Transformer The working principle of a transformer The magnetic field produced by coil A varies at the same frequency with the a.c. source. An a.c of the same frequency is induced in coil B. coil A coil B a.c. source galvanometer

The structure of a transformer soft iron core secondary coil a.c. source primary coil output Almost all the magnetic field lines produced by primary coil guide through the secondary coil.

Changing voltage input voltage number of turns in secondary coil number of turns in primary coil output voltage The output voltage depends on the number of turns in the primary and secondary coils.

number of turns in the primary coil = number of turns in the secondary coil 1 input voltage = output voltage number of turns in the primary coil < number of turns in the secondary coil 2 input voltage < output voltage It is called step-up transformer.

number of turns in the primary coil > number of turns in the secondary coil 3 input voltage > output voltage It is called step-down transformer.

where Vp = input voltage Vs = output voltage turn ratio where Vp = input voltage Vs = output voltage Np = number of turns in the primary coil Ns = number of turns in the secondary coil Simulation: Transformer

 For ideal transformer, power input = power output where Ip = input current Is = output current 

Efficiency of transformers Real transformers convert 90 % to 99 % of input power to the output power. Efficiency = 90% to 99 %

Main energy loss in transformers output a.c. input a.c. The coil is heated up when current flows through it. Energy is required to magnetize and demagnetize the core solid core Eddy current heats up the coil.

Ways for improving the efficiency output a.c. input a.c. Using thicker wire to reduce resistance of the coil Using soft iron to reduce energy loss Using a laminated core to reduce eddy current. laminated core

That’s the end of Section 21.3 Key Ideas Previous Page Section 21.4 Exit