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MANAGERS AND PROJECTIONISTS 1133
control box are three vacuum tubes. Two of these act as rectifier tubes, changing the pilot generator current to D. C. for supplying the regulating field on the motor. The amount of current these tubes will pass, however, depends on their grid bias, which is controlled by the third tube, which in turn is operated as an amplifier by the tuned circuit. At speeds below 1200 R. P. M. the grids of the rectified tubes are negatively biased, therefore the rectified current through the motor regulating field is small, permitting it to speed up. At 1200 R. P. M., the tuned circuit functions and the negative grid bias of the rectified tubes is decreased, causing the regulating field to strengthen so that no further rise in speed can take place. If the speed tends to go above 1200 R. P. M., this effect becomes still more pronounced.
The motor control box circuit functions in this manner when the regulating switch is in the "Reg" position. When it is in the "Var" position, the tuned circuit and vacuum tubes do not function ; the regulating field winding is supplied with D. C. from the mains through a rheostat connected with the control knob, which therefore regulates the speed as desired when it is turned.
The motor used on A. C. supply is of the repulsion type. Such motors have two windings, the stator winding, which is fixed and receives power from the mains, and the rotor winding, which is on the revolving part of the motor and is not connected to the powrer supply. This winding is connected to a commutator which has two brushes. If these brushes be connected through a circuit so that current can flow from one to the other, then the speed of the motor will depend on the amount of this current, hence by regulating the latter we can regulate the speed of the motor.