THE L3 SYSTEM — DESIGN 825 



The terminal equipment is normally mounted on standard 11' 6" duct- 

 type bays. The submastergroup and mastergroup equipment required 

 to handle 1,860 channels occupies two complete bays and portions of 

 two others. One carrier and pilot supply is mounted in three bays. 



3.6 POWER GENERATION AND TRANSMISSION 



Power is transmitted to the auxiliary repeater over the inner con- 

 ductors of each pair of coaxials as noted earlier. A maximum of twenty- 

 one auxiliary repeaters can be fed from a main repeater. This limit is 

 determined by the maximum potential the cable can safely withstand. 

 Shorter spacings are dictated by geographical and plant layout con- 

 siderations. 



The power supplied to the coaxials at the main stations is generated 

 by a motor-alternator set which consists of the alternator, an induction 

 motor, a dc motor and its exciter, all coupled together on the same shaft. 

 Normally, commercial power is used to drive the induction motor. When 

 this source fails or the voltage goes out of prescribed Umits the drive 

 is transferred to the dc motor w^hich operates from a 130-volt battery. 

 If the commercial power is unusable for more than 23-^ minutes an 

 emergency engine alternator is started and after a five minute warm-up 

 period it replaces conmiercial power in driving the regular induction 

 motor. 



The constant current to the coaxials is supplied through a power con- 

 trol circuit which accurately regulates it to within =bl per cent of the 

 desired value. A simplified schematic of the power control unit is shown 

 on Fig. 22. The unit consists of two motor driven continuously variable 

 transformers which supply power to the line transformer. The course 

 control variable transformer is relay controlled and maintains the line 

 current within ±3 per cent of the prescribed value. The range of this 

 transformer is sufficient to permit reducing the voltage to zero in order to 

 turn down power on the system for maintenance purposes. The fine 

 control variable transformer is regulated by an electronic regulator to 

 maintain the line current within ±1 per cent of the desired value. 



The change in the line current in response to commercial power 

 transients and transients introduced by changes in the motors driving 

 the alternator requires careful consideration. By increasing the inertia of 

 the motor alternator set with a fly wheel and carefully designing the 

 frequency response of the above described power control regulator a 

 satisfactory transient response has been obtained. 



For the maximum length power section the potential appHed to 



