Page 825 - Basic Electrical Engineering

When the coil alongwith its pointer moves because of deflecting torque, it
should be free from oscillations, i.e., the pointer should quickly come to its
deflected position over a graduated scale so that the reading can be taken. To

reduce any oscillation of the pointer, a damping mechanism is provided
which produces a damping torque which damps or reduces the oscillation.

Figure 11.15 (a) PMMC-type instrument; (b) moving system

Thus, there exists three types of torques on the moving system, i.e.,
deflecting torque, control torque, and damping torque. Note that if the
control torque is not provided for, then for any amount of deflecting torque

the pointer will give full-scale deflection.

Equation for deflecting torque

Let I be the current flowing through the moving coil N be the number of turns
of the moving coil B be the flux density due to the magnetic field created by
the magnet in which the moving coil is placed ℓ be the length of one side of

the coil and r be the radial distance of the coil from its axis of rotation
(deflection).

Then, force on each coil side is BI ℓ