’t get that info.

So the way I do it…did it recently when my only choice in transformers due to needing it ASAP and it being a really oddball voltage combination was delta delta. I put in high resistance grounding. I installed a zig zag transformer (277 V x 15 A = 4155 VA or about 5 kVA). These are pretty small. This creates a neutral from the delta. Then I connected that to a resistor then the resistor to my grounding system. I also installed a CT and a trip relay that sets off an alarm and flashing light. I had enough room to put it all in the main panel. So the way this works is the resistor limits ground faults to 15 A or less. The trip/alarm relay lets the plant know if they have a ground fault but since the current is so low as long as you shut down the faulty equipment quickly (treat like a fire alarm) it doesn’t shut down the plant. A second fault on another phase however is very, very bad. The whole thing is so small it fits in the bottom of a fused disconnect. And with this system the VFD issue is solved.
 
Ok so my building is already corner grounded. Hence the l to n is 480 480 0. This reading going into the new CNC freaked the install tech out which made him call Haas who then informed me that warranty void if problems arise while leg to ground shows those readings. To get the required 277 277 277 I have to switch to wye from corner grounded delta. Spending the money on transformers is well worth the safety of my new CNC and the 250k investment. Spend the 6 to 8 k now to save myself the 50k headache later and the time waiting on voided warranty parts seems ok with me.. the rest of what you described made my head explode so not sure on how to even do that part 😂
 
Oh yes among other things the thought of having to make a pseudo fire alarm to alert me when I drop a leg seems overtly complicated to me and I think I might just sit there and worry about it however it's probably for more peace of mind however I would not even know where to begin since I'm not an electrician. I have no doubt that what you describe worked flawlessly. With that being said being the business owner and a mechanical engineer the thought of buying two pre-made gray boxes that get me there seems a whole lot less scary. Don't get me wrong I love to prototype solutions for customers and invent new things. I make a living on that. Thats why I came to this forum. This is not my area of expertise. SME I am not when pertaining to building power. If you want a below the hook device that makes magic happen or customer handling solution then I'm you guy.
 
High resistance grounding is not complicated but it is different. It is the standard in many countries and cheaper in the long run. You can trip like normal with it…better actually. In solidly grounded systems ground faults are so violent the breaker needs to trip immediately. With a high resistance system you don’t even have enough current to cause fibrillation so the shock hazard isn’t there. And you have the choice of tripping, or just alarming. It is the best hybrid of both ungrounded and grounded systems. But again it’s different and requires different maintenance techniques. The application is a water plant where continuity of service is very important. This system is common where continuity of service or safety are high priorities. It is common in mining, chemical plants, and paper mills.

Corner grounding has its own problems because of the unusual voltages and you can’t trip one phase leg at the main service panel or lose your grounding.
 
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