Deflecting torque vs Controlling torque vs Damping torque

Analog meters rely on a delicate balance of forces to accurately display measurements. These forces are generated by three distinct torques: deflecting torque, controlling torque, and damping torque. Let’s explore each of these in detail.

What is Deflecting Torque?

Deflecting torque is the driving force behind the pointer’s movement on the meter’s scale. It’s the torque that responds directly to the electrical quantity being measured, such as current, voltage, or power.

This torque initiates the movement of the meter’s moving system, causing the pointer to move away from its zero position. The amount of deflection is directly proportional to the magnitude of the electrical quantity being measured.

$$ TD ∝ Measurable quantity $$

Where:

• TD represents the Deflecting Torque.
• Measurable Quantity represents the electrical quantity being measured.

What is Controlling Torque?

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.

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.

At equilibrium, $Tc = TD$

Where:

• Tc represents the Controlling Torque
• TD represents the Deflecting Torque

What is Damping Torque?

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.

Summary

All three torques work in concert to ensure accurate and stable readings in analog meters. Without the proper application of each, the meter would fail to provide meaningful data.