The deflecting torque in a moving iron meter:

Deflecting torque:

The torque needed to move the pointer over a calibrated scale is known as deflecting torque and it can overcome the inertia of the moving system, controlling torque and damping torque.

In moving iron instruments, the deflecting torque is given by

\({T_d} = \frac{1}{2}{I^2}\frac{{dL}}{{d\theta }}\)

I is the operating current in Ampere

L is the inductance in Henry

θ is the deflection angle in radians

As the operating torque is directly proportional to the square if the operating current. Hence, this instrument possesses high operating torque.

Important Points:

• In moving-coil instruments, the deflecting torque is directly proportional to the average value of current
• In moving iron instruments, the deflecting torque is directly proportional to square of the rms value of the current
• In hot wire instruments, the deflecting torque is directly proportional to the square of the rms value of the current
• In induction type instruments, the deflecting torque is directly proportional to the frequency