In a moving iron instrument, if the operating current is increased from 1 A to 3 A, the torque will be _________. 

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MPPGCL JE Electrical 28 April 2023 Shift 2 Official Paper
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  1. increased to 9 times
  2. increased to 3 times
  3. reduced to one third
  4. reduced to one nineth

Answer (Detailed Solution Below)

Option 1 : increased to 9 times
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Detailed Solution

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Concept

The torque in the MI instrument is given by:

\(T_d={1\over 2}I^2{dL\over dθ}\)

In a moving iron (MI) instrument, the torque is proportional to the square of the current.

T α I2

\({T_2\over T_1}=({I_2\over I_1})^2\)

Calculation

Given, I2 = 3 A 

I1 = 1 A

\({T_2\over T_1}=({3\over 1})^2\)

\(T_2=9T_1\)

The torque increased to 9 times of initial value.

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