Operation principle of current transformer

What is a current transformer?

Current transformer is a simple component made according to the magnetic effect principle of current, which can accurately measure the communication AC current.

Efficacy and principle

The key of current transformer is often used to convert the large current in communication AC circuit into a certain proportion of small current (the specification in China is 5 ampere Shinzo), so as to provide accurate measurement and relay protection devices. We should understand that in the whole process of power generation, transformer, transmission and distribution equipment, because the power consumption machines and equipment are different, the current usually ranges from tens of thousands of a to tens of thousands of yuan a, and this circuit will change with the high voltage. In order to monitor and accurately measure the circuit of this route, and deal with the risks caused by high voltage and high current, current transformer must be used at this time. Some people will have seen the clamp meter used by welders. It is a machine and equipment used to accurately measure the communication AC current. Its "clamp" is a through core current transformer.

Current transformer

Current transformer can be used to expand the measurement range of AC ammeter. During application, its primary coil shall be connected in series with the load route of the measured current, and the secondary electromagnetic coil shall be connected in series with the ammeter to form a closed loop, as shown in the circuit diagram on the right side of the figure.

The primary coil of current transformer is wound with thick transmission line, and its coil turns are only one or several turns, so its characteristic impedance is very small. When the primary coil is connected in series in the circuit under test, the current on both sides is very small. Although the number of coil turns of the secondary coil is large, under all normal conditions, its induced electromotive force E2 is not high, about only a few volts.

Because I1 / I2 = ki (KI is called converter ratio), I1 = kii2

It is not difficult to see the wiring plan of current transformer in the application of ammeter. The current according to the load is equivalent to the multiplication of the current measured by the secondary electromagnetic coil and the conversion ratio ki. If the ammeter is used with a special current transformer supporting facilities, the scale of the ammeter can be marked according to the current value in the high current circuit. The maximum current of secondary coil of current transformer is 5A in general design scheme. The current transformers applicable to circuits with different currents are different, and their conversion ratios are 10 / 5, 20 / 5, 30 / 5, 50 / 5, 75 / 5 and 100 / 5. For the sake of safety, one end of the secondary coil of the current transformer and the housing must be grounded