An Audio-Frequency Dynamometer Wattmeter

Negative-feedback amplifiers are used to supply the currents to the coils of a nickel-iron-cored dynamometer, and by this means, indications proportional to current, voltage or power in an external circuit may be obtained from the dynamometer at any frequency between 50c/s and 20kc/s. The signal voltages required at the amplifier input terminals for full-scale instrument deflection are 2 volts. A voltage divider, having a resistance of 1000 ohms/volt and time-constant ratios of less than 0.02¿H/ohm, is used to give eight voltage ranges up to 500 volts. The current ranges are obtained by using four terminal resistance standards of different values and having time-constants of less than 0.02¿H/ohm. The overall instrument error at any power factor and any frequency within the working range does not exceed 0.6% of full-scale deflection. Internal switching is provided so that the voltage, current and power in a load may be determined without disturbing the load conditions. The upper limit of frequency is 20kc/s for power measurements, except near unity power factor, but voltage and current may be measured at frequencies up to 30kc/s.