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Current transformer working principle
Apr 07, 2018

In the power generation, transformation, transmission, distribution and power lines, the currents vary widely in size from several to several tens of thousands of amps. In order to facilitate measurement, protection, and control, it is necessary to convert to a more uniform current. In addition, the voltage on the line is generally higher. If direct measurement is very dangerous. Current transformers perform current conversion and electrical isolation [1].

For pointer-type ammeters, the secondary current of the current transformer is mostly in the ampere class (eg 5A, etc.). For digital instruments, the sampled signals are typically in the milliampere range (0-5V, 4-20mA, etc.). The secondary current of the miniature current transformer is in the order of milliampere, and it mainly acts as a bridge between the large transformer and the sampling.

Miniature current transformers are also known as "instrument current transformers." ("Instrument current transformer" has a layer of meaning is used in the laboratory more than current precision current transformer, generally used to expand the instrument range.)

Similar to current transformers and transformers is based on the principle of electromagnetic induction, the transformer is the voltage and the current transformer is the current flow. The current transformer is connected to the winding of the measured current (the number of turns is N1), which is called the primary winding (or the primary winding, the primary winding); the winding connected to the measuring instrument (the number of turns is N2) is called the secondary winding (or secondary winding) Winding, secondary winding).

The current ratio of the primary winding current I1 to the secondary winding I2 of the current transformer is called the actual current ratio K. The current of the current transformer working at the rated current is called the current ratio of the current transformer, which is expressed by Kn.


Current transformer (referred to as CT) can convert the primary current with a relatively large value into a secondary current with a smaller value for protection and measurement purposes. For example, a current transformer with a ratio of 400/5 can convert an actual current of 400 A into a current of 5 A.