In a permanent magnet moving coil instrument, the deflecting torque is

Question

Question

This question was previously asked in

WBPSC JE Electrical 2018 (Held on 18th Feb 2018) Official Paper

directly proportional to both number of turns and flux density.
directly proportional to the number of turns and inversely proportional to the flux density.
inversely proportional to the number of turns and directly proportional to the flux density.
inversely proportional to both number of turns and flux density.

Answer 1

Concept:
In moving coil instruments, deflecting torque is given by

Td = NBAI

Here, N = number of turns

B = Magnetic flux density

A = Area of cross-section

I = current

So, the directly proportional to both the number of turns and flux density.

Important Points:

In thermocouple instruments, the deflection is directly proportional to the square of the current. i.e. θ ∝ I2
In moving iron instruments, the deflection is directly proportional to the square of the current. i.e. θ ∝ I2
In hotwire instruments, the deflection is directly proportional to the square of the current. i.e. θ ∝ I2
In moving-coil instruments, the deflection is directly proportional to the current. i.e. θ ∝ I