200 ALTERNATORS. 



The steeper the inclination of the first portion of the 

 curve, the less is the reluctance of the magnetic circuit. 

 At a higher induction, the curve bends decidedly to the right, 

 and then again becomes comparatively straight, making a 

 much smaller inclination with the horizontal. When working 

 upon this part of the curve, the effect of small variations in 

 the excitation upon the voltage of the alternator is com- 

 paratively slight, and the ratio of magnetising current to 

 magnetic flux is considerably greater, owing to the decreased 

 permeability of the iron at the higher saturation. 



Magnetic Leakage. The magnetic leakage increases at 

 higher magnetic densities in practically the same propor- 

 tion as the excitation, since the reluctance of the leakage 

 path is practically constant, and, consequently, the leakage 

 flux varies directly with the field ampere turns. The effect 

 of the leakage upon the curve will be to cause it to incline 

 slightly to the right. 



Magnetisation Curve under Load. The effect of a current 

 flowing in the armature upon the terminal voltage is to pro- 

 duce a loss of voltage due to the following causes : 



(1) Armature resistance. 



(2) Armature self-induction. 



(3) Eddy currents in pole faces. > 



(4) Demagnetising and distorting action of armature 



currents. 



The first two causes produce a loss of voltage numerically 

 equal to the product of armature current and armature 

 impedance. 



The voltage overcoming impedance will not usually be in 

 phase with the total voltage generated. This must be borne in 

 mind in subtracting the armature impedance volts from the 

 total voltage. 



The third cause is one depending largely on the construc- 

 tion of the machine and the method of winding. It should 

 not contribute very much to the armature drop. 



The last cause of loss of voltage might at first appear 

 incongruous since the current in the armature is an alter- 

 nating one, and it might be thought that it would consequently 

 not affect the permanent field. It must be remembered, 

 however, that the rate of alternation of the current is the 

 same as the rate at which an armature coil moves from the 

 centre of one pole to the centre of the next. Hence, if the 



