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GATE 2018 EE – Question 37

Electrical and Electronic Measurements · Instrument transformers, Digital voltmeters and multi-meters, Phase, Time and Frequency measurement · 2 marks · Multiple choice

A 0-1 Ampere moving iron ammeter has an internal resistance of 50 m$\Omega$ and inductance of 0.1 mH. A shunt coil is connected to extend its range to 0-10 Ampere for all operating frequencies. The time constant in milliseconds and resistance in m$\Omega$ of the shunt coil respectively are

  1. 2, 5.55
  2. 2, 1
  3. 2.18, 0.55
  4. 11.1, 2

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Correct answer: (A) 2, 5.55

Explanation

For the shunt to divide the current equally at all frequencies, it must have the same time constant as the meter: $\tau=\frac{L}{R}=\frac{0.1\text{ mH}}{50\text{ m}\Omega}=2$ ms. The shunt carries 9 A against 1 A in the meter, so $R_{sh}=\frac{50}{9}=5.55$ m$\Omega$.