CONVERSION OF ALTERNATING CURRENT. 



171 



is not suited to heavy service, that is to say, it cannot rectify a 

 large alternating current at a moderately high voltage because of 

 its excessive tendency to spark. 



(b) The ntotor-generator. Fig. 145 shows a 500 kilowatt 

 compound wound direct-current generator directly coupled to a 

 three-phase synchronous motor with a small direct-current ex- 



500 Kw., 575 Volt, Compound Wound Generator and 1100 Volt, Three-Phase, 3000 Alternation, 

 Synchronous Motor; 375 R.P.M., with .A.C. Starting Motor and D.C. Exciter. ( Westinghouse 

 Company. ) 



Fig, 145. 



citer on one end of the main shaft, and a small induction starting- 

 motor on the other end of the main shaft. The synchronous 

 motor takes power -from a three-phase alternating-current supply, 

 and drives the direct-current generator which delivers direct cur- 

 rent. Generally the field excitation of the synchronous motor is 

 supplied from the main direct-current generator without the use 

 of a small auxiliary exciter ; and whenever direct current is 



