The basic function of current transformer (2)

July 13 , 2020

What is the role of the current transformer? It is a device that transforms a large current into a second small current, and then displays the information on the second small current according to the secondary display information table, and you can understand the measured current. Carefully observe the current in each phase to see if the three-phase electricity is in equilibrium. Then there is another point to note is that the secondary side of the current transformer is strictly forbidden to open. Its primary winding resistance is connected in series in the measured route, and the secondary winding resistance is connected in series with the secondary display information table.

Discuss why the current transformer is strictly forbidden to open

When the current transformer is working normally, the magnetic flux potential caused by the secondary side magnetic induction current has a demagnetizing effect, the total magnetic flux of the iron core is small, and the induced electromotive force of the secondary side is also small. According to the calculation of the induced electromotive force formula, E=n*ΔΦ/Δt, E is the induced electromotive force, n is the number of secondary turns, and ΔΦ/Δt is the rate of change of magnetic flux.

Current Transformer

When the secondary side of the current transformer is open, according to the knowledge, the demagnetization function of the secondary side disappears, the primary current becomes the excitation adjustment current, and the total magnetic flux of the iron core increases, the rate of change of ΔΦ/Δt increases. As known from the magnetic induction potential formula, the induced electromotive force will become larger and larger, resulting in a higher working voltage. There is a serious risk for the machinery and equipment connected to the secondary side and related workers.

Therefore, the current transformer has a second open circuit, the secondary operating voltage is high, and the iron core is saturated again. The magnetic induction strength is limited. As is known from the magnetic flux formula Φ=BS, a certain cross-sectional area S in the direction of the magnetic field will not change, and the magnetic induction When the strength B is raised to the limit, the Φ induced electromotive force is getting higher and higher, the iron core is very hot, and the insulation of the secondary side electromagnetic coil is damaged.

According to the simple indication that it is strictly forbidden to open the current transformer, it is found that the secondary open circuit is actually very dangerous. Once the actual operator of the open circuit touches the secondary side, it is very easy to cause a safety accident.

Seeing here, do you want to ask, when the load current is 100A, how high the voltage is on the secondary side open circuit for a short time?

If the transformation ratio of the commonly used current transformer is 200A/5A, and there is no open circuit on the secondary side of the power circuit, the load current is 100A and the secondary side current is 2.5A. The transformation ratio is that the rated input is 200A, and the rated output is 5A, it is not difficult to see that the transformation ratio is 40. E=KN2A√f√I1N1/L, if you want to measure the operating voltage of the secondary side open circuit, you should understand that the number of secondary side turns, the reasonable total area of the core, the switching power supply frequency, the average length of the core magnetic circuit, the rated value of the primary current And the rated value of the number of turns and the core K index, you can calculate the highest value of the operating voltage E.

It is known from the above that the resulting peak operating voltage is related to the accurate measurement of the current transformer used in the power supply circuit. The actual parameters are the main parameters, and finally the peak operating voltage E is calculated.

Summary, when the secondary side is open, it will induce a very high operating voltage and will burn the current transformer. Therefore, it is strictly forbidden to open the secondary side. Under normal circumstances, the secondary open circuit also has noise. Because the core is magnetically saturated and the ferrite core oscillates unevenly, it will cause noise. Secondly, it can also carry out temperature detection according to the handheld infrared thermometer, and can be distinguished if it is open.

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