7.2. Induction Wattmeters: Induction wattmeters can be used on A.C. circuit only (in
contrast with dynamometer wattmeters can be used both on D.C. and A.C. circuits) and are useful only when the frequency and supply voltage are constant.
The operation of all induction instruments depends on the production of torque due to reaction between a flux ɸ1 (whose magnitude depends on the current or voltage to be measured) and eddy currents induced in a metal disc or drum by another flux ɸ2 (whose magnitude also depends on the current or voltage to be measured). Since the magnitude of eddy currents also depends on the flux producing them, the instantaneous value of the deflecting torque is proportional to the square of the
current or voltage under measurement and the value of mean deflecting torque is proportional to the mean square of the current or voltage.
Fig. 28 shows an induction wattmeter. It has two laminated electromagnets one is excited by the current in the main circuit-its exciting winding being joined in series with the circuit, hence it is also called series magnet.
The other electromagnet is excited by current which is proportional to the voltage of the circuit. Its exciting coil is joined in parallel with the circuit, hence this magnet is sometimes referred to as shunt magnet.
A thin aluminium disc is mounted in such a way that it cuts the fluxes of both the magnets. Hence two eddy currents are produced in the disc. The deflection torque is produced due to the interaction of these eddy currents and the inducing fluxes. Two or three copper rings are fitted on the central limb of the shunt magnet and can be so adjusted as to make the resultant flux in the shunt magnet lag behind the applied voltage by a suitable angle.
Fig. 28. Induction wattmeter.
This instrument is spring controlled, the spring being fitted to the spindle of the moving system which also carries a pointer. The scale is uniformly even and extends over 3000.
Such wattmeters can handle current upto 100 A. For handling greater currents they are used in conjunction with current transformers.
(i) Fairly long scale (extending over 3000).
(ii) Free from the effects of stray fields.
(iii) Good damping.
(iv) Practically free from frequency errors.
Sometimes subjected to serious temperature errors.