# Test and Measurement World

## Test and Measurement World Terminology Section

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## difference between Deflecting torque vs Controlling torque vs Damping torque

This page compares definitions of Deflecting torque vs Controlling torque vs Damping torque and mentions difference between Deflecting torque,Controlling torque and Damping torque.

In order to ensure proper operation of electrical indicating instruments, these torques are required. These are used in analog meters.

### Deflecting torque

• Definition: It is the torque which deflects the pointer on a calibrated scale according to the electrical quantity passing through the instrument.
• The deflecting torque causes moving system and hence pointer attached to it moves from zero position to indicate electrical quantity being measured on graduated scale.
➤ TD ∝ Measurable quantity

### Controlling torque

• Definition: It is the torque which controls movement of the pointer on particular scale according to the quantity of electricity passing through it.
• If deflecting torque acts alone, pointer would continue to move indefinitely and would swing over to the maximum deflected position irrespective of magnitude of current (or voltage or power) to be measured.
• The controlling torque is opposite to deflecting torque. When deflecting torque equals to controlling torque, pointer comes to final steady state position.
➤ At equilibrium, Tc = TD
➤Controlling torque is also used to bring the pointer in zero initial position, if there is no deflecting torque.
➤Controlling torque is provided by spring control and gravity control.

### Damping torque

• Definition:This torque is used to damp out oscillation at the final steady state position. The time response of the instrument depends on damping torque.
• Under the action of deflecting torque, moving system of the instrument defects, while the controlling torque is applied on it in the opposite direction so that deflection may be proportional to the quantity under measurement.
• Under the action of these two torques the pointer oscillates at the mean position and delays to come to the final position.
• Damping torque is provided in the instrument which help the moving system of the instrument to reach to the final position at the earliest.

Damping torque is provided by following:
➤Air friction damping: It is used where low magnetic fields are produced.
➤Fluid friction damping: It is used where deflecting torque is minimum.
➤Eddy current damping: It is used where permanent magnet produces the required deflecting torque.