Question: Q. 8. (i) Draw a labelled diagram of a step-down transformer. State the principle of its working.
(ii) Express the turn ratio in terms of voltages.
(iii) Find the ratio of primary and secondary currents in terms of turn ratio in an ideal transformer.
(iv) How much current is drawn by the primary of a transformer connected to $220 \mathrm{~V}$ supply when it delivers power to a $110 \mathrm{~V}-550 \mathrm{~W}$ refrigerator?
U [O. D. I, II, III 2016]
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Solution:
Ans. (i) Labelled diagram :
Principle :
$1 / 2+1 / 2$
When the current flowing through the primary coil changes, an emf is induced in the secondary coil due to the change in magnetic flux linked with it.
[Note : Give $1 / 2$ mark to the student who writes only mutual induction.]
(ii)
$$ \frac{V_{s}}{V_{p}}=\frac{1}{1} $$
(iii) For an ideal transformer,
$\therefore \quad \frac{i_{p}}{i_{s}}=\frac{V_{s}}{V_{p}}=\frac{N_{s}}{N_{p}}$
(iv) We have
$$ \begin{aligned} i_{p} V_{p} & =i_{s} \mathrm{~V}{s}=550 \mathrm{~W} \ V{p} & =220 \mathrm{~V} \ i_{p} & =\frac{550}{220}=\frac{5}{2}=2.5 \mathrm{~A} \end{aligned} $$
[CBSE Marking Scheme 2016]
