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Calculate the current required to produce a deflection of 100°, if the coil of a PMMC instrument has 60 turns on a former 18 mm wide, effective length of conductor = 25 mm, moves in a uniform field of flux density 0.5 T, control spring constant = 1.5 × 10-6 Nm/degree.


1. 4.56 mA
2. 11.11 mA
3. 2.56 mA
4. 12.43 mA

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Correct Answer - Option 2 : 11.11 mA

Concept:

PMMC torque equation

Td = Tc

Td = deflecting torque

Td = BINA

where B = flux density

I = current in moving coil

N = number of turns of the coil

A = effective Area of coil

Tc = controlling torque

Tc = Kθ

K = spring constant

θ = deflection Angle.

Kθ = BINA

Calculation:

Given:

θ = 100°, N = 60, B = 0.5T, l = 25mm, W = 18 mm

Area (A) = l × W = 25 × 18 = 450 mm2 = 450 × 10-6 m2

K = 1.5 × 10-6 N-m/degree

By torque equation

Kθ = BINA

1.5 × 10-6 × 100 = 0.5 × I × 60 × 450 × 10-6

I = 11.11 mA

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