5 YNCHRONIZA TION S YSTEM 



473 



Control Circuit 



Figure 2 shows a schematic diagram of the control circuit. When 

 the motor is operating at full speed the pilot generator delivers approx- 

 imately 1 watt of power at 300 volts, 1275 cycles to the plates of a pair 

 of push-pull detector tubes. The grids of these tubes are supplied 

 with an e.m.f. of the same frequency from an oscillator or other 

 source of power having a sufficiently constant frequency. The amount 

 of power required for this grid circuit is only a few thousandths of a 

 watt. The detector tubes rectify the plate voltage producing a po- 

 tential drop across the coupling resistance R]. If the plate and grid 

 voltages are in phase, so that the grids of the tubes are positive at the 



SLOT INSULATION 



Fig. 3 — Damping winding preventing shifting of field flux. 



same instant that the plates are positive, the plate current will be a 

 maximum. If the grid voltage is negative when the plate voltage is 

 positive the plate current is practically zero, so that the magnitude of 

 this current is a function of the phase relationship between the grid 

 and plate voltages as shown in Fig. 5. 



The voltage drop across the coupling resistance i?i is applied to the 

 grid circuits of three regulator tubes. These tubes derive their plate 

 voltage supply from a three phase transformer fed with power from the 

 three slip rings provided on the motor. These tubes act as a rectifier 

 whose output is controlled by the potential impressed upon the grids 

 from the coupling resistance R\. The current of the regulator tubes 

 is passed through the regulating field provided on the motor. This 

 field is in a direction to aid the shunt field and series fields of the motor. 



