Deflecting, Controlling and damping torque - Engineering notes

Deflecting, Controlling and damping torques

Deflecting Torque:

The deflecting torque is produced by making use of one of the magnetic , heating, chemical electrostatic and electromagnetic induction effects of current or voltage and causes the moving system of the instrument  to move from its zero position when the instrument is connected in an electrical circuit to measure the electrical quantity. The method of producing this torque depends 

upon the type of the instrument.

Controlling Torque:

Under the action of deflecting torque, the pointer will continue to move indefinitely and shall be independent of the value of electrical quantity to be measured. This controlling torque should oppose the deflecting torque and should increase with the deflection of the moving system so that the pointer is brought to rest at a position when the two opposing torques are equal. The controlling torque performs the following functions:

• It opposes the deflecting torque and increases with the deflection of the moving system. It thus limits the movement of the pointer so that the magnitude of deflection is always the same for the given value of electrical quantity to be measured.
• If brings the pointer to the zero position when the deflecting torque is removed. If it were not provided, the pointer once deflected would not return to zero position on removing the deflecting torque.

The controlling torque is provided either by spring control or gravity control. A hair spring usually of phosphor bronze is attached to the moving system of the instrument. With the movement of the pointer, the spring is twisted in opposite direction. This twist in spring produces restoring torque. Gravity control is obtained by attaching a small adjustable weight to the moving system. Such that the two exert torques in opposite directions.

Damping Torque:

If the moving system is acted upon by deflecting and controlling torques alone, then the pointer due to its inertia will oscillate about final position before coming to rest. These oscillations are undesirable and must be prevented. In order to avoid these oscillations of the pointer and to bring it quickly to its deflected position, damping torque is provided which opposes the movement of the pointer and operates only when the system is moving.