310 ELECTRICAL MACHINERY 



it reads positive power or power output. Now when the steam 

 is shut off from the prime mover of No. 2 it will be noticed 

 that W indicates negative power or power input, that is, 

 generator No. 2 is running as a motor and when so running 

 it is styled a synchronous motor. 



Speed of Synchronous Motor. If an alternator is to be 

 used as a synchronous motor it has some features of design 

 different from those of a generator but essentially a syn- 

 chronous motor is nothing but an alternating current 

 generator operating in a manner opposite to its normal 

 operation. In two respects the synchronous motor and 

 generator are identical; they both have separately excited 

 fields (generally rotating) and they both run at synchronous 



I Feeders 

 1 Supplying Load 



I I Bus Bars 



Prime Mover "3t/7$v Prime Mover 



gjV^^^Q 



No.l No.2 



FIG. 200. Alternators Operating in Parallel. 



speed, no matter what the load may be. A 10-pole syn- 

 chronous motor supplied with 25-cycle power would run 

 300 r.p.m. no load, 300 r.p.m. half load, and 300 r.p.m. 

 full load, and the same speed even if overloaded. But a 

 10-pole generator running 300 r.p.m. would generate an 

 e.m.f. of 25 cycles per second, hence the significance of the 

 term " synchronous " motor. 



80. Starting Characteristics. The synchronous motor 

 may be started by some auxiliary driver and, after being 

 brought up to synchronous speed and proper voltage, it 

 may be connected to the line just as though it were an 



