Wednesday, December 18, 2013

Deflecting System

       In most of the indicating instruments 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 and the torque is denoted as The deflecting torque overcomes,
1) The inertia of the moving system
2) The controlling torque provided by controlling system.
3) The damping torque provided by damping system.
      The deflecting system uses on of the following effects produced by current or voltage, 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 it. This effect is mostly used in many instruments like moving iron attraction and repulsion type, permanent magnet moving coil instruments etc.
2) Thermal Effect: The current to be measured is passed through a small element which heats it to cause rise in temperature which is converted to an e.m.f. by a thermocouple attached to it.
When two dissimilar metals are connected end to end to form a closed loop and the two junctions formed are maintained at different temperatures, then e.m.f. is induced which causes the flow of current through the closed circuit which is called a thermocouple.
3) Electrostatics Effects: When two plates are charged, There is a force exerted between them, which moves one of the plates. This effect is used in electrostatic instruments which are normally voltmeters.
4) Induction Effects: When a non-magnetic conducting disc is placed in a magnetic field produced by electromagnets which are excited by alternating currents, an e.m.f. is induced in it.
       If a closed path is provided, there is a flow of current in the disc. The interaction between induced currents and the alternating magnetic fields exerts a force on the disc which causes to move it. This interaction is called an induction effect. This principle is mainly used in energymeters.
5) Hall Effect: If a bar of semiconducting material is placed in uniform magnetic field and if the bar carries current, then an e.m.f. is produced between two edges of conductor. The magnitude of this e.m.f. depends on flux density of magnetic field, current passing through the conducting bar and hall effect co-efficient which is constant for a given semiconductor. This effect is mainly used in flux-meters.
       Thus the deflecting system provides the deflecting torque or operating torque for movement of pointer from its zero position. It acts as the prime mover for the deflecting of pointer.

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