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A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side for measurement based on the principle of electromagnetic induction. A current transformer consists of a closed iron core and windings. Its primary winding has a small number of turns, which are strung on the current line that needs to be measured.

Therefore, it often has the full current of the line flowing, and the secondary winding has a large number of turns. It is connected in series with the measuring instrument and the protection circuit. When the current transformer is working, its secondary winding is always closed, so the impedance of the series coil of the measuring instrument and the protection circuit is small, and the working state of the current transformer is close to short circuit. The flow transformer converts the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.

1. The functions of current transformers

The main function of the current transformer is to convert the large current in the AC circuit into a certain proportion of the small current (the Chinese standard is 5 amperes) for measurement and relay protection.

In the process of power generation, transformation, transmission and distribution, due to different electrical equipment, the current is usually from tens of amps to tens of thousands of safe, and these circuits may also be accompanied by high voltage. Therefore, in order to monitor and measure the circuits of these lines, while addressing the dangers posed by high voltages and high currents, current transformers are required at this time. Electrician's clamp meter, which is a device used to measure alternating current, its clamp is a through-core current transformer.

2. The current transformer working principle

Power generation, substation, transmission, distribution, transmission, distribution and power lines, ranging from a few amps to tens of thousands of amps. In order to facilitate measurement, protection and control, it is necessary to convert the current into a relatively uniform current. In addition, the voltage on the line is generally relatively high, such as direct measurement is very dangerous. Current transformers play the role of current conversion and electrical isolation.

For pointer galvanometers, the secondary current of the split core current transformer is mostly ampere level (such as 5A, etc.). For digital instruments, the sampled signal is generally at the milliamp level (0-5V, 4-20 mA, etc.). The secondary current of the micro-current transformer is milliampere, which is mainly used as a bridge between large transformers and sampling.

Miniature current transformers are also known as instrument current transformers. Instrument current transformers are multi-current ratio precision current transformers used in laboratories, and are generally used to expand the scope of instruments.

The current transformer similar to the transformer also works according to the principle of electromagnetic induction. The transformer converts the voltage, and the current transformer converts the current. The current winding (the number of turns is N1) connected to the measured current is called the firstly winding (or the ordinary winding, the primary winding); the winding connected to the measuring instrument (the number of turns is N2) is called the secondary winding (or the accessory winding, subordinate winding).

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