Saturable Reactors

Although the nonlinear magnetic characteristics of ferromagnetic materials produce such undesirable effects as distortion of voltage and current waveforms, they are the basis for successful operation of certain types of devices. The saturable reactor is such a device, as it depends upon a high value of permeability in the unsaturated region and a low value of permeability in the saturated region of the magnetization curve. In general, the greater the departure of the magnetization from linearity, the more effective is the saturable reactor. An ideal magnetization curve for saturable reactor operation is shown in Fig. 7-1(b).

The nearly rectangular hysteresis loops of grain-oriented alloys of silicon-iron and nickel-iron, therefore, lead to operating characteristics that are superior to those of saturable reactors using core materials with the more rounded knees in the saturation curve. Figure 7-1 (a) shows a hysteresis loop for a nickel-iron alloy. Although the saturable reactor has found some applications since about the turn of the century its use did not become widespread until the advent of the magnetic alloys mentioned above. The improvement in dry-type rectifiers by the use of such materials as silicon and selenium compounds gave further impetus to its development.

Figure 7-1. (a) 60-cycle hysteresis loop; (b) simplified magnetization curve of cold rolled 50%-nickel, 50%-iron alloy

One of the widest applications of the saturable reactor is that of a magnetic amplifier, by means of which large amounts of electric power can be controlled, with rapid response, by making use of much smaller amounts of control power. This makes for smaller control equipment and greater flexibility than if the output power were controlled directly. Some of these applications are motor speed control, thyratron-grid phase control, current in electric welders, amplification of small d-c voltages from thermocouples, and d-c transformers for measuring very large values of direct current. Saturable reactors also make it possible to multiply frequency with relatively simple circuitry and without rotation or electronic equipment. This text includes only a brief introduction to this subject, which has been treated thoroughly by other textbooks.