Iron-Core Reactors

A reactor is a coil that has substantial inductance. Reactors that operate at and below audio frequencies usually have magnetic cores of ferromagnetic material such as laminated silicon steel. Such cores may or may not contain one or more appreciable air gaps in series with the flux path. Iron-core reactors are used for a variety of purposes, among which are suppressing the a-c flux ripple in rectifier circuits, compensating long telephone circuits, limiting starting currents in motors, and stabilizing the arc in sun lamps, arc furnaces, and electric welders.

The ferromagnetic core is used because of its comparatively high relative permeability μ_r, so that even in combination with appreciably long air gaps a magnetic circuit with high permeance is obtained. This results in the desired value of inductance with a much smaller size than if an air core were used. Inductance is related to permeance as follows

[5-1]

where

L = the inductance in henrys
N = the number of turns
P = the permeance in webers per ampere turn

Reactors with a more-or-less fixed value of self-inductance are also known as chokes or choke coils. One of the advantages of reactors is that the energy loss is usually small compared with the energy loss of resistors that have the same ohmic value. Generally, for a given volt-ampere rating, the size of the reactor decreases with increasing frequency, whereas the size of a resistor must be the same regardless of the frequency for a given power rating. In many cases, one or more air gaps are used in series with the iron to overcome the undesirable effects of nonlinearity in the B vs H characteristic of the iron. This nonlinearity causes the inductance to vary with saturation. It also causes the wave form of the current through the reactor winding to be different from that of the flux in the core. In some cases where the current has an appreciable d-c component, an air gap is used to prevent the d-c component from saturating the core and thus reducing the reactance. In the case of an air-core reactor, the B-H characteristic is linear as B = μ₀H and the current has the same wave form as the flux.