Quadrature current transformer



Quadrature current transformer, quadrature transformer, air-gap transformer , compensator , transactor or transreactor - a type of current transducer with magnetic core which has a significant air gap . Reluctance of the air gap is much greater than that of the core. The output signal of such a transformer is voltage (rather than current as it is for the ordinary current transformer).

The flux density in the magnetic circuit is almost directly (linearly) proportional to the magnetising primary current. The output voltage across the secondary winding is proportional to the rate of change of the flux density, hence also to the rate of change of primary current.

For sinusoidal signals the secondary voltage lags the primary current by 90 or those signals are in quadrature, hence the name.

From theoretical point of view operation of quadrature current transformer is similar to that of Rogowski coil. For this reason if not countermeasures are taken the accuracy of current measurement is reduced for distorted waveforms due to the influence of higher harmonics. This can be compensated by using for instance passive or active integrators.

Quadrature transformers are used in protection of electric circuits, especially the high-power grid.

In some applications the air gap can be distributed, for instance by employing magnetodielectric materials.

The invention of "quadrature transformer" is credited to Wiliam Edward Sumpner, who used this device as a part of his wattmeter in 1907.