98 ALTERNATING CURRENTS 



whereas with a lead of 90, Fig. 79 (c), there is an equally strong 

 magnetizing effect. For any intermediate value of the phase differ- 

 ence, the distorting and demagnetizing (or, for a leading current, 

 magnetizing) effects are both present. 



In the case of a single-phase alternator, the armature current gives 

 rise to a simple alternating flux wave ( 18). Such a wave may, 

 however, be resolved into two rotating waves ( 19), travelling with 

 equal speeds in opposite directions. One of these waves will be 

 stationary with respect to the field-poles, and will produce the various 

 effects (according to the phase difference of the current) already con- 

 sidered, while the other will move relatively to the field-poles with 

 a speed equal to twice that of the poles themselves. Since the 

 effect of this second rotating wave alternates from a magnetizing to a 

 demagnetizing one as it sweeps past the poles, its mean reaction is 

 zero, and it merely reduces the efficiency of the machine by causing 

 additional eddy-current losses. 



48. Walker Compounded Alternator 



With lagging currents such as have generally to be dealt with in 

 practice, armature reaction is a troublesome feature, as it reduces the 

 e.m.f. and thereby renders satisfactory regulation difficult. An 

 extremely ingenious method of utilizing armature reaction in order 

 to produce an automatic increase of e.m.f. with increase of load has 

 recently been devised by Mr. Miles Walker,* and is shown diagram- 



MIXIIIAHY POlt 

 FIG. 80. To illustrate Principle of Walker Compounded Alternator. 



matically in Fig. 80. Each field-pole consists of two parts, a larger 

 one, which is surrounded by the exciting coil, and is strongly saturated, 

 and a smaller one, which is unwound, and at no load is practically 

 unmagnetized. In Fig. 80 are shown two such compound poles, and 

 the dotted curve indicates the position of the rotating wave of magnetic 

 flux due to slightly lagging currents. The field is supposed to rotate 

 counter-clockwise. As will be seen, the armature currents tend to 

 demagnetize the saturated portion of the pole, and to magnetize the 



* Journal of the Institution of Electrical Engineers, vol. xxxiv. [Part 2] p. 432 (1905). 



