Current Sense Transformer Guide


As their name implies, current sense transformers “sense,” or detect, and measure current flow in an isolated conductor or a primary winding and produce a proportional alternating current in a secondary winding.

The current sense transformer produces an alternating current in the secondary that is proportional to the current in the primary. By having a known turns ratio, the current in the primary and secondary are linked and if the current in the secondary is measured, then the current flowing in the primary can be determined.

When only current sensing is needed, the primary winding is a conductor integral to the application that passes directly through the resistor. If the electrical current needs to be transformed, the current sense transformer may have both primary and secondary windings, or it may utilize a wire from the circuit to serve as the primary winding.

Current sense transformers “transformer” AC electrical currents from larger-current values into smaller currents that are better suited for instruments, protective relays, and control circuits. This is achieved by developing a current in the secondary winding that is proportional to that of the primary. The isolation aspect of the transformer keeps the high voltage of the primary system separate from the instrument, safety circuit, or control circuit on the other side, thereby protecting them.

Current sense transformers must draw on some of the primary circuit’s energy to complete tasks, but the load is negligible. Plus, if the loss becomes a serious concern, the output voltage can be amplified as needed. Current sense transformers can also be used to trigger circuit events when the sensed current reaches a specified level. Large current transformers are an essential component in power systems as a whole and are often used in generating stations, electrical substations, and industrial and commercial electrical power distribution.

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