Quadrature current transformer, quadrature transformer¹⁾, air-gap transformer²⁾, compensator³⁾, transactor⁴⁾ or transreactor⁵⁾ - a type of current transducer with magnetic core which has a significant air gap⁶⁾ The output signal of such a transformer is voltage (rather than current as it is for an ordinary current transformer).

Simplified configuration of a quadrature current transformer with a significant air gap in the magnetic core⁷⁾

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

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.⁸⁾

From theoretical viewpoint the operation of a quadrature current transformer is similar to that of a Rogowski coil. For this reason if countermeasures are not taken and the output voltage is measured directly 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 passive⁹⁾ or active integrators.¹⁰⁾ Quadrature transformers are used in protection of electric circuits, especially the high-power grid.¹¹⁾¹²⁾

In some applications the air gap can be distributed, for instance by employing magnetodielectric materials.¹³⁾

The invention of a “quadrature transformer” is credited to Wiliam Edward Sumpner, who used this device as a part of his wattmeter in 1907.¹⁴⁾

• Quadrature voltage transformer
• Linear coupler
• Current transformer
• Air gap
• Rogowski coil