Induction Electrical Measuring Instrument

a device for measuring electrical quantities in AC circuits. Induction instruments differ from other types of instruments in that they can be used in AC circuits only for one specific frequency: small changes in frequency cause large errors in the readings. In the USSR induction ammeters and voltmeters are not widely used; since the early 1950’s, wattmeters also have not been produced. Modern induction measuring instruments are used only as energy meters in single-phase and three-phase AC circuits of industrial frequency (50 hertz).

The principle of operation of induction measuring instruments is analogous to that of an asynchronous motor: the load current, passing through the operating circuit of the instrument, generates a traveling or rotating magnetic field, which induces a current in the movable part, thus causing its rotation. Induction measuring instruments are classified as single-flux or multiflux instruments, depending on the number of alternating magnetic fluxes inducing a current in the movable part of the instrument.

The main components of an induction instrument are a magnetic system, a movable part, and a permanent magnet. The magnetic system consists of two electromagnets with cores of complex shape, on which are arranged windings connected in series and in parallel with the load circuit. The movable part is a thin brass or aluminum disk located within the field of the magnetic system. A permanent magnet provides braking torque. An induction instrument is not sensitive to the influence of external magnetic fields and can withstand significant overloads.