Canadian Patents & Development Ltd.

Canadian Patents & Dev. Limited

Norbert L Kusters

William J Moore

Eddy So

Malcolm P MacMartin

Glen A Herriot

Louis G Cox

A differential thermal wattmeter of the type disclosed in U.S. Pat. No. 4,045,734 is modified to remove undesirable in-phase a.c. ripple at the fundamental frequency in the feedback path. This result is achieved by a closed loop system in which the in-phase signal in the feedback path is detected and driven to zero by injecting sufficient signal in opposite polarity into the feedback path to cancel the ripple. The advantages are reduction of error and the ability to use standard, low cost components.

Wattmeter

The ac current comparator bridge measures the resistance of an unknown resistor at 50-60 Hz with an accuracy in the order of 10 parts per million. The current in the unknown resistor is compared, using a current comparator, to the current in a reference resistor for the in-phase component, and to the current in a reference capacitor for the quadrature component or phase defect. Once an ampere-turn balance is achieved in the comparator by adjusting the winding turns, the unknown resistor's values are a function of the reference resistor, the reference capacitor, the voltage sources driving the currents through the unknown resistor and the references, as well as the turns of the windings.

Alternating current comparator bridge for resistance measurement

This reference source provides a current which is of relatively high magnitude and which leads the voltage by substantially 90.degree.. It includes a reference capacitor which is highly stable and pure but of low capacitance, and a power capacitor which has a high capacitance. The capacitors are connected together at one end for coupling to a voltage source. A current comparator compares the currents flowing through the reference capacitor and the power capacitor, and provides a difference signal. A circuit generates a current in response to the difference signal and adds the generated current to the current through the power capacitor resulting in a current having a waveform and phase substantially identical to the reference capacitor current which leads the voltage source voltage by substantially 90.degree..

Active quadrature current reference source

The differential thermal wattmeter for the ac/dc transfer of power includes a differential thermal converter as a 'sum and difference' multiplier, with both ac and dc voltage and current signals simultaneously applied to both heaters. DC feedback to the current input speeds up the response and maintains thermal balance between the heaters, so that the output meter becomes a null indicator. This reduces and can eliminate the need for quadratic response. The requirement for exact matching of the thermal converters is also eliminated by effective interchange of the heaters.

Differential thermal wattmeter

A novel method and apparatus for monitoring an isolated system for ground faults which provides greater sensitivity and accuracy and at the same time does not provide a detector hazard index which would add to the fault hazard index. The monitor connects one of the lines in the isolated system to ground through a non-linear detector. The detector allows current below a predetermined level to flow freely to ground and prevents current which is above a predetermined level from flowing to ground. This is achieved by immediately disconnecting the line in question from ground, by limiting the amplitude of the ground current to the predetermined level, or both. This predetermined current level sensed may be adjusted by the operator. In addition, the monitor includes an alarm circuit which indicates the existence of an unacceptable ground fault in the isolated system.

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Ottawa, Canada

Norbert L Kusters