Current transformers are very similar to transformers. Transformers are mainly used to change the voltage of a line, while current transformers are mainly used to change the current of a line. Therefore, current transformers were also called converters in the past. Later on, instruments and equipment that convert direct current into alternating current were commonly called inverters. Electrical appliances that change the magnitude of current on a circuit were called current transformers based on their working principle of passing through transformers.

Why is it necessary to change the current on the line? This is because the current on the line varies greatly depending on the different situations of power generation and consumption, with some only a few amperes and others as high as tens of thousands of amperes. To directly measure these large and small currents, it is necessary to make many ammeters and other electrical instruments ranging from several amperes to tens of thousands of amperes according to the size of the line current. This will bring great difficulties to instrument manufacturing. In addition, some power lines are high-voltage, such as 220000 volts or 10000 volts high-voltage transmission and power supply lines. It is extremely dangerous and absolutely not allowed to directly measure the current on the high-voltage lines with electrical instruments.

If a current transformer is connected to the line to convert the current, the large and small currents on the line can be uniformly converted into currents of similar sizes according to different proportions. As long as an electrical instrument with a current specification, such as a universal current of 5A, can be used to measure currents ranging from a few amperes to tens of thousands of amperes on the circuit through a current transformer. At the same time, the basic structure of a current transformer is very similar to that of a transformer. It also has two windings, one called the primary winding or the primary winding; One is called the secondary winding or secondary winding. There is insulation between the two windings, which provides electrical isolation between them. When the current transformer is in operation, the primary winding W1 is connected to the line and the secondary winding W2 is connected to the electrical instrument. Therefore, when measuring the current on the high-voltage line, although the primary voltage is high, the secondary voltage is very low, and both the operator and the instrument are safe.

From this, it can be seen that current transformers can not only convert currents of different sizes on the line into a certain size for measurement, but also serve as insulation from the line to ensure the safety of operators and instruments.

So, then,Current transformerHow does it work?

In the power supply and consumption lines, the current varies greatly, ranging from a few amperes to tens of thousands of amperes. For the convenience of measuring instruments and surfaces, it is necessary to convert them into a relatively uniform current. The voltage on other power supply lines is relatively high, and direct measurement is very risky for instruments, surfaces, and people. The current transformer planned for this purpose serves as a converter and electrical isolation.

The appearance of pointer type current display is mostly in ampere level, so the secondary current of current transformers is mostly in ampere level (such as * * *, etc.), while the sampled signal of computers is generally in milliampere level (0-5V, 4-20mA, etc.). For this purpose, a miniature current transformer (with a secondary current of milliampere level) is used as a bridge between the current transformer and the computer sampling signal for secondary current conversion.

A current transformer consists of a primary coil, a secondary coil, an iron core, insulation support, and output terminals. The iron core of a current transformer is made of stacked silicon steel sheets, and its primary coil is connected in series with the main circuit. Through the measured current I1, it generates alternating magnetic flux in the iron core, causing the secondary coil to induce a corresponding secondary current I2 (with a rated current of * * *). If the excitation loss is ignored, then I1n1=I2n2, where n1 and n2 are the turns of the primary and secondary coils, respectively. The conversion ratio of the current transformer is K=I1/I2=n2/n1. Due to the primary coil of the current transformer being connected to the main circuit, insulation materials suitable for the voltage of the primary line must be used to ensure the safety of the secondary circuit and personnel. The secondary circuit is composed of the secondary coil of the current transformer, the outer surface, and the current coil of the relay connected in series.