216 BELL SYSTEM TECHNICAL JOURNAL 



variation in the actual voltage regulation for any two load conditions 

 amounted to a change in heating time of approximately one minute. 

 This small difference permits the use of one heating period for all 

 cable as some variation in final temperature is permissible. As will 

 be noted, the generators gave a slightly rising voltage as the current 

 dropped, which contributed to a shortening of the heating period. 



The control panel located at the dryers contains switches for starting 

 and stopping the motor generator set, as well as indicating and control 

 equipment for properly conducting the drying operation, Fig. 2. In- 

 cluded in the indicating equipment is a voltmeter and ammeter for 

 showing the potential and current readings for the cable at any time. 

 At the beginning of the heating cycle the operator adjusts the rheostat 

 controlling the field amperage in the generator until the proper voltage 

 is obtained across the cable. This reading is determined from the 

 cable length since the voltage for the specified current density is 

 directly proportional to the cable length. Having established the 

 proper voltage the operator then checks the ammeter for the proper 

 current reading which, in turn, is determined by the size and number 

 of conductors in the cable. To simplify the work, charts are prepared 

 which show the voltage and corresponding starting current for all 

 lengths and sizes of cable to be dried. 



As the cable increases in temperature the current falls in direct 

 proportion to the increase in conductor resistance, which in turn is 

 determined by the temperature coefficient of copper (voltage con- 

 stant). A chart can be used, therefore, to determine the final current 

 for the desired drying temperature. As noted above, the voltage rises 

 slightly as the current falls and the corresponding correction is made 

 in the chart for final current readings for the various sizes of cable. 



Such a control by final current readings is all that would be needed 

 if the set were operated manually. However, it is not only more 

 economical to release the operator from watching the ammeter but it 

 has proved advisable for the safety of the product to control the 

 drying cycle automatically. This is accomplished by a time relay and 

 a temperature controller, either of which operates at a predetermined 

 setting to open the field circuit on the direct current generator. The 

 opening of the field circuit operates a signal device to let the operator 

 know that the heating cycle is over so that the next one can be started 

 without delay. The time relay is set from actual experience for a 

 period slightly longer than the required heating time and serves as a 

 protection in case of failure in the heating control circuit. The tem- 

 perature controller is operated from a thermocouple embedded be- 



