Question

Describe, with the help of  a suitable diagram, the working principle of a step-up transformer. Obtain the relation between input and output voltages in terms of the number of turns of primary and secondary windings and the currents in the input and output circuits.

Answer 1

Principle − It works on the principle of electromagnetic induction. When current in one circuit changes, an induced current is set up in the neighbouring circuit.

Construction

Working

• Alternating emf is supplied to the primary coil PP’. The resulting current produces an induced current in secondary.

• Magnetic flux linked with primary is also linked with the secondary. The induced emf in each turn of the secondary is equal to that induced in each turn of the primary.

Let

E_P − Alternating emf applied to primary

n_P − Number of turns in the primary

`(dø)/dt - `Rate of change of flux through each turn of primary coil

∴`E_p = - n_p (dø)/dt  .......... (1)`

E_s− Alternating emf of secondary

n_s − Number of turns in secondary

∴`E_s = - n_s (dø)/dt  .......... (2)`

Dividing equation (2) by (1),

`E_s/E_p = n_s/n_p = k`

For step-up transformer, K > 1

∴ E_s > E_p

• According to law of conservation of energy,

• Input electrical power = Output electrical power

  E_pI_p = E_sI_s

∴ `E_s/E_p = I_p/I_s`