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DRIVES 609
through the circuits of this cabinet to control the inductance of L-2, and hence to control the motor speed.
The analysis of those internal circuits will unfortunately make somewhat hard reading. There is no help for that, no matter what pains are taken to make the account clear and simple. These particular vacuum tube circuits are altogether different from those we have previously traced in amplifying equipment. We will now have to encounter positive grid bias, grid-controlled rectifiers, and other strange animals. All resting, of course, on the same electrical fundamentals, but still perhaps a trifle confusing in their newness. If the reader will carefully refuse to let himself feel puzzled, remembering that if he understands the elements of electricity and of vacuum tube action he must be able to understand these circuits, he should find the paragraphs that follow much easier to digest.
The Impedance of Coil L-2
L-2 is wound on an iron core (not shown in the drawing). The same core also carries the coil drawn just above L-2 at right angles with it. We shall call this second winding the control coil. The two together, with their core, make up the retard coil, 144-A.
Direct current provided by the full wave rectifier (tubes V-l and V-2) flows through the control coil, creating a magnetic field that very nearly saturates the iron core on which it is wound. The impedance of L-2 depends upon the degree of saturation of that core. A fully saturated magnetic core cannot absorb an additional field (cannot be magnetized further). If this core were completely saturated the impedance of L-2 would be comparatively small. An increase in the direct current through the control coil increases the saturation of the core, reduces the impedance of L-2, and permits the rotor to speed up. Conversely, a decrease in the direct current through the control coil raises the impedance of L-2 and compels the motor to slow down.
The 720-cycle current generated in the inductor arma