Question

Show that the torque produced in a permanent-magnet moving-coil instrument is proportional to the area of the moving coil. A moving-coil voltmeter gives full-scale deflection with a current of 5 mA. The coil has 100 turns, effective depth of 3 cm and width of 2.5 cm. The controlling torque of the spring is 0.5 cm for full-scale deflection. Estimate the flux density in the gap.

Answer 1

The full-scale deflecting torque is Td = NBIA N-m

where I is the full-scale deflection current ;

I = 5mA = 0.005A

Td = 100 × B × 0.005 × (3 × 2.5 × 10−4 ) = 3.75 × 10−4 B N-m

The controlling torque is

Tc = 0.5 g-cm = 0.5 g. wt.cm

= 0.5 × 10−3 × 10−2 kg wt-m

= 0.5 × 10−5 × 9.8

= 4.9 × 10−5 N-m

For equilibrium, the two torques are equal and opposite.

∴ 4.9 × 10−5 = 3.75 × 10−4 B

∴ B = 0.13 Wb/m2