232 ELECTRICAL MACHINERY 



Frequency Fixed by the Speed and the Number of Poles. 

 The frequency of the e.m.f. which an alternator generates 

 depends upon its speed and upon the number of poles. A 

 complete cycle is generated in any conductor on the arma- 

 ture when a pair of poles have moved past it, hence to find 

 the frequency of an alternator it is only necessary to calculate 

 the number of pairs of poles passing any armature conductor 

 in one second. 



Typical Cases. Take, for example, a four-pole machine 

 turning 1800 r.p.m.; there are two pairs of poles, so that 

 every time the field makes a complete revolution there are 

 two cycles generated in any armature conductor. Now 

 this machine is turning 30 rev. per sec. and hence the fre- 

 quency is equal to 30X2 = 60 cycles per second. Suppose 

 a 10-pole machine turns 300 r.p.m.; it generates 5 cycles 

 per revolution and makes 5 rev. per sec., therefore it 

 generates a frequency of 5X5 = 25 cycles per second. 

 A certain turbo-alternator has two poles and turns 1500 

 r.p.m.; it generates therefore 25 cycles per second. 



58. Excitation. Attempts have been made to build 

 self-exciting alternators but they have been failures as 

 evidenced by the fact that there are practically none in 

 service, except an occasional machine of very small capacity. 

 These small alternators with self excitation are sometimes 

 called double current generators. 



Separate Excitation from Small C-C. Machines. All 

 alternating current stations have two or more small, self- 

 exciting, shunt-wound (or compound-wound), continuous 

 current machines whose sole purpose is to furnish the field 

 current for the alternators. As the amount of power used 

 in the field coils of an alternator is generally less than 5% 

 of its capacity, the exciters may be comparatively small 

 machines. An alternator having a capacity of 1000 kw., for 

 example, would require an exciter of about 25 kw. capacity. 



Arrangement of Exciters in a Station. Sometimes each 

 alternator is furnished with its own exciter (either belted 



