The current transformers convert through the phenomenon of electromagnetic conversions, high currents, that circulate in their primary in small secondary currents, according to a transformation relation. The primary current measured by circulating in the primary windings creates an alternating magnetic flux which deflects the electromotive forces Ep and Es at the same time in the primary and secondary windings.
Thus, if a current of 100 A is circulated in the primary terminals of a current transformer with a nominal transformer ratio of 20, the current of 5 A is reached in the secondary, ie 100/200 = 5 A.
Current transformers are devices that allow the measuring and protection devices to operate perfectly without the need for nominal currents, such as the load current of the circuit to which they are connected.
In their simplest form they have a primary, usually with few turns, and a secondary one in which the modified rated current is, most of the time, equal to 5A. In this way, the measuring and protection devices are dimensioned in reduced sizes, with the coils of chains consisting of yarns with small amounts of copper.
Current transformers are used to provide devices with low electrical resistance, such as ammeters, induction relays, differential relay current coils, power meters, power meters, etc.