TRANSATLANTIC CABLE POWER SYSTEM 155 



over 60 degrees. Fig. 7 also shows the characteristics at the extremes of 

 gain control, the range of control being about 8 db. 



Fig. 8 shows the measured performance of the dc regulators, the servo 

 system being disabled in order to obtain a plot of the performance of the 

 dc amplifier and associated circuits. Twenty-two regulator units were 

 manufactured and measured and the curves of Fig. 8 show the extreme 

 limits observed, the differences between individual regulators being due 

 primarily to differences between electron tubes. The measured range of 

 source impedance, 130,000 to 170,000 ohms, allows margin for regulator 

 tube aging above the 100,000-ohm objective. 



AC Servomechanism 



As noted earlier, the servo system shown in Fig. 2 is part of the cur- 

 rent regulating scheme and holds the series tube plate potential within 

 reasonable limits by adjusting the rectifier input voltage. In an emer- 

 gency, a "turndown" feature, operated from several remote points, 

 either manually or automatically, will reduce the autotransformer output 

 to zero in less than two seconds. For simplicity, only the manual turn- 

 down feature is shown in Fig. 2. It operates simply by switching one end 

 of the motor control winding from one corner of the bridge to the other, 

 thus applying half of the input voltage to the control winding. 



Manual operation of the autotransformer tap is provided to raise the 

 cable current slowly, either initially or after a turndown. In manual 

 operation a dynamic brake, consisting of a short circuit on the motor 

 control winding, prevents the motor from creeping or coasting when the 

 operator releases the hand wheel, as it otherwise would since the fixed 

 phase of the two-phase motor is always energized. The turndown feature 

 takes precedence over the short circuit of the motor control winding, 

 automatically, to energize the motor should the operator inadvertently 

 cause abnormally high cable voltage or current. 



One essential feature of the servo design is the dead band of the series 

 tube plate voltage in which the servo remains stationary, even though 

 there are small changes in the incoming signal. This band can be varied 

 from 10 to 100 volts under control of a gain-adjust potentiometer across 

 the control winding of the two-phase motor. Without this dead band, 

 the servo would be constantly in operation correcting for small random 

 variations in line voltage or earth potentials. Furthermore, since it is 

 extremely difficult to set the current regulators at the two ends of a 

 cable to exactly the same current, the servo dead band permits some 

 margin of error. Otherwise the servo associated with the current regu- 



