Effect of Air Gaps

An iron-core reactor is much smaller than an air-core reactor of the same value of inductance and current rating. However, the inductance of an air-core reactor is constant regardless of the degree of the magnetic flux density. The inductance of an iron-core reactor varies with flux density, its value decreasing with increasing magnetic saturation.

Figure 5-11. Reactors with tapped windings, (a) Shell-type; (b) core-type.

Reactors are used for controlling current in apparatus such as arc furnaces and a-c welders. In such arrangements the winding of the reactor is generally provided with taps, so that different numbers of turns may be excited and different values of inductance or reactance obtained thereby. However, for a given tap setting and at constant frequency, the inductance should not vary excessively with variations in the voltage across the reactor. Figures 5-11 (a) and 5-11(b) are schematic representations of reactors with tapped windings such that three different values of inductance may be had in each. One or more air gaps in series with the flux path will make the characteristic of the reactor more nearly linear.

In some applications the distortion in the current, caused by the nonlinear characteristic of the iron, is objectionable and the reactor is built with a small air gap in the iron circuit. The effect of the air gap is to increase the fundamental component of the magnetizing current without materially affecting the core-loss current. The air gap then has the effect of smoothing out the current waves.

Reactors that are used for decreasing the ripple in rectified current generally have air gaps in their cores to prevent the d-c component of the current from saturating the iron and making for a low value of inductance, thus imparing the effectiveness of the reactor.