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1134 HANDBOOK OF PROJECTION FOR
The A. C. motor control box, Fig. 412, contains four vacuum tubes. Of these, one is used to supply rectified current for the field of the pilot alternator. Two more tubes act as rectifiers, supplying current to one winding of a special choke coil. This coil has a second winding placed in the circuit which connects the motor brushes. When the current through the first winding is large the choking action of this coil is not very pronounced ; therefore a relatively large rotor current can pass and the motor can speed up. As the current through the first winding is decreased, the choking action of the coil is increased, hence the motor speed is retarded. It then follows that the motor speed can be controlled by regulating the output of the rectifier tubes.
The output of the rectifier tubes depends on the bias supplied to the grids and this, in turn, is controlled by the fourth vacuum tube which is operated as an amplifier by the tuned circuit. At speeds below 1200 R. P. M. the grids of the two rectifier tubes have very little negative bias. These tubes therefore pass a relatively large current through the first winding of the choke coil, therefore a large rotor current circulates, allowing the motor to speed up. At 1200 R. P. M. the tuned circuit functions, causing the negative bias of the rectifier tubes to be increased, which decreases their output and therefore causes the choke coil to cut down the rotor current so that no further speed increase can take place. If the speed tends to go above 1200 R. P. M., this effect becomes still more pronounced.
The A. C. 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 is opened and does not function ; the vacuum tubes