Principle of transformer:
It is based on the principle of mutual induction i.e., whenever the magnetic flux linked with a coil changes, an e.m.f. is induced in the neighbouring coil.
Construction:
i. A transformer consists of two sets of coils P and S insulated from each other. The coil P is called the primary coil and coil S is called the secondary coil.
ii. The two coils are wound separately on a laminated soft iron core.
iii. The a.c input voltage is applied across the primary and the induced output a.c voltage is obtained across the secondary, which is used to drive current in the desired circuit.
iv. The two coils are electrically insulated from each other but they are magnetically linked.
v. To minimise eddy currents, the soft iron core is laminated.
Working:
i. When an alternating voltage is applied to the primary coil the current through the coil goes on changing. Hence, the magnetic flux through the core also changes.
ii. As this changing magnetic flux is linked with both the coils, an e.m.f is induced in each coil.
iii. The amount of the magnetic flux linked with the coil depends upon the number of turns of the coil.
iv. Let, ‘ϕ’ be the magnetic flux linked per turn with both the coils at certain instant ‘t’.
v. Let ‘NP' and ‘NS’ be the number of turns of primary and secondary coil,
NPϕ = magnetic flux linked with the primary coil at certain instant ‘t’
NSϕ = magnetic flux linked with the secondary coil at certain instant ‘t’
vi. Induced e.m.f produced in the primary and secondary coil is given by,
vii. Dividing equation (2) by (1),
∴ \(\frac{e_s}{e_p}=\frac{N_s}{N_p}\) .............(3)
Equation (3) represents equation of transformer.
The ratio \(\frac{N_s}{N_p}\) is called turns ratio (transformer ratio) of the transformer.
viii. For an ideal transformer, Input power = Output power
∴ ePIP = eSIS
∴ \(\frac{e_s}{e_p}\) = \(\frac{l_s}{l_p}\) .........(4)
ix. From equation (3) and (4),
∴ \(\frac{e_s}{e_p}\) = \(\frac{N_s}{N_p}\) = \(\frac{l_s}{l_p}\)
