Help!! Delta... 3 phase 240v!!

TNGuy81

Member
ok, figured it out.

ok, figured it out.

This was not the main panel it was a sub panel(someone asked). I read it wrong being this is just one of a number of other projects im working on at this plant. Its 4 wire, b phase reading is 240v, A-B= 240, B-C= 240v, A-C= 240v, A- neutral = 120, b- neutral =240, c-neutral 120v. I never looked at main im assuming its wye, than hit a transformer to delta sub. Anyway, wired motor low voltage terminals and all worked just fine. Many thanks to all for the time and info. And no, I really didn't have a vast grasp on the delta system but now, I have a better understanding. It was marked black, orange, BLUE also.
 
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ActionDave

Moderator
Staff member
Anyway, wired motor low voltage terminals and all worked just fine. Many thanks to all for the time and info. And no, I really didn't have a vast grasp on the delta system but now, I have a better understanding. It was marked black, orange, BLUE also.
Were you nervous when you threw the switch? :)
 

jim dungar

Moderator
Staff member
This was not the main panel it was a sub panel(someone asked). I read it wrong being this is just one of a number of other projects im working on at this plant. Its 4 wire, b phase reading is 240v, A-B= 240, B-C= 240v, A-C= 240v, A- neutral = 120, b- neutral =240, c-neutral 120v. I never looked at main im assuming its wye, than hit a transformer to delta sub. Anyway, wired motor low voltage terminals and all worked just fine. Many thanks to all for the time and info. And no, I really didn't have a vast grasp on the delta system but now, I have a better understanding. It was marked black, orange, BLUE also.


It is physically impossible for the B-N reading to be 240V when the A-N and N-C readings are 120V.
As mentioned earlier the nominal B-N voltage must be 208V.
 

ggunn

PE (Electrical), NABCEP certified
This was not the main panel it was a sub panel(someone asked). I read it wrong being this is just one of a number of other projects im working on at this plant. Its 4 wire, b phase reading is 240v, A-B= 240, B-C= 240v, A-C= 240v, A- neutral = 120, b- neutral =240, c-neutral 120v.
There is still something wrong. If A-B, B-C, and C-A are all 240V, and A-N and C-N are both 120V, then B-N cannot be 240V. This is what we have been saying all along.

If N is a center tap between A and C, then B-N must be 208V. If N is the center of a wye, then A-N, B-N, and C-N all must be 138.6V. The only way that B-N can be 240V is if the system is corner grounded, and in that case there is no neutral, and either A or C is 0V to ground.

Or am I missing something?
 

roger

Moderator
Staff member
Its 4 wire, b phase reading is 240v, A-B= 240, B-C= 240v, A-C= 240v, A- neutral = 120, b- neutral =240, c-neutral 120v.
Let me suggest that you get another meter, what you are posting is impossible period.


Roger
 

TNGuy81

Member
..

..

Never had a problem with my meter. Those are the numbers, motor works. This building was built back in the 40's and has had much work of who knows what added and we and continuing to add. I understand the math doesn't add up but, those are the readings. My boss put his fluke on it and same reading as my ideal. Also the 120v branches work correctly on A and C phase. Not sure on the hows and either, though in not a master electrician either. Thanks again
 

TNGuy81

Member
perhaps??

perhaps??

http://en.m.wikipedia.org/wiki/High-leg_delta

corner grounded as stated before..?..
 
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ggunn

PE (Electrical), NABCEP certified
Never had a problem with my meter. Those are the numbers, motor works. This building was built back in the 40's and has had much work of who knows what added and we and continuing to add. I understand the math doesn't add up but, those are the readings. My boss put his fluke on it and same reading as my ideal. Also the 120v branches work correctly on A and C phase. Not sure on the hows and either, though in not a master electrician either. Thanks again
The only way I can think of that you could be getting anything like those readings is if you have a high leg 240V system with a corner grounded instead of the center tap between A and C, but if that is the case your neutral will have 120V on it relative to ground, which would be exceedingly dangerous. Also, you'd have to be reading the B phase relative to ground (not the "neutral") and the A and C phases relative to the "neutral".

Something is definitely screwy and your measurements make no sense.

Measure voltage to ground (not neutral) from A, B, C, and N and tell us what you see. And what color is that fourth wire?
 

ggunn

PE (Electrical), NABCEP certified
You can not have a corner grounded and a grounded center tap (high-leg)at the same time.
If you have 120V L-G measurements then it is not corner grounded.
What if he has a high leg transformer with a corner grounded instead of the center tap? His 120V loads would still work but he would have 120V on the "neutral". His readings would make sense except that he would have to be reading B relative to ground while measuring A and C relative to the "neutral".
 

roger

Moderator
Staff member
What if he has a high leg transformer with a corner grounded instead of the center tap? His 120V loads would still work but he would have 120V on the "neutral". His readings would make sense except that he would have to be reading B relative to ground while measuring A and C relative to the "neutral".
No, with 240 volt windings you would read 240 volts from the ends of the windings to the grounded phase. A Delta with a high leg is simply a set of two or three windings connected in a Delta with one center tapped.

Reading from point to point will give you full voltage of the winding being read.

If you were to read half of any winding you would read 120 volts, but that brings center tapping back into the show.

Roger
 

jim dungar

Moderator
Staff member
What if he has a high leg transformer with a corner grounded instead of the center tap? His 120V loads would still work but he would have 120V on the "neutral". His readings would make sense except that he would have to be reading B relative to ground while measuring A and C relative to the "neutral".
Which is why I talked specifically said L-Ground measurements of 120V.
 

kwired

Electron manager
What if he has a high leg transformer with a corner grounded instead of the center tap? His 120V loads would still work but he would have 120V on the "neutral". His readings would make sense except that he would have to be reading B relative to ground while measuring A and C relative to the "neutral".
There is a possibility, the center tap would then be a floating ungrounded conductor , would still have 208 volts to B phase and 120 to A and C.

If you measured each conductor to ground you would have 240 on A and C, 0 on B, and 208 on the center tap.
 

kwired

Electron manager
This was not the main panel it was a sub panel(someone asked). I read it wrong being this is just one of a number of other projects im working on at this plant. Its 4 wire, b phase reading is 240v, A-B= 240, B-C= 240v, A-C= 240v, A- neutral = 120, b- neutral =240, c-neutral 120v. I never looked at main im assuming its wye, than hit a transformer to delta sub. Anyway, wired motor low voltage terminals and all worked just fine. Many thanks to all for the time and info. And no, I really didn't have a vast grasp on the delta system but now, I have a better understanding. It was marked black, orange, BLUE also.
Motor worked just fine because it still sees 240 volts with 120 degree phase displacement between each input lead - it doesn't care what is grounded and what isn't grounded just wants correct input voltage.

Do you know this source is coming directly from a delta secondary? Maybe you have some phase conversion equipment involved throwing voltages off, though from my experiences I would expect even stranger readings then you have with phase conversion equipment, or maybe an autotransformer in the supply somewhere could also give unexpected readings, still is grasping at straws though as it should throw off more then just the one voltage reading.
 

ggunn

PE (Electrical), NABCEP certified
There is a possibility, the center tap would then be a floating ungrounded conductor , would still have 208 volts to B phase and 120 to A and C.

If you measured each conductor to ground you would have 240 on A and C, 0 on B, and 208 on the center tap.
What if A or C were the grounded phase? If he measured B to ground and A and C to the floating (actually it wouldn't be floating but 120V relative to ground) center tap, wouldn't he see the voltages he reported? I'm grasping at straws here trying to build a scenario that fits the allegedly observed facts.
 

ggunn

PE (Electrical), NABCEP certified
What if A or C were the grounded phase? If he measured B to ground and A and C to the floating (actually it wouldn't be floating but 120V relative to ground) center tap, wouldn't he see the voltages he reported? I'm grasping at straws here trying to build a scenario that fits the allegedly observed facts.
<stupid comment deleted>
 
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