The working principle and selection points of current transformer

The working principle of current transformer

The principle of current transformer is the same as that of transformer and reactor, which is electromagnetic induction. Electromagnetic induction refers to the induction of electromagnetism in the alternating current wire due to the change of the current (primary side), and the induced electromagnetism will generate current (secondary side) after passing through the closed wire. There is a very critical formula: the ratio of the primary side current I1 and the secondary side current I2 is inversely proportional to the ratio of the primary side turns W1 and the secondary side turns W2. Using the formula, I1/I2=W2/W1.

Current transformers generally use a closed silicon chip as an iron core. The magnetic field φ generated by the primary current I1 circulates inside the iron core, and the magnetic field can drive the closed coil wound on the iron core to form a secondary current I2.

Knowing from I1/I2=W2/W1, controlling the turns ratio of the primary and secondary wires can change the current ratio of the primary and secondary sides.

In actual application, there is generally one primary wire passing through the center hole of the current transformer, so the number of turns of the secondary wire wound on the iron core is the actual transformation ratio. For example, the ratio of 40 turns on the iron core is 40. If the maximum current on the primary side is 200A, the maximum current on the secondary side is 5A. The ammeter uses the 0-5A variation to display the 0-200A primary side. The current realizes the numerical display of the current.

The purpose of using a current transformer is to proportionally transform the large current on the primary side to the small current on the secondary side for measurement. At the same time, the transformer achieves a good line isolation between the primary side and the secondary side.

The selection points of current transformer

The selection of current transformer is mainly determined from three aspects: primary current, transformation ratio, and hole size.

Primary side current: the current through which the wire needs to be measured, and the maximum current value of the wire is used when selecting the model.

Transformation ratio: the ratio of the primary side current to the secondary side current. Conventional current transformer primary side rated current standards are 20, 30, 40, 50, 75, 100, 150, 200, 300, 400, 600, 800, 1000 (A)... etc. The conventional secondary side current is 1A , 5A, etc.

Perforation size: The transformer installation needs to be perpendicular to the wire to be measured, and the wire passes through the core. Therefore, it is necessary to ensure that the hole is large enough when selecting the type, especially when the copper or aluminum wire is inserted in the center of the transformer.

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