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DRIVES 613
Grid Bias Circuits of V-l, V-2 and V-3
The positive bias of the full-wave rectifier is obtained by means of a voltage drop in the field exciting current that is provided by the half-wave rectifier, V-3. The circuits of this tube are conventional, and need not detain us long. Its filament is heated (in series with the filament of V-4) by the third secondary of the power transformer T-l. The bottom secondary provides plate power for V-3. From the top of that plate secondary the circuit may be traced to the plate of V-3, thence to the filament of that tube, down through R-8 and R-9, left across the drawing and through the field of the induction machine, up and around the edge of the drawing to the bottom of fourth secondary of T-l. There is a voltage drop through R-8 and R-9, which as we have just seen are positive at top and negative at bottom. The grids of V-l and V-2 connect to the top or positive side of R-8 through resistors R-6 and R-7. The filaments of V-l and V-2 connect, through the milliammeter and the arrowhead contact, to a more negative point, lower down on R-8 or R-9. The grids of V-l and V-2 are therefore positive with respect to the filaments of those tubes.
Now the speed of our motor is controlled by the space current through V-l and V-2, and therefore by the bias of the grids of those tubes. Whenever that motor departs from proper speed the alternating current generated in the induction machine armature will become either more or less than 720 cycles, and an appropriate correction must be applied to the grids of V-l and V-2. This is done by a change in the voltage drop through R-6 and R-7, which voltage drop is in series with the source of grid bias through V-l and V-2. Therefore, the object of the 720-cycle a. c. developed in the armature of the inductor is to control the direct current through R-6 and R-7, since the drop across these resistors depends upon the amperage flowing through them (Ohm's Law).
The Circuits of V-4
The filament of V-4 is lit in series with that of V-3,