Does transformer polarity change secondary rotation?

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11bgrunt

11bgrunt

Pragmatist
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Electric Utility Reliability Coordinator
Utility tansformer bank question.
3 transformers existing in service on a pole.
240 delta secondary. All 3 transformers are additive polarity.
Take them down dead and replace all 3 with larger transformers, but all new transformers are subtractive polarity.
Connections on both banks are identical. Nothing changes but polarity.
Does the 3 phase secondary rotation change?
 
The polarity will not affect the internal construction of the transformer windings but only with the routing of leads to the bushings.

I am ignorant of the secondary rotation, is this the same as angular displacement between the primary and secondary?

If your transformer are connected with subtractive polarity where correspondingly marked terminals for the primary and secondary windings opposite each other, I don't see how that would affect any rotation.
 
11bgrunt said:
Utility tansformer bank question.
3 transformers existing in service on a pole.
240 delta secondary. All 3 transformers are additive polarity.
Take them down dead and replace all 3 with larger transformers, but all new transformers are subtractive polarity.
Connections on both banks are identical. Nothing changes but polarity.
Does the 3 phase secondary rotation change?
Click to expand...
The following is true of distribution transformers regardless of polarity...

What goes into H1 goes out of X1 ? the voltage at the X1 bushing
is in-phase with the voltage at the H1 bushing.

H1 is the left primary bushing.

X1 is the right secondary bushing.
 
Smart $ said:
The following is true of distribution transformers regardless of polarity...

What goes into H1 goes out of X1 ? the voltage at the X1 bushing
is in-phase with the voltage at the H1 bushing.

H1 is the left primary bushing.

X1 is the right secondary bushing.
Click to expand...

Transformer data plates agree when polarity is additive.
With Subtractive polarity X1 is the left secondary bushing, drawn under H1.
 
shamsdebout said:
The polarity will not affect the internal construction of the transformer windings but only with the routing of leads to the bushings.

I am ignorant of the secondary rotation, is this the same as angular displacement between the primary and secondary?

If your transformer are connected with subtractive polarity where correspondingly marked terminals for the primary and secondary windings opposite each other, I don't see how that would affect any rotation.
Click to expand...
The apparent paradox of the question is what happens to a connected motor when this change is made, with no other changes, such that each secondary voltage with respect to a fixed chosen secondary lead now appears to be opposite in phase to the primary voltage. Consider two different ways of analyzing the problem:

1. If you shift the phase of all secondary voltages (relative to some constant reference signal) by 30 degrees, it is clear that neither the amplitude nor the phase rotation direction change. Now increase the phase change to 60 degrees, then 90 and so on. Eventually you reach a phase change of 180 degrees on all three phase leads and yet the direction of motor rotation has not changed. Seems simple and very hard to misinterpret, therefore probably correct.

2. Alternatively, we know that reversing the position of two phase leads as connected to the motor will reverse the direction of rotation. Reverse a second pair and you will go back to the original rotation. Make a third reversal and you are in opposite rotation again. The argument now goes that reversing the polarity of all three phase leads is equivalent to interchanging leads three times, and so should cause opposite rotation. I know that this argument is wrong, and the problem now becomes to determine what is wrong with the model I just started out with. :)
I have some pretty clear ideas about this, in particular the realization that if I switch the polarity of one secondary winding it is NOT the same as reversing two of the leads connected to the motor (and quite spectacular in result). Let's see what others have to say!
(As math textbooks often say, the proof is left as an exercise for the reader.) :)
 
11bgrunt said:
Transformer data plates agree when polarity is additive.
With Subtractive polarity X1 is the left secondary bushing, drawn under H1.
Click to expand...
Upon checking a second source, I have two contradicting sources. The second one I checked is aligned with yours and the one I consider more authoritative. Should have checked it first. :ashamed1:

"What goes into H1 goes out of X1 ? the voltage at the X1 bushing is in-phase with the voltage at the H1 bushing" supposed to hold true regardless.
 
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Smart $ said:
Connections identical in what manner... same bushing markings (X1 vs. X2) or same orientation (i.e. left vs. right).
Click to expand...

Orientation. Wire to bushing lug position remains the same. No reference to X1. If it came from the left bushing in the old pot, put it back in the left bushing of the new pot.

Drawing the vectors is looking like the way to simply explain why polarity does not change the rotation when all three transformers are the same.
Working with meter techs in the past, I remember if the CTs got polarity wrong, the meter would turn backwards.
Maybe it is something totally different.
 
Motor Phase Rotation

Motor Phase Rotation

I was just wondering if the motors in question ran backwards....?
 
StarCat said:
I was just wondering if the motors in question ran backwards....?
Click to expand...

Yes. Customer reported three days later that he made changes in the plant to correct rotation.
This discussion is to gather information to teach why it happened.
 
11bgrunt said:
Orientation. Wire to bushing lug position remains the same. No reference to X1. If it came from the left bushing in the old pot, put it back in the left bushing of the new pot.

Drawing the vectors is looking like the way to simply explain why polarity does not change the rotation when all three transformers are the same.
Working with meter techs in the past, I remember if the CTs got polarity wrong, the meter would turn backwards.
Maybe it is something totally different.
Click to expand...
Yes, I had a job where I sale a 2300v synchronous motor starter for a compressor in a remote part of a plant where the the phases were reversed. The starter faulted out on reverse phase. I worker with my design engineers to revise the starter wiring and then made the recommending wiring changes myself to get the starter to see the right phase rotation. It was simpler to do that than to reconnect the line conductors.
Incidently, the motor may have been 3' long and at least 6' in diameter as I can recall. It had open connecting rods driving the compressor.
 
11bgrunt said:
Orientation. Wire to bushing lug position remains the same. No reference to X1. If it came from the left bushing in the old pot, put it back in the left bushing of the new pot.
Click to expand...
Were the transformers re-hung the same, like on the same side of the pole and oriented the same? If the transformers were hung differently, it might look the same but might not really be the same. If the rotation changed and the secondaries were hooked up the same then the primary had to change.


11bgrunt said:
Working with meter techs in the past, I remember if the CTs got polarity wrong, the meter would turn backwards.
Maybe it is something totally different.
Click to expand...
Same general idea, but the CTs are modifying only the currents (the current was changed but the voltage wasn't).
 
StarCat said:
I was just wondering if the motors in question ran backwards....?
Click to expand...

Thank you to everyone that contributed information.

When I started this thread, the information I had to work with originally was provided and I wanted to see how the community responded.
I made the error that affected the rotation.
At that job, after reviewing transformer data plates and drawing the H1s and X1s, I recommended that the primary rotation serving only this bank be changed to offset the perceived rotation change caused by the difference in polarity between the two banks.
The resulting change in rotation showed that if all else was exactly the same between the two banks, then the change in polarity would not have caused a change in the secondary rotation.
Hard lesson for me but at the time what I thought was the right thing to do.:ashamed:
 
Last edited:
11bgrunt said:
Thank you to everyone that contributed information.

When I started this thread, the information I had to work with originally was provided and I wanted to see how the community responded.
I made the error that affected the rotation.
At that job, after reviewing transformer data plates and drawing the H1s and X1s, I recommended that the primary rotation serving only this bank be changed to offset the perceived rotation change caused by the difference in polarity between the two banks.
The resulting change in rotation showed that if all else was exactly the same between the two banks, then the change in polarity would not have caused a change in the secondary rotation.
Hard lesson for me but at the time what I thought was the right thing to do.:ashamed:
Click to expand...

The guys that I know to check rotation they would take a broom stick handle an actually push the contactor in manually which essentially bumps the the motor in order to see which direction the motor is going to rotate. A bit crude but it worked.
Remember that with hard lessons we have all been there and done that in one manor or another. Anyone who hasn't has to a lier or from another planet.
 
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