transformer

Hi irfan, the reason for change in the curent from primary to the secondary is due to the principle of conservation of energy. since Transformer is the passive device except little loss due to the Hysterysis in the core, the output power is nearly equal to the input power. Accordingly Io=( Vin / Vo)* Iin. In stepup transformer Vin/Vo is < 1. Therefore Io is < Iin. In stepdown I o > Iin. In simple issolation transformers Io=Iin.

I'd say same magnetic field cross the primary wiring, cross the secondary too, being the "voltage" in each loop proportional to the speed of change of the magnetic field dB/dt (supose same size for every loop),you think those are like batteries in serie conection.So will result V1/V2=n1/n2.Aditional hypo is no energy leakings so V1*I1*COS A1=V2*A2*COS A2, A second aditional hypo to complete an ideal transformer is COS A1=COS A2. So don't think no energy losses=ideal transformer.After that just rest to solve a basic problem V1=E, where E is a "f.e.m."or "voltage source of generator".Let be now i2*r=V2=n2V1/n1=(n2/n1)*E, THEN i2=(n2/n1)E/r.Since we said E*I1=V2*i2, I1=V2*i2/E=((n2/n1)*E*(n2/n1)*E/r)/E=((n2/n1)**2)*E/r.So is like in the primary circuit you have a resistor r1=((n1/n2)**2)*r.This is an important result and tell you the impedance in the entrance is (n1/n2)**2 times the impedance in the check out used for example in audio check out transformers calculations.(correct my english please..).-

The basic idea is the conservation of energy. Energy into the transformer equals the energy out of the transformer. (Remember, not all of the energy that leaves the transformer is electrical energy, but as heat. For many applications this is not significant.) When you get down to the real nitty gritty details how that energy gets conserved, you have to get into the three dimensional tensor equations simplified by Maxwell. These equations now bare his name.

Current change is proportional to voltage change. There are 3 main reasons why transformers are used.

1) Voltage change step up or down

2) isolation of a electrical system

3) to add or decrease impediance to a electrical system

The KVA rating is equal Voltage x current amps (1phase)

or KVA = Volts X Amps x 1.732 (3phase)

Most (not all) transformers step down voltage which leads to increased current availablity on the secondary side.