Case Study Based Question on Transformers
For many purposes, it is necessary to change (or fransform) an alternating voltage from one to another of geater or smaller value. This is done with a device called a transformer.
When an alternating voltage is applied to the primary, the resulting current produces an alternating magretic flux which links the secondary and induces an emf in it. The value of this emf depends on the number of turns in the secondary. We consider an ideal transformer in which the primary has negligible resistance and all the flux in the core links both primary and secondary windings..
(i) A step-down transformer increases :
(a) Voltage
(b) Current
(c) Power
(d) Frequency
(ii) A step-up transformer has________ number of turns on primary winding and _______ number of turns on secondary winding.
(a) less, more
(b) more, less
(c) less, less
(d) more, more
(iii) If the primary coil of a transfonner has 100 turns and the secondary
has 200 turns, Ns/Np=2. Thus, a 220 V input at 10A will step-up :
(a) to 220 V output at 20 A
(b) to 440 V output at 10 A
(c) to 440 V ouput at 5 A
(d) to 110 V ouput at 10 A
(iv) A transforrner is used to reduce the mains supply of 220 V to 22 V. If the currents in the primary and secondary are 2A and 15 A respectively, then the efficiency of the transformer is .
(a) 65%
(b) 75%
(b) 80%
(d) 90%
Doubt by Guni
Solution :
(i) (b) Current.
A step-down transformer reduces voltage but increases current in the same ratio.
(ii) (a) less, more.
A step-up transformer has less turns on the primary and more turns on the secondary.
(iii) (c) to 440 V output at 5 A.
Transformation ratio (Ns/Np) is 2, it means voltage becomes double and current reduces to half the input values.
(iv) (b) 75%
Output Power = ε2I2 = 22×15
Input Power = ε1I1 = 220×2
Efficiency = (Output Power / Input Power) × 100%
= (22×15/220×2) ×100%
= (3/4)×100%
= 75%
