How does a transformer work?

[JustStartingOut]

Foo

I find Apaches way of thinking helpful.

If you assume that the transformer is 100% efficient (we know this isn't reality) then the electrical power in the primary is equal to the electrical power in the secondary. Power (primary) = Power (secondary)

We know Power=Current x Voltage P=IV, so the following is true:

Ip x Vp = Is x Vs

I(primary) x V(primary) = I(secondary) x V (secondary)

So if this is a step-down transformer, the smaller that V(secondary) gets, the larger I(secondary) has to be so that the power in the secondary remains constant and equal to the power in the primary.

For example if I(primary)=65A and V(primary)=1000V we can then have the following results depending on how much the secondary voltage is changed by.

Step-Up

V(secondary)=10,000V

I(secondary)=6.5A

Step-Down

V(secondary)=100V

I(secondary)=650A

V(secondary)=10V

I(secondary)=6500A