Electromagnetic Induction (A2)

Np = number of turns in the primary coil
Ns = number of turns in the secondary coil
Es/ Ep = Ns/ Np
Vs/ Vp  = Ns/ Np

Case I
If Ns >Np
Then, Vs > Vp
i.e. the voltage increases and the transformer is called step up transformer.

Case II
If Ns < Np
Then, Vs < Vp
i.e. the voltage decreases and the transformer is called step down transformer.

If the transformer is 100 % efficient
Input Power= Output Power
Vp* Ip = Vs* Is
Vs/ Vp  = Ip/ Is
Therefore, Vs/Vp = Ns/ Np = Ip/ Is

Transformer’s Energy loses: Although transformers are very efficient devices small energy losses do occur in them due to the four main causes
  1. Resistance of the windings:
The copper wire used for the windings has resistance and so heat losses occur. These are minimized by using thick wire.
  1. Hysteresis
The magnetization of the core is repeatedly reversed by the alternating magnetic field. The resulting expenditure of energy in the core appears as heat. It can be minimized by using a magnetic material which has a low hysteresis loss.
  1. Edd current
The alternating magnetic flux induces Edd currents in the iron core and causes heating. The effect is reduced by having a laminated core.
  1. Flux leakage
The flux due to the primary may not all links the secondary coil if the core is badly designed or has air gas in it. Very large transformers have to be oil cooled to prevent over heating.

The supply to the primary coil must be A.C. not D.C. Explain why?

The magnetic flux in the core must be changing to induce an e.m.f. in the secondary coil, which is possible in case of alternating current. If D.C. is used no flux change takes place and hence no induction will take place.
For constant input power the output current must decrease if the output voltage increases.