212 ALTERNATORS. 



assisting the main field as indicated by the axis n' s'. Later 

 in the revolution the field will also be less, but will be 

 directed so as to oppose the main field, its direction being, for 

 instance, n" s". 



The average effect of the armature currents is thus to 

 distort, but neither to strengthen nor weaken the main 

 field.* 



Case II. Current Lagging. Suppose the current only 

 attains its full value after the position of maximum voltage 

 has been passed, as, for instance, after the conductors under 

 s and n have moved into the position s" n". The maximum 

 current will then produce a field in the direction s" n". 

 Positions of the armature on either side of this will corre- 

 spond to weaker fields, which will be of equal value at equal 

 angles on either side of n" s". Hence the average effect of the 

 armature currents will be the production of a field along 

 n" s", which will partly weaken and partly distort the main 

 field. This may be briefly stated thus : A lagging current 

 tends on the whole to weaken the main field. 



Case III. Current Leading. Suppose that the current 

 leads, so that it attains its maximum value before the position 

 of the conductors producing maximum voltage, for instance 

 when the conductors shown under n s are at n' s'. Then 

 the maximum armature field will be induced in the direction 

 n' s', and the average magnetic reaction will be such that 

 the main field is both strengthened and distorted. Hence a 

 leading current will on the whole strengthen the main field 

 of an alternator. 



Exactly similar reasoning applies to the case of a multi- 

 polar alternator, whether with fixed or rotating armature. 

 This action necessarily makes it more difficult to regulate 

 an alternator with varying load when the power factor of 

 the circuit is low, i.e., when there is considerable phase 

 difference between current and voltage. The amount of 

 the effects to be expected may be found as a result of the 

 following experiments. 



The foregoing reasoning may be put into more exact 

 form by the statement, which will be readily seen to be true, 

 that if the ampere turns of the main .magnetic circuit be 

 N C and the product of current and windings in the armature 



* The effect of decreased permeability owing to the concentration 

 of the lines under one pole tip is here neglected. 



