Transformer polarity

[xformer]

Forgive my ignorance, but what is the reason for transformers 200KVA or less having voltages of 7500 or less have additive polarity, while all others have subtractive polarity?

 

[jbelectric777]

What do you mean by "additive" polarity ? Polarity is a "method" of connection, it is descriptive of flow. Direct current and Alternating current both can have proper and/or reverse polarity. Are you sure your using the correct terminology ? Are you refering to the connection of line and load for backfeeding ? Like in the case where a transformer works both ways i.e 277/480 step down to 120/208 will work in reverse by input of 120/208 to get high output off 277/480 ? They are not UL listed for those applications, electricians do that in a pinch only until they can get the right transformer.

 

[jim dungar]

Forgive my ignorance, but what is the reason for transformers 200KVA or less having voltages of 7500 or less have additive polarity, while all others have subtractive polarity?

Tradition.

That's the way 'grandpa' did it, so now we are stuck.

 

[ohmhead]

Transformer polarity

READ THIS www.kilowattclassroom.com/Archive/TransfPol.pdf

 

[LarryFine]

It seems to me that knowing the polarity doesn't matter, almost like 3-phase rotation, as long as all transformers in the group have the same polarity and you wire them consistently.

 

[jcormack]

It seems to me that knowing the polarity doesn't matter, almost like 3-phase rotation, as long as all transformers in the group have the same polarity and you wire them consistently.

It is important when using three single phase cans to make a three-phase bank, or when replacing an old can of a three-phase bank. You need to check the polarity marks (dots on diagram) for both primary & secondary to make sure they are the same.

 

[broadgage]

For a single transformer, or a single 3 phase bank of transformers, the polarity is of no importance.
If however a transformer is required to operate in parralell with others, all MUST be of the same polarity.

In theory, "wrong" polarity may be corrected by altering the external connections, but only if each end of each winding is available, in practice most transformers have some internal connections that cant be altered by the installer.

UK 3 phase transformers normally only have 4 terminals on the LV side.
Neutral, and phases A, B, C.
Incorrect phase rotation is easily rectified by swapping any two.

Incorrect polarity can only be rectified by altering which ends of the three windings are connected together as the neutral.
If these connections are internal and sealed, as is the norm, then it cant be altered.
In stand alone use, it matters not, but 3 phase transformers of different polarity can never be paralelled.

"Universal" transformers used to be manufactured that could be connected for either polarity. They were costly items since they required 6 output bushings insulated for line voltage, instead of 3.
The transformer windings were also more costly since thicker insulation was required at each end of each winding in order to withstand switching surges, rather than at one end only.

For his reason British standards stipulate a standard polarity for all power transformers.
Standards in other countries may differ, but the basic principle is the same.

 

[BJ Conner]

TRansformer Reference

TRansformer Reference

Here is a good reference for transformer connections and polarity.

http://www.cooperpower.com/library/pdf/r201902.pdf

Somewhere in stone age electricity transformers wern't marked with H1,H2,X1,X2 etc. Polarity was important then. :roll:

 

[jim dungar]

Somewhere in stone age electricity transformers wern't marked with H1,H2,X1,X2 etc. Polarity was important then. :roll:

Polarity is still important, it tells you the relationship between H1 and X1.

 

[BJ Conner]

Your Right

Your Right

Polarity is still important, it tells you the relationship between H1 and X1.

IT's just easier now.