Wye or Delta serving motor loads

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11bgrunt

11bgrunt

Pragmatist
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TEXAS
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Electric Utility Reliability Coordinator
Years ago 480 volt grounded corner delta was the REA POCO choice for serving 480 motor load. Today I see 480 volt wye as the choice offered by the POCO and maybe even the utility industry. I have been reading through many of the threads but I still have trouble understanding how the wye or delta source would make any difference to the motor. If, on Monday the POCO served the plant with 480 grounded corner delta or ungrounded 480 delta, and on Tuesday they replaced it all with a Y-Y padmount 277/480, I would not expect any of the motors to be touched and connected any different than they were the day before.
When does it make a difference, if at all?
Thanks guys.
 
ActionDave said:
The motor does not care a whit about how the transformer is grounded.
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The motor does not care because all of its windings are insulated above 480V to ground and so 277 to ground is fine too.

There will be a difference in the number of fuses or breaker poles required/allowed though.
 
ActionDave said:
The OP asked about motors not code.
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But OP stated...
11bgrunt said:
... If, on Monday the POCO served the plant with 480 grounded corner delta or ungrounded 480 delta, and on Tuesday they replaced it all with a Y-Y padmount 277/480, I would not expect any of the motors to be touched and connected any different than they were the day before.
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...and then asked...
When does it make a difference, if at all?
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In short, the motors may have to be connected differently by Code.
 
No, it makes zero difference to the motors themselves.

But delta power systems are NOT friendly to anything electronic. So VFDs, DC drives, electronic ballasts, power supplies for servos, UPS systems and such will all be subjected to a much higher likelihood of failure. Most if not all power electronic systems using rectifiers are designed for Wye power systems where the maximum voltage reference to ground is 58% of the line to line voltage. The components they are made from are expecting that and the surge protection devices used in them are designed based on that. If you use Delta, you must investigate how the SPDs are connected and remove ALL ground reference connections or they will very briefly attempt to be the Wye point of your entire system, until they vaporize and cause collateral damage in the process.

Bottom line if you have a Delta power system, put Delta-Wye isolation transformers ahead of any power electronics. You'll find though that it is far far cheaper to just use a Wye system to start with. Power electronics are everywhere now.
 
GoldDigger said:
Where he is going, I think, is that if POCO changed an existing service, changes in wiring would be necessary.
And if a new site is installed, wiring the same way as an existing site will be close but probably no cigar.
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GoldDigger said:
But the motors will not know the difference. :)
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Correct.. sort of. If the three wires powering it are 480v 3Ø 3W, the motors will not know the difference. But getting this to happen isn't always going to be easy peasy.

Let's say the service gets changed overnight from 480V 3Ø 3W corner-grounded delta (grounded conductor white/gray and no inline OCPD) to a 480/277V 3Ø 4W configuration, you certainly cannot just hook up this new system to the existing without a major change in equipment and wiring.
:happyno:;)
 
iwire said:
So let's make up odd situations that won't happen in real life just to give reason to nit pick. :D
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The POCO didn't change the source overnight or without warning, but I have seen this at least five times where a delta existed and the POCO changed to a wye. Usually at the request of their customer. I have always said the motor doesn't care, just give me the volts on my dataplate. Yes, as some of you have said, it won't be easy or cheap but that wasn't my question. I read a thread here that talked about delta run and wye start. I didn't understand what they were talking about. I had not seen a motor dataplate like that before. It was 200 volts, so maybe outside the USA for sure but it was Ingersoll Rand. I know right now there is a customer that is served by the POCO with ungrounded wye primary - corner ground 480 volt delta secondary, three transformers totaling 1000kVA. The plant maintenance manager has said, " That delta will go away and I will get wye or I will go away. I will not continue to be served by the corner grounded delta." The POCO is preparing to make that change.
 
11bgrunt said:
The POCO didn't change the source overnight or without warning, but I have seen this at least five times where a delta existed and the POCO changed to a wye. Usually at the request of their customer.
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Always at the requests of the customer or the power company would have to pay for any required changes.


I have always said the motor doesn't care, just give me the volts on my dataplate. Yes, as some of you have said, it won't be easy or cheap but that wasn't my question. I read a thread here that talked about delta run and wye start. I didn't understand what they were talking about.
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Delta run, wye start has to do with the motor configuration not the source. A delta run, wye start motor will be supplied by 3 phase from either delta or wye the motor will not care.
 
iwire said:
So let's make up odd situations that won't happen in real life just to give reason to nit pick. :D
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:lol: Looks like you picked the wrong day to take that one-in-a-million chance...
11bgrunt said:
... I know right now there is a customer that is served by the POCO with ungrounded wye primary - corner ground 480 volt delta secondary, three transformers totaling 1000kVA. The plant maintenance manager has said, " That delta will go away and I will get wye or I will go away. I will not continue to be served by the corner grounded delta." The POCO is preparing to make that change.
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Smart $ said:
... Let's say the service gets changed overnight from 480V 3Ø 3W corner-grounded delta (grounded conductor white/gray and no inline OCPD) to a 480/277V 3Ø 4W configuration, you certainly cannot just hook up this new system to the existing without a major change in equipment and wiring.
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Electric motors are color-blind; they're going to run the same no matter what color the third-phase insulation is.

I have seen 480-volt motors on corner-grounded delta service that not only don't have an OCPD on the third phase, they didn't even switch it -- the motor starters had only two line contacts. (likely installed between 1941-1945 to conserve resources) That could be a nasty surprise: not only would the white wire be hot, it'd remain hot when the switch is turned off, locked out & tagged out.

I think every panel changed from CGΔ to wye should be labeled with a sign warning "This panel was originally wired as corner-grounded delta. Vestigial hazards may still exist."
 
drcampbell said:
...
I have seen 480-volt motors on corner-grounded delta service that not only don't have an OCPD on the third phase, they didn't even switch it -- the motor starters had only two line contacts. ...
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That would be code compliant even today. However you would be required to have an overload device looking at the grounded conductor current under current codes.
 
drcampbell said:
..That could be a nasty surprise: not only would the white wire be hot, it'd remain hot when the switch is turned off, locked out & tagged out.

I think every panel changed from CGΔ to wye should be labeled with a sign warning "This panel was originally wired as corner-grounded delta. Vestigial hazards may still exist."
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don_resqcapt19 said:
That would be code compliant even today. However you would be required to have an overload device looking at the grounded conductor current under current codes.
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While switching only two poles might be compliant, there'd be major problems with OCPD in the "now" ungrounded conductor, disconnecting means disconnecting all ungrounded conductors, and of course the LOTO process.

Where conductors are larger than #6, running around and changing out all the white or gray phase tape wouldn't be a huge burden... but if there's a lot of #6 and smaller white- or gray-insulated wire.... need I say more?

So we're back to my comment "you certainly cannot just hook up this new system to the existing without a major change in equipment and wiring."
 
Smart $ said:
While switching only two poles might be compliant, there'd be major problems with OCPD in the "now" ungrounded conductor, disconnecting means disconnecting all ungrounded conductors, and of course the LOTO process.

Where conductors are larger than #6, running around and changing out all the white or gray phase tape wouldn't be a huge burden... but if there's a lot of #6 and smaller white- or gray-insulated wire.... need I say more?

So we're back to my comment "you certainly cannot just hook up this new system to the existing without a major change in equipment and wiring."
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I have never seen a 3 phase corner grounded system that used a white wire....yes I am well aware that code requires that, but have never seen it in the real world.
 
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