What will be the direction of deflecting torque in a moving iron instrument if the direction of current in the coil is reversed at the same magnitude?

The correct answer is option 4.

Deflecting torque in MI instrument is given by:

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

If the direction of the current in the coil is reversed at the same magnitude, then I will be -I, then the new deflecting torque will be:

\(T_{d(new)}={1\over 2}(-I)^2{dL\over d\theta}\)

\(T_{d(new)}=T_d\)

The direction of deflecting torque remains the same.

Moving iron instrument

MI instruments are used for measuring the RMS value of the current or voltage.

Torque in MI instrument

1.) Deflecting torque

Deflecting torque in MI instrument is given by:

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

Since deflection is nonlinear, the scale of a moving-iron meter must be nonlinear. The numbers at the low end of the scale are crowded, and they are farther and farther apart toward the high end of the scale where deflection is greater.

2.) Controlling torque

Controlling torque in horizontally mounted MI instruments is provided by spring control.

For a vertically mounted instrument gravity control is used.

\(T=k\theta\)

At equilibrium: TC = Td

\(k\theta={1\over 2}I^2{dL\over d\theta}\)

\(\theta={I^2\over 2k}{dL\over d\theta}\)

3.) Damping torque

Damping torque is used to enable fast and accurate reading of an object that undergoes oscillation. Due to inertia, an object in motion tends to stay in motion, thus requiring a counteractive force to bring it to its final rate of oscillation in a short period.

It is provided by eddy current or air friction damping.