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608 RICHARDSON'S BLUEBOOK OF PROJECTION
The stator is supplied with a. c. from a 110-volt line through the tumbler switch D-l in the lower left-hand corner of the control cabinet drawing. (In physical fact this switch is located conveniently at the top of the cabinet.) The same switch also supplies line a. c. to the primary of the control cabinet's power transformer, T-l, to which we shall return in a moment. The power supply to the inductor generator cannot be fully understood until the construction of that machine is explained.
The inductor generator consists of two coils, the field and the armature, both wound on iron. The field coil is excited by direct current which is provided by a rectifier tube inside the control cabinet (tube V-3). As the iron wheel revolves with the rotor its projections pass through a gap left for that purpose in the core of the field winding. The magnetic flux of the field winding is thus made to vary in strength, and an alternating current is generated in the stationary armature. The frequency of the armature current is governed by the rapidity with which the iron teeth replace each other in the gap of the field core, and therefore by the speed of the motor. At the rated speed of 1200 r. p. m., the armature frequency is 720 cycles.
We have therefore two a. c. generators in this machine, one being the armature just mentioned, and the other the rotor.
The alternating current generated in the rotor completes its circuit through the rotor brushes and the choke coil L-2 inside the control cabinet, just left of the milliammeter.
Coil L-2 is therefore the "load" upon the rotor, exactly as an arc lamp and its ballast resistor constitute the "load" on the d. c. output of an arc supply generator. The arc generator, however, is compounded to maintain constant speed under variations of load. The motor of Figure 149 is so designed that its speed is very sensitive to changes in the rotor load. Any change in the impedance of L-2 will vary the speed of this motor.
The 720-cycle current generated in the armature acts