Saturable Reactors

Author: E.E. Kimberly

Fig. 28-11. Circuit With Saturable Reactor

Because of the non-linear shape of the saturation curve of iron, the flux per ampere, and hence the inductance of an iron-core reactor, is a complex function of the flux density in the core. As the flux density increases beyond the first approximately straight portion of the magnetization curve, the reactance decreases rapidly. By superposing a uni-directional but variable mmf on the core, a variable reactor without moving parts may be obtained. Fig. 28-11 shows one of the simpler methods of doing this. The a-c coils produce simultaneous mmf's, and hence flux, around the long outer magnetic circuit but none of appreciable magnitude in the middle leg. In this way, no a-c voltage is induced in the d-c control coil on the middle leg. Flux produced by the d-c control coil divides between the outside two legs and produces a variable "pre-saturation" in them. The core is said to be "biased" by the d-c ampere-turns. The effect of bias on the reactance of the coil is evident in Fig. 28-12. If the core is biased by 2 ampere-turns per inch and an alternating current producing NIm = 0.5 is passed through the a-c coil, the reactance of the coil will swing from 27.5 ohms when the alternating current is negative to only 13 ohms when the current is positive. The average reactance is about 20.2 (from the graph).

Fig. 28-12. Reactance Variation in a Saturable Reactor

However, the average reactance affecting the negative part of the a-c cycle is about 14.5 ohms and that affecting the positive half is about 16.5 ohms. Therefore, when the core is biased to a point on the bend of its magnetization curve, the reactance is not the same on both halves of the current cycle and the current will be distorted. If the bias were 5 ampere-turns per inch, the average reactance would be only 3 ohms and the current would be distorted less than with a 2-ampere-turn bias. With a bias of only 1 ampere-turn, the maximum reactance would be 27.5 ohms and both halves of the current would be equally affected. In Fig. 28-12 a change in reactance in the ratio of 10 to 1 is accomplished by a change in direct control current of less than 4 to 1. For some applications the distortion of current caused by biasing the core to a point near the curved part of the magnetization curve is not objectionable. In other cases such distortion is objectionable and the use of the saturable reactor must be restricted to the more nearly linear parts of the curve.