ALTERNATING CURREXT AM) VOLTAGE 



is, after it has passed from 1 to 5, it has completed one electrical 



cycle or 360 electrical time-degrees, as is shown in Fig. 6 (6). 



Mechanically, it has completed one-half a revolution, or ISOspace- 



es, so that in one revolution, or 360 space-degrees, the einf. in 



onductor will have completed two cycles, and will have gone 



through 720 electrical time-degrees. Therefore, in this case one 



space-degree corresponds to two electrical time-degrees. That 



is, for every space-degree that the coil passes through, the voltage 



wave completes two electrical time-degrees. This conductor 



- to make only 30 r.p.s. or 1,800 r.p.m. in order to generate 



a 60-cycle electromotive force. Likewise for a B5-cycle elec- 



tromotive force, this conductor needs to revolve at only 12.5 



or 750 r.p.m. For a given frequency, as the number of 



])(!(< increases, the mechanical speed decreases proportionately. 



The relation between speed, poles and frequency maybe written 



in the form of an equation: 



PXS PXS 



J ' o xx an ~ i on v^/ 



2 X 



120 



where/ is the frequency in cycles per second, P is the number of 

 poles, and S is the speed in revolutions per minute. 



The table shows the relation of speed, frequency and number 

 of poles for a few typical cases. 



r.p.m. 



>I>1>-. A 60-cyclc, <*n^m'-<lriv<Mi alternator !i:is ;i speed of 1'JO r.p.m. 



many |>ol> 



im for /' 



p. .1?0^60. eOpo.ee. A*. 



In prad :rly all alternators have stationary armatures 



and ti'lds, and the above equations apply. 



