Question

A current of – 8 + 6\(\sqrt{2}\) (sin (ωt + 30°)) A is passed through three meters. The respective readings (in ampere) will be?

(a) 8, 6 and 10

(b) 8, 6 and 8

(c) – 8, 10 and 10

(d) -8, 2 and 2

The question was asked during a job interview.

My doubt stems from Advanced Problems on Indicating Instruments in portion Extension of Instrument Ranges of Electrical Measurements

Answer 1

The correct choice is (c) – 8, 10 and 10

To explain: PMMC instrument reads only DC value and since it is a centre zero type, so it will give – 8 values.

So, rms = \(\sqrt{8^2 +(\frac{6\sqrt{2}}{\sqrt{2}})^2}\) = 10 A

Moving iron also reads rms value, so its reading will also be 10 A.