473 



FARM MOTORS 



638. Magneto alternator. 



Fig. 358 shows a magneto 

 armature with the wires off. 

 This is probably the most 

 simple commercial electrical- 

 current generator used. It 

 is only applicable for such 

 uses as cigar lighters, tele- 

 phone calls and line testers. 

 FIG. 359 SYSTEM OF WIRING FOR A For large purposes it is too 

 MULTIPOLAR ALTERNATOR inefficient. 



639. Multipolar alternator. The number of alternations 

 in a dynamo as just described is 4,000 a minute with a 

 speed of 2,000 revolutions a minute. This speed is as 

 high as advisable, but the number of alternations is only 

 about half as high as is considered good practice. For 

 this reason large commercial dynamos are built with 

 several poles, as shown by Fig. 359, and the number 

 of revolutions reduced. The dotted lines in Fig. 359 

 represent the directions and paths of the lines of force. 

 The full lines indicate the windings, and the arrowheads 

 the direction of current. By carefully following out the 

 direction of the induced current it will be seen that the 

 coils passing beneath the north poles have a current set 

 up in them which is opposite in direction to that set up 

 in the coils passing under the south poles. By inspecting 

 the windings it will be noted that the direction is reversed 

 between each set of poles, hence the current set up 

 through the system is the sum of all the currents set up 

 at each pole. As the coils of the armature pass across 

 the points midway between the poles, the direction of 

 current is alternated. The number of alternations to the 

 minute is found by multiplying the number of poles by 

 the number of revolutions to the minute. 



