Electrical Engineering ⇒ Topic : Induction Type Instruments and its principle

Induction Type Instruments

This class of instruments is suitable only for a.c. measurements. These instruments may be used either as ammeter or voltmeter or wattmeter or energy meter. Perhaps the widest application of induction principle is in watt-hour or energy meter.

Principle. Fig. (a) illustrates the principle of induction type instruments. Two alternating fluxes Φ₁ and Φ₂ (whose magnitudes depend upon the current or voltage to be measured) having a phase difference θ pass through a metallic disc, usually of copper or aluminium. These alternating fluxes induce currents in the disc. The current produced by one flux reacts with the other flux, and vice-versa, to produce the deflecting torque that acts on the disc. It can be proved that net deflecting torque on the disc is given by ;

                                                                   figure (a)

Obviously, to obtain maximum deflecting torque, the angle θ (i.e., phase angle between Φ₁ and Φ₂) should be 90⁰ .