How to use a multimeter to measure the polarity of a CT
The electromagnetic current transformer (CT) is widely used in power systems, and its working principle is similar to that of transformers.
The characteristics of a 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 depends entirely on the load current of the tested circuit, and is independent of the secondary current; (2) The impedance of the current coil 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 state close to a short circuit.
The ratio of the primary and secondary rated currents of a current transformer is called the rated 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 uniformly set to 5 (1 or 0.5) amperes, the rated inductance ratio of the current transformer has also been standardized. Kn can also be approximated as the ratio of turns between the primary and secondary coils of a transformer, i.e. kn ≈ kN=N1/N2, where N1 and N2 are the turns of the primary and secondary coils.
The function of a current transformer is to measure relatively large currents.
Simply put, if you want to measure a 400A current, what should you do if you don't have a 400A electricity meter? So, you need to use a current transformer to convert the large current through the transformer into a small current when entering the electricity meter. The transformer has a transformation ratio, for example, a 200/5 is 40 times the electricity meter, which means that if you have one degree of electricity on your electricity meter, you have actually used 40 degrees of electricity. There are many levels of transformers, including 15/5 30/5, 50/5, 75/5, etc,,,,,
The best way to determine the polarity of a transformer on site is to use a dry battery or a pointer type multimeter MF47. With the addition of wires, it is the most direct and easy to see, with an accuracy of over 90%. The method is very simple. The primary side of the transformer has a standard secondary polarity of P1 P2, such as S1S2 K1K2. Connect the positive pole to the secondary side of P1 and then connect it to the negative pole of S1. Connect the secondary side directly, and then use the battery to short-circuit the primary side of the transformer to apply DC current. At this time, you will see the pointer of the multimeter swing. If the polarity is clockwise, it is positive, and counterclockwise, it is negative, indicating an internal winding error. At this point, it is necessary to observe in detail because the pointer does not swing much. It is recommended to use two 1 # batteries using the same method, as having too many batteries is too dangerous, And after about 10 tests, stop for half an hour or replace the battery, because the battery heats up very much during the test. It is recommended not to be lazy and use AC power to turn it into 12V 6V, thinking that you have saved the battery, but the error has increased and the safety has decreased,
