A 15-volt moving iron voltmeter has a resistance of 300 ohms and an inductance of 0.12 Henry.
A 15-volt moving iron voltmeter has a resistance of 300 ohms and an inductance of 0.12 Henry. The instrument reads correctly on DC and on AC at 36 Hz when it shows a voltage of 14.75 V. What will it's reading for the same voltage at 100 Hz?
Right Answer is:
14.5 V
SOLUTION
The impedance at 100 Hz
On D. C., only ohmic resistance is involved and the voltmeter reads correctly. But on AC it is the impedance of the instrument which has to be taken into account.
When the frequency is 100 Hz
Z2 = R2 + XL2 = R2 + 2πfL2
= 3002 + (2 × π × 100 × 0.12)2
Z = 300.6Ω
Voltmeter reading on A.C = Voltmeter voltage × voltmeter resistance ⁄ Impedance
= (15 × 300) ⁄ (309.3) = 14.55V