Induction Type Wattmeter
Induction type wattmeter works on induction principle.
Construction of induction type wattmeter
Induction type wattmeter consists of two laminated electromagnets. One is called series magnet and other is called shunt magnet. Series magnet is connected in series with the supply and carries the load current. Series magnet is made highly non-inductive so that angle of lag or lead is wholly determined by the load. Shunt magnet carries the current which is proportional to load voltage. This magnet is made highly inductive. So that the current in it lags behind the supply voltage by 90O.
A thin aluminum disc mounted on the spindle of the induction type wattmeter. It is mounted in such a way that it cuts the fluxes of both magnets. Hence, two eddy currents are produced in the disc. The controlling torque is provided by spiral springs. In this wattmeter, electro magnetic damping torque is provided. Shading rings are provided on the central limb of the shunt magnet.
When induction type wattmeter is connected in the circuit to measure ac power, the current start flowing through both magnets. The shunt magnet carries the current proportional to the voltage across load while series magnet carries the load current. The fluxes produced by the series and shunt magnets induced eddy currents in the aluminum disc. The deflection torque is produced due to interaction of these eddy current and inducing fluxes. The deflecting torque on the disc causing the pointer connected to the moving system to move over the scale. The pointer comes to rest at a position where deflection torque becomes equal to controlling torque provided by spiral spring.
Td α VI cos Φ
This is used to measure ac power.
Induction type wattmeter free from the effects of story field and have good damping.
Induction type wattmeter has fairly long scales.