Feedback electrostatic wattmeter using a noncontact moving-coil actuator

An electrostatic wattmeter employing automatic torque balance about a taut-band suspension has been developed. The only direct physical electrical connection to the suspension assembly is the normal load voltage connection to the vane of the quadrant electrometer. The feedback torque is provided by a noncontact precision actuator which consists of an inductive coupling to a moving coil fixed on the suspension assembly, and which uses magnetic field variation. Suspension angular displacement is determined by using the quadrant structure as a push-pull capacitance transducer. The experimental instrument provides a d.c. output proportional to load power whether the supply is a.c. or d.c. It has linearity of at least ±0.5% over its measuring range, and the instrument calibration is essentially independent of power supply voltages and load power factors. It is easy to set the zero, but zero drift has a temperature sensitivity of about 1% per degree centigrade. No special antivibration mount is used, and typical noise at the instrument output (without a lowpass filter) has a peak-peak value of about 5% of full-scale reading. The transducer bridge circuit limits the range of power frequencies that can be measured. Improvements to reduce drift and noise effects are proposed, and the use of an optical position detector is suggested.