First slide

Electrical machines and devices

Question

A step-down transformer is connected to main supply 200V to operate a 6V, 30W bulb. The current in primary is

Moderate

A

3 A
B

1.5 A
C

0.3 A
D

0.15 A

Solution

$\begin{array}{l} V_{p} = 200 V, V_{s} = 6 V \\ P_{out} = V_{s} i_{s} \Rightarrow 30 = 6 \times i_{s} \Rightarrow i_{s} = 5 A \\ From \frac{V_{s}}{V_{p}} = \frac{i_{p}}{i_{s}} \Rightarrow \frac{6}{200} = \frac{i_{p}}{5} \Rightarrow i_{p} = 0 .15 A \end{array}$

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