Question

A relay coil having voltage of magnitude 210 V and frequency 50 Hz is connected to a 210 V, 50 Hz supply. If it has resistance of 50 Ω and an inductance of 0.2 H, the apparent power is _____________

(a) 549.39 VA

(b) 275.6 VA

(c) 157 VA

(d) 187 VA

I had been asked this question during an interview.

Origin of the question is Problems Involving Standard Time Functions in portion Application of the Laplace Transform in Circuit Analysis of Network Theory

Answer 1

The correct choice is (a) 549.39 VA

Explanation: Z= 50 + j (0.2) (2π) (50) = 50 + 62.8

S = \(\frac{|V|^2}{Z} = \frac{210^2}{50 – 62.8}\)

Apparent Power |S| = \(\frac{210^2}{\sqrt{50^2 + 62.8^2}}\)

= 549.39 VA.