ALTERNATORS. 213 



be n c, the effective (R.M.S ) value of the magnetising 

 action of the armature currents will be -7 n c, and the total 

 effective demagnetisation of the fields is due to 



N C -1 n c sin </> ampere turns|, 



i.e., the armature current diminishes (or increases 



if be negative) the effective ampere turns of the field 



by the amount -7 n c sin <f> J, where < is the angle of lag of 

 the current in the armature. 



Characteristic of the Alternator. The determination of 

 the loss of the voltage produced by a given current is a very 

 important matter, and the result of such a determination 

 is one of the most important records of the behaviour of 

 the machine. 



The curve comparing the voltage and load current is 

 called the " characteristic curve " of the machine, and forms 

 the subject of the next experiment. 



After determining the variation in voltage produced by 

 the armature current, it will be possible to partially separate 

 the loss of voltage into that due to the various factors 

 enumerated above. 



f A multiplying factor must be introduced if the relative positions 

 of windings and pole make the armature windings not all equally 

 effective. The value of n may usually be taken to be 

 total number of armature conductors 

 number of poles 



J This is obtained as follows : 



The actual current in the armature conductors may be written 

 c = C sin ( - <j> ) where <f> is the angle of lag of the current behind 

 the voltage, and 6 is the angular distance of the conductors from the 

 point corresponding to minimum electromotive force, C being here 

 maximum value of current in conductor. 



The demagnetising action of the current when the conductors are 

 in any position is proportional to the resolved component of the 

 current parallel to main field, i.e., to cos0, since the magnetic field pro- 

 duced is perpendicular to the main field. Thus the field due to the con- 

 ductors opposes the main field, when = 90, and assists it when 

 6 = - 90. 



Hence the demagnetising effect of the current in the conductors at 

 any instant is 



7/1 n c cos 6 = n C sin (0 - <) cos 

 = n C (sin cos <p - cos sin <f>) cos 

 = n C (sin cos cos <f> - cos <2 6 sin <f>) 

 the average value of sin cos 6 O 



cos 2 = \. 



Hence the average value of m= -\n C sin <f> 



= -^ n \2* c sin <f> 

 -Inc. sin (p. 



