A self-balancing direct current comparator for 20 000 amperes

The direct current comparator is a current ratio device which uses a two-core magnetic modulator as an ampere-turn balance detector. The applications of this comparator to precision measurements are very limited, unless it is made self-balancing by feedback. A transistorized self-balancing direct current comparator for 20 000 amperes which employs two stages to obtain a maximum ratio of 20 000/1 is described. The self-balancing circuit has a range of ±10 percent and an open loop gain of over 10 000. An analysis is made of the errors of the comparator including the effects of large external magnetic fields, magnetic memory, and alternating currents in the ratio windings. The total error increases with the amount of self-balancing required, but does not exceed 10 parts per million, provided the comparator is operated near its rated ampere-turn level. Applications of the comparator to the calibration of transductors (tic current transformers) and dc shunts at full rated current are discussed.