It was found in practical operation that the current value was always very small, about 27A, while the actual working current of the primary side was measured as 82A using a clamp ammeter, which was significantly different. Moreover, the conditions of the three motors were basically the same. We replaced the CT current transformer, secondary circuit, and ammeter for one of the motors, but the situation remained the same.
After a careful analysis, we found a common rule that the measured currents of the primary and secondary sides were nearly three times different, which made us vigilant. Upon closer inspection of the nameplate of the through-core CT current transformer, we found an important issue neglected: the ampere-turn capacity was marked as 300 A-turn, which means it should be wound three times for 100/5 circuit instead of the common one-turn through-core.
We wound the primary circuit three times around the through-core current transformers, and the measured current was 81A, which was consistent with the 82A measured by the clamp ammeter for the primary side. This indicates that we should not neglect this issue.
Through-core current transformer is a common electrical device that is widely used for measuring, testing, and maintaining circuits due to its simple wiring and easy installation. However, if not properly used, it can cause significant errors, resulting in inaccurate measurement, maintenance failure, and even electrical accidents, which are related to the ampere-turn capacity of the through-core current transformers.
The so-called ampere-turn capacity refers to the rated current value of the primary side of the through-core current transformers when a single core is passed through, which is the product of the rated current and the number of turns. For example, if the model is LMZJ--0.5, 400 A-turn, the single-core passing current is 400A, and if two turns are used, the rated current of the primary side is 200A, which is commonly used with the measuring current, indicating the rated current working range of the primary side of the through-core current transformers as well as the wiring method. If this issue is neglected, unexpected problems may occur as mentioned above.
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