ALTERNATORS. 



197 



Let n = revolutions per minute at normal speed. 

 n l = revolutions per minute observed. 

 V = required voltage corresponding to -normal speed. 

 F 1 = voltage actually observed at speed n l . 



*W 



Then F = F 1 



n l 



DETERMINATION OF MAGNETISATION CURVE AT NO-LOAD. 



Observer Date 



Alternator No Type 



Normal output volts, amps., at revs, per 



minute. 



In the table of results given above, the column headed 

 " Corrected Voltage " is obtained from the preceding column 

 headed " Actual Voltage," by correcting for speed varia- 

 tions. 



The readings obtained should form two curves, the curve 

 with decreasing excitation being slightly higher than the 

 curve with increasing excitation forming the first series of 

 readings. This is due to the hysteresis of the iron forming 

 the magnetic circuit. The final curve is the mean of the 

 two, corresponding to the mean of the two voltages 

 observed for each value of the exciting current. 



The curve will probably not pass through zero on account 

 of the residual magnetism which will remain in the magnets, 

 and enables the machine to generate a low voltage without 

 excitation. 



The curve in Fig. 92 is the magnetisation curve of a 6 kw. 

 4-pole alternator. In this case the rotating field showed no 



