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 in an interview for job.

This intriguing question originated from Power and Energy Signals in portion Fourier Series of Signals and Systems

Answer 1

Correct answer is (a) 549.39 VA

The best I can explain: 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.