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Transformers

Transformers allow AC voltage to be stepped up or down (and current to go down or up

respectively).

Two or more coupled coils on a magnetic core. I will draw the core as a square, with the coils on

the two vertical limbs, but in reality, the two coils are often wound one on top of the other.

One winding will be called the primary (1) and the other the secondary (2). Sometimes, winding

are labelled with "X" for the low voltage winding, and "H" for the higher voltage winding. Ther

are N1 turns on the primary, and N2 turns on the secondary.

Each winding has a polarity mark. The voltage will have its positive referred to this mark, and

current will flow into the polarity mark. In many power systems applications, the secondary

current is treated as flowing out of the secondary winding polarity mark.

Ideal Transformer Relations

We say transformer is ideal is there is no winding resistance, the core is a perfect magnetic material.

V1/V2 = N1/N2  or V1/N1= V2/N2

and

N1.I1 =N2.I2  or  N1.I2 + N2.I2 =0  or I1/I2 = N2/N1

If we treat I2 as flowing out of the polarity mark, then the negative sign goes away.

Combining these: S1 =V1. I1^*

                              S2 =V2 .I2  -= * V1. I1 *