Electrical Engineering ⇒ Topic : Controlling Torque

Controlling torque is essential in an indicating instrument in order to get a steady deflection of the pointer. If this is not present, the pointer will continue to move forever since there is no force to oppose the deflecting force. When the deflecting torque (T_d) is equal to the controlling torque (Te), the pointer will indicate a steady deflection. The importance of this agency can be best understood again by referring to a cyclist. Imagine a cyclist who is riding a bicycle on a perfectly frictionless glass surface. Once the cyclist starts pedalling the bicycle, it will start moving and will continue to move forever because there is no force opposing the motion (Newton's first law). If there is friction on the surface similar to the friction on the ground, then the bicycle will come to a stop at some point since the frictional force always opposes the motion. If the pedalling force is equal to the frictional force, then the cyclist will move at a uniform velocity. In a similar way, when the controlling torque is equal to the deflecting torque (i.e. T_c= T_d), the pointer will come to rest and will indicate a steady deflection. Controlling torque (T_e) can be produced in two ways. They are

(a) by spring control and (b) by gravity control.

Controlling Torque

If deflecting torque were acting alone, the pointer would continue to move indefinitely and would swing over to the maximum deflected position irrespective of the magnitude of current (or voltage or power) to be measured. This necessitates to provide some form of controlling or opposing torque (T_c). This controlling torque should oppose the deflecting torque and should increase with the deflection of the moving system. The pointer will be brought to rest at a position where the two opposing torques are equal i. e. T_d=T_c. The controlling torque performs two functions :

(1) It increases with the deflection of the moving system so that the final position of the pointer on the scale will be according to the magnitude of current (or voltage or power) to be measured.

(2) It brings the pointer back to 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 in indicating instruments may be provided by one of the following two methods :

(1) By one or more springs                                       .....             Spring control

(2) By weight of moving parts                                   . ...              Gravity control