Calculation of the torque-RPM characteristics of power-meters

A method based on variational principles has been developed for the computation of the eddy current distribution and hence the driving and the braking torque-RPM characteristics of single-phase power meters. The air-gap field due to the voltage and current coils and the permanent magnet in the absence of the disk is presumed to be given, and the resultant air-gap field, as well as the eddy currents induced by it, are determined. Motional induction is taken into account. Global approximation is used to obtain the field quantities as expansions in terms of suitable analytical functions. A comparison is made between computed and measured torques, showing a maximum relative error of 3.1%