(1) The primary winding of a current transformer is connected in series with the circuit under test, and it typically has a small number of turns. As a result, the current flowing through the primary coil is directly determined by the load current of the circuit, and it is not affected by the secondary current. This ensures that the primary current accurately reflects the actual current being measured.
(2) The secondary winding of a current transformer is connected to devices such as ammeters or relays, which have very low current requirements. Therefore, under normal operating conditions, the current transformer functions almost like a short-circuited device. This near-short condition helps maintain a stable and accurate current transformation ratio.
The ratio between the rated primary current (Iâ‚n) and the rated secondary current (Iâ‚‚n) of a current transformer is known as its rated transformation ratio, denoted as kn = Iâ‚n / Iâ‚‚n.
Since the primary rated current (Iâ‚n) is standardized, the secondary rated current (Iâ‚‚n) is typically set to a fixed value, such as 5 A, 1 A, or 0.5 A. This standardization ensures consistency across different current transformers. In many cases, the rated transformation ratio can also be approximated as the turns ratio of the primary and secondary windings, expressed as kn ≈ kN = Nâ‚ / Nâ‚‚, where Nâ‚ and Nâ‚‚ represent the number of turns in the primary and secondary coils, respectively.
Understanding these principles is essential for correctly selecting and using current transformers in electrical measurement and protection systems. Proper application ensures accurate current monitoring and safe operation of electrical equipment.
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