Canadian Patents and Development Limited/societe Canadienne Des Brevets

Petar N Miljanic

William J Moore

A transconductance amplifier for emitting an output current that is a replica of a input voltage has its accuracy enhanced by a current comparator having a first magnetic core, a second magnetic core, a pair of ratio windings electromagnetically coupled with both the cores, and a detection winding electromagnetically coupled with the first core but not with the second core. A first of the ratio windings is connected in series with the output current of the transconductance amplifier. A high value resistor is connected in series with the second of the ratio windings across the input voltage of the transconductance amplifier whereby the current in the second ratio winding accurately reproduces the waveform of the input voltage. A further amplifier has its input connected to the detection winding. The output of this amplifier is so connected as to add a correcting current to the output current of the transconductance amplifier and thus provide the necessary accuracy enhancement.

Transconductance amplifier with transformer coupled forward feed

A current ratio device for use in constructing a current transformer comprises a first magnetic core, a second magnetic core, a measuring winding electromagnetically coupled with the first core but not with the second core, and a compensating winding electromagnetically coupled with both cores. An externally powered amplifier has its input connected to receive the voltage waveform across the measuring winding. Its output provides a compensating current through the compensating winding equal to a magnetizing current required to magnetize the second core. The amplifier output also provides a measuring current through the measuring winding in the direction opposite that of the compensating current and equal to the compensating current multiplied by the ratio of turns in the compensating and measuring windings. This device is a self-contained article of commerce that, when electromagnetically coupled (without electrical connection) to conventional primary and secondary windings and a burden, provides a current transformer with improved qualities. In particular, the magnetizing current needed by the transformer is provided by the separate current ratio device, thus avoiding the error inherent in current transformers that arises from the need to supply the magnetizing current from the primary circuit. Another disclosed feature is the ability to examine the waveform of the magnetizing current and in particular to detect the presence of even harmonics and hence a probability of direct current in the primary circuit.

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Gloucester, Canada

Petar N Miljanic