Study Material For Engineering Students: Deflecting torque and controlling torque

Deflecting system: The mechanical force proportional to the quantity to be measured is generated. This force or torque deflects the pointer. The system which produces such a deflecting torque is called deflecting system.

The deflecting torque overcomes 1) The inertia of the moving system 2) controlling torque provided by controlling system 3) Damping torque provided by a damping system.

As most of the meters are generally current sensing meters so Td is directly proportional to the current I.

The deflecting system uses one of the following effects to produce deflecting torque.

1) Magnetic effect: When a current carrying conductor is placed in a uniform magnetic field it experiences a force which causes to move conductor.

2) Thermal effect: The current to be measured is passed through the element which heats up and causes a rise in its temperature which is converted to an emf by a thermocouple.

3) Electrostatic effect: When two plates are charged electrostatically then there is a force exerted between them which moves one of the plates.

4) Induction effect: When a nonmagnetic conducting disc is placed in a magnetic field produced by electromagnets excited by AC then the disc will produce torque

5) Hall effect: if a bar of semiconducting material is placed in the uniform magnetic field and if the bar carries current, then an emf is produced between the two edges of the conductor. the effect is mainly used in flux meters.

Thus the deflecting system provides the deflecting torque or operating torque for movement of the pointer from its zero position. it acts as the prime mover for the deflection of the pointer.

Controlling torque:

1) it produces a force equal and opposite to the deflecting force in order to make the deflection of a pointer at a definite magnitude. If this system is absent then the pointer will swing beyond its final position for the given magnitude and deflection will become indefinite.

2) It brings the moving system back to zero position when the force which causes the movement of the moving system is removed. It will never come back to its zero position in the absence of the controlling system.

There are 2 types of control systems.

1) Gravity control:

This type of control consists of a small weight attached to the moving system whose position is adjustable. This weight produces a controlling torque due to gravity. This weight is called control weight.

As shown in fig at the zero position of the pointer The controlling torque is zero. if the system deflects, the weight position also changes. The system deflects through an angle Theta, The control weight acts at a distance l from the center. The sine component is responsible to bring back the pointer to the initial position. this is called controlling torque.