Electromagnetic current transformer (hereinafter referred to as current transformer) is widely used in power system. Its working principle is similar to that of transformer.
Current transformers are characterized by:
1. The primary coil is connected in series in the circuit with few turns. Therefore, the current in the primary coil depends entirely on the load current of the circuit under test, but has nothing to do with the secondary current.
2. Current coil impedance of instrument and relay connected by secondary coil of current transformer is very small, so under normal circumstances, current transformer operates in near-short-circuit state.
The ratio of primary and secondary rated current of current transformer is called rated mutual inductance ratio of current transformer: kn = I1n/I2n
Because the rated current I1n of primary coil has been standardized and the rated current I2n of secondary coil has been unified to 5 (1 or 0.5) amperes, the rated mutual inductance ratio of current transformer has also been standardized. Kn can also be approximately expressed as the turn ratio of the primary and secondary coils of the transformer, i.e. the turn number of the first and second coils in kn_kN=N1/N2.
The function of current transformer is to measure relatively large current.
Popularly speaking, if you want to measure the current of 400A, then what should you do if there is no 400A meter, so you need to use current transformer to change large current through transformer into small current and then input the meter. Transformers have variable ratio, for example, 200/5 is 40 times of the meter, that is to say, on your ammeter. When the surface shows 1 ampere current, the actual current you measured is 40 amperes. There are many levels of transformers, such as 15/5, 30/5, 50/5, 75/5, etc.
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