The characteristics of the current transformer are: (1) The primary coil is connected in series in the circuit, and the number of turns is very small. Therefore, the current in the primary coil completely depends on the load current of the circuit under test. It has nothing to do with the secondary current; (2) The current coil impedance of the instrument and relay connected to the secondary coil of the current transformer is very small, so under normal circumstances, the current transformer operates in a near short-circuit state.
The ratio of the primary and secondary rated currents of the current transformer is called the rated mutual inductance ratio of the current transformer: kn=I1n/I2n
Because the rated current I1n of the primary coil has been standardized and the rated current I2n of the secondary coil is unified to 5 (1 or 0.5) A, the rated mutual inductance ratio of the current transformer has also been standardized. kn can also be approximately expressed as the turns ratio of the primary and secondary coils of the transformer, that is, kn≈kN=N1/N2, where N1 and N2 are the number of turns of the primary and secondary coils.
In layman's terms, if you want to measure a 400A current, what should you do if you don't have a 400A watt-hour meter? So you have to use a current transformer to convert the large current into a small current through the transformer and input it into the watt-hour meter. Transformers have variable ratios. For example, 200/5 is 40 times that of the watt-hour meter. That is to say, your watt-hour meter has one kilowatt hour, but you have actually used 40 kilowatt hours. There are many levels of transformers. There are 15/5...30/5,,50/5,,75/5,,,etc.,,,,,
The best way to determine the polarity of the transformer on site is to use a dry battery or an analog multimeter MF47. Adding wires is the most direct and easiest to see. The accuracy can reach more than 90%. The method is very simple. The primary side has P1 and P2, and the standard secondary ones are mostly S1S2, K1K2, etc. Connect the positive electrode to P1. The positive electrode of the secondary side is connected to S1. The negative electrode is connected to another section. The secondary side is connected directly, and then the battery is directly short-circuited. When a DC current is applied to the primary side of the transformer, you will see that the pointer of the multimeter swings clockwise to the positive polarity and counterclockwise to the negative polarity, indicating that the internal winding is wrong. At this time, you need to observe it carefully because the pointer swing is not Large, it is recommended to use 2 1# batteries for the same method, because too many batteries are too dangerous, and the test should be stopped for half an hour or the battery should be replaced after about 10 times, because the battery heats up very much during the test. Again, it is recommended not to be lazy in using it. The alternating current becomes 12V 6V. I think you save the battery, but the error increases and the safety decreases.
