A current transformer is an instrument that uses the principle of electromagnetic induction to convert a large current on the primary side into a small current on the secondary side. A current transformer consists of a closed iron core and windings. Its primary winding has several turns, strung on the current line that needs to be measured.

　　The principle of the current transformer is based on the principle of electromagnetic induction. It's primary winding often carries all the current of the line through it. When the current transformer is working, its secondary circuit is always closed. Therefore, the measuring instrument and the protection circuit are connected in series with the coil. The impedance is very small, and the working state of the current transformer is close to a short circuit.

　　In an ideal current transformer, assuming no-load current I0=0, the total magnetomotive force I0N0=0. According to the law of conservation of energy, the magnetomotive force of the primary winding is equal to the magnetomotive force of the secondary winding, that is, I1NI=-I2N2, that is, the current of the current transformer is inversely proportional to its turns, and the ratio of the primary current to the secondary current I1/I2 is called is the current ratio of the current transformer. When the secondary current is known, the primary current can be obtained by multiplying the current ratio. At this point, the phasor of the secondary current differs from the phasor of the primary current by 1800.

　　These transformers are basically step-up transformers, i.e. step up the voltage from the primary to the secondary. Therefore, the current decreases from primary to secondary.

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