# Understand AC Transformer Polarity for Three-Phase Connections

• Leyden
In summary, the terminals on a transformer are labeled H, X, T, U, V, W in the order of decreasing voltage.
Leyden
my transformer book does not cover an area i am interested in and was wondering if someone would direct me to literature that would help me understand something and if you wouldn't mind trying to explain it to me yourself that would also be appreciated.

you have three single phase transformers connected for three-phase, you have to connect the primary properly per there polarity. is this because if hooked up with opposing polarity the magnetic fields will negatively effect each other and lower the impedance/inductive reactance?

any information or direction is greatly appreciated, if your not sure of the answer but have a theory please post it while including something to the affect of " i believe it is that...".thanks,
another unimportant question
is there a reason why primaries are H, secondaries X, motor leads T, U, V, W other than they have to be called something?

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Can you give a diagram of what you are actually proposing?

Did you try a search ?
https://www.electronics-tutorials.ws/transformer/three-phase-transformer.html
Leyden said:
you have three single phase transformers connected for three-phase, you have to connect the primary properly per there polarity...
You connect them per polarity marking for the sake of orderliness. The guy behind you will expect to find them hooked up according to convention.
...is this because if hooked up with opposing polarity the magnetic fields will negatively effect each other and lower the impedance/inductive reactance?
Each transformer's magnetic field is almost entirely constrained to inside its respective core.

.

Teslascience, Leyden, sophiecentaur and 2 others
jim hardy said:
https://www.electronics-tutorials.ws/transformer/three-phase-transformer.html
Nice reference. Thanks Jim.

jim hardy
Leyden said:
is there a reason why primaries are H, secondaries X, motor leads T, U, V, W other than they have to be called something?
From: https://www.schneider-electric.us/en/faqs/FA108808/
"The H terminals are always on the coil with the highest maximum line-to-line voltage, and the X terminals are always on the lower, regardless of which coil is designed to be connected to the source. Per the NEC, the side of the transformer connected to the source is always the primary, regardless of the design of the transformer."

"Here is a quote from a textbook that I used during my MS-EE degree program (Turan Gonen, �Electric Power Distribution System Engineering,� McGraw-Hill, 1986.):

�According to NEMA and ASA standards, the higher voltage winding is identified by HV or H, and the lower-voltage winding is identified by LV or x. Transformers with more than two windings have the windings identified by H, x, y, and z, in the order of decreasing voltage.�"

Cheers,
Tom

Teslascience, Leyden and jim hardy
Tom.G said:
From: https://www.schneider-electric.us/en/faqs/FA108808/
"The H terminals are always on the coil with the highest maximum line-to-line voltage, and the X terminals are always on the lower, regardless of which coil is designed to be connected to the source. Per the NEC, the side of the transformer connected to the source is always the primary, regardless of the design of the transformer."

"Here is a quote from a textbook that I used during my MS-EE degree program (Turan Gonen, �Electric Power Distribution System Engineering,� McGraw-Hill, 1986.):

�According to NEMA and ASA standards, the higher voltage winding is identified by HV or H, and the lower-voltage winding is identified by LV or x. Transformers with more than two windings have the windings identified by H, x, y, and z, in the order of decreasing voltage.�"

Cheers,
Tom
step-up xfrm's use H for lower voltage side(primary)

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jim hardy said:
Did you try a search ?
https://www.electronics-tutorials.ws/transformer/three-phase-transformer.html
You connect them per polarity marking for the sake of orderliness. The guy behind you will expect to find them hooked up according to convention.
Each transformer's magnetic field is almost entirely constrained to inside its respective core.

.
i have been told by a very experienced (50 years in field believe it or not) tech, that it will cause problems and burn up the xfmr's

sophiecentaur said:
Can you give a diagram of what you are actually proposing?
-----H1-H2-------H2-H1-------H1-H2------ middle xfmr is backwards

Leyden said:
i have been told by a very experienced (50 years in field believe it or not) tech, that it will cause problems and burn up the xfmr's

What will ?

Hi again, @Leyden,
Leyden said:
is there a reason why primaries are H, secondaries X, motor leads T, U, V, W other than they have to be called something?
I finally realized that I overlooked that question earlier, so here are a couple links that may help. The first one refers to the standard IEC 60034-8 "Terminal markings and direction of rotation". It also mentions wiring/labelling of 3-phase transformers.
The second link is a bit longer and mentions several different 'standards' for labelling the phases.

The above were found with: https://www.google.com/search?source=&q=r+s+t+electrical+phases

Cheers,
Tom

Leyden and Asymptotic
jim hardy said:
What will ?
being reverse polarity of the other two transformers, he didn't go into the science of it. not that i think he would know that anyway.

That is not 3 phase - that is three transformers connected in parallel, that causes other issues if you also connect the secondaries in parallel.

E> Came back to say that all three phase systems shoud be drawn in a vector diagram, and in a way realized one way to settle the classic split-phase vs "2 phase" debate - IMO - If you draw a vector diagram and it is a straight line... it is single phase.

As usual - it depends. Here is one connection diagram:

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Leyden
That is not 3 phase - that is three transformers connected in parallel, that causes other issues if you also connect the secondaries in parallel.

E> Came back to say that all three phase systems shoud be drawn in a vector diagram, and in a way realized one way to settle the classic split-phase vs "2 phase" debate - IMO - If you draw a vector diagram and it is a straight line... it is single phase.

delta 3 phase

---a phase--H1(xfmr 1)H2---b phase----H2(xfmr 2)H1----c phase---H1(xfmr 3)H2----a phase--
middle xfmr is backwards

The middle leg of the transformer connected backwards is only done with a 3 phase shell type xfmr. By doing so the flux density is reduced by a factor of sqrt 3.
With 3 legged E core type 3 phase xfmr, the 3 windings are wound in the same direction, no need to reverse polarity.
Same for 3 single xfmrs connected for 3 phase. There is a code for polarity & phase. In North America, wye-wye, & delta-delta xfmr must have 0 degree phase from primary to secondary.
But for wye-delta & delta-wye xfmr, the high voltage side shall lead the low voltage side by 30 degrees phase. This has beern to standard practice for a very long time.
Did I help?

Claude

Leyden, Tom.G and jim hardy
cabraham said:
The middle leg of the transformer connected backwards is only done with a 3 phase shell type xfmr. By doing so the flux density is reduced by a factor of sqrt 3.
With 3 legged E core type 3 phase xfmr, the 3 windings are wound in the same direction, no need to reverse polarity.
Same for 3 single xfmrs connected for 3 phase. There is a code for polarity & phase. In North America, wye-wye, & delta-delta xfmr must have 0 degree phase from primary to secondary.
But for wye-delta & delta-wye xfmr, the high voltage side shall lead the low voltage side by 30 degrees phase. This has beern to standard practice for a very long time.
Did I help?

Claude

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• 20180226_134621.jpg
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jim hardy
cabraham said:
The middle leg of the transformer connected backwards is only done with a 3 phase shell type xfmr. By doing so the flux density is reduced by a factor of sqrt 3.
With 3 legged E core type 3 phase xfmr, the 3 windings are wound in the same direction, no need to reverse polarity.
Same for 3 single xfmrs connected for 3 phase. There is a code for polarity & phase. In North America, wye-wye, & delta-delta xfmr must have 0 degree phase from primary to secondary.
But for wye-delta & delta-wye xfmr, the high voltage side shall lead the low voltage side by 30 degrees phase. This has beern to standard practice for a very long time.
Did I help?

Claude
thanks for the input and information, but I'm asking about 3 single phase transformers. do you think the flux density or any impedance would be reduced in a scenario with 3 single phase transformers? I'm not asking if there is a reason to do it, I'm asking if there is a problem if it is accidentally done. i have been told there is (but with no scientific reasoning)

thanks

I think there is no scientific reason for it, because each transformer has it's own independent closed magnetic fluxes and in that case only the stray fluxes can interact with each other which can't be so significant to cause a great disturbing effect on each's operations.
The only reason for it - I suppose from my practice - that there can be an insulation problem and that's why one shall connect they in the right polarity. Of course I suppose that all the transformers are identical.
(If I used some words not correctly please forgive me I'm not a native speaker of english.)

Leyden

## 1. What is the purpose of understanding AC transformer polarity for three-phase connections?

Understanding AC transformer polarity is crucial for maintaining the correct phase relationship between the primary and secondary windings. This ensures that the transformer operates efficiently and safely, and prevents equipment damage.

## 2. How do I determine the polarity of a three-phase transformer?

The polarity of a three-phase transformer can be determined by using a polarity tester or by conducting a polarity test. This involves connecting the primary and secondary windings of the transformer in a specific configuration and measuring the voltage output to determine the polarity.

## 3. Can the polarity of a three-phase transformer be reversed?

Yes, the polarity of a three-phase transformer can be reversed by swapping the connections of any two of the three phases. However, it is important to consult the manufacturer's instructions before attempting to reverse the polarity as it may not be recommended for all transformers.

## 4. What happens if the polarity of a three-phase transformer is incorrect?

If the polarity of a three-phase transformer is incorrect, it can cause the transformer to overheat, which can lead to equipment failure and potential safety hazards. It can also result in incorrect voltage outputs and cause damage to connected equipment.

## 5. Are there any safety precautions to take when dealing with AC transformer polarity for three-phase connections?

Yes, it is important to follow all safety precautions when working with AC transformers, including wearing appropriate protective gear and ensuring that the transformer is de-energized before conducting any tests or making any changes to the wiring. It is also recommended to consult a professional electrician if you are unsure about the polarity of a three-phase transformer.

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