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Current transformer working principle

2025-05-23

A current transformer (CT) operates on the principle of electromagnetic induction. It uses a core and two windings, a primary and a secondary, to convert a high current into a smaller, more manageable current for measurement or protection. The primary winding is connected to the circuit carrying the high current, and the secondary winding has a smaller number of turns, resulting in a stepped-down current suitable for meters and relays.

Elaboration:

Electromagnetic Induction:

The AC current flowing through the primary winding generates a magnetic field in the core. This changing magnetic field induces a current in the secondary winding, which is proportional to the primary current.
Primary and Secondary Windings:

The primary winding typically has a few turns and carries the high current of the circuit being measured. The secondary winding has more turns, creating a lower, more manageable current.
Core Material:

The core is usually made of a ferromagnetic material like iron or steel, which helps to concentrate the magnetic flux and efficiently induce the current in the secondary winding.
Ratio:

The ratio of turns between the primary and secondary windings determines the current transformation ratio. For example, a 100:5 ratio CT will reduce a 100-ampere current in the primary to a 5-ampere current in the secondary.
Measurement and Protection:

The stepped-down current in the secondary winding is used for various applications, including measuring the current with ammeters, triggering protection relays, or monitoring the power system.
Safety:

CTs are also important for safety, as they allow measurement and protection devices to be connected to the high-current circuit without directly handling the high current.
Key Points:
The secondary winding should never be open-circuited when the primary is carrying current, as this can lead to dangerous overvoltages and core saturation.

The secondary winding should always have a load connected to it to ensure proper operation and prevent the core from saturating.