current comparator
Current comparator
An instrument for determining the ratio of two currents, based on Ampère's laws. The application of the current comparator, unlike the voltage ratio transformer, is not limited to alternating currents but can be used with direct currents as well.
The current comparator is based on Ampère's circuital law, which states that the integral of the magnetizing force around a closed path is equal to the sum of the currents which are linked with that path. Thus, if two currents are passed through a toroid by two windings of known numbers of turns, and the integral of the magnetizing force around the toroid is equal to zero, the current ratio is exactly equal to the inverse of the turns ratio.
For alternating-current comparators, the ampere-turn unbalance is given by the voltage at the terminals of a uniformly distributed detection winding on a magnetic core. Direct-current comparators use two magnetic cores which are modulated by alternating current in such a way that the dc ampere-turn unbalance is indicated by the presence of even harmonics in the voltage. Neither method is an exact measure of the integral of the magnetizing force, and various design features are added to overcome this deficiency. The most important of these are the magnetic shields, which are configured as hollow toroids. They protect the magnetic core and detection winding from the leakage fluxes of the current-carrying windings and ambient magnetic fields, and they are responsible for an improvement in accuracy of about three orders of magnitude. The copper shields supplement this action at higher frequencies and also provide mechanical protection for the magnetic steels.
Applications of the ac comparator include current and voltage transformer calibration, measurement of losses in large capacitors, inductive reactors and power transformers, resistance measurement, and the calibration of active and reactive power and energy meters. Further, the current comparator, like the current transformer, has the advantage of being applicable to measurements involving high-voltage, high-power circuits as well as being the basis for very accurate high-current-transconductance amplifiers.
In dc applications, the current comparator provides the means to resolve the first three or four most significant digits of a balance by turns-count, thus eliminating problems associated with switch contact resistance and thermal electromotive forces. Its uses include eight-decade resistance and thermometry bridges, a seven-decade potentiometer, and high current ratio standards. It also provides the means for generating highly accurate direct-current voltages and currents. See Current measurement, Electrical measurements