234 



ALTERNATING CURRENTS 



have advanced 90 electrical time-degrees, but the rotating field 

 has advanced only 45 space-degrees. That is, the advance of the 

 rotating field in space-degrees is equal to one-half the advance 

 of the currents in electrical time-degrees. Therefore, the speed 

 of such a field in revolutions per second is equal to one-half the 

 circuit frequency in cycles per second. 



In general it may be stated that in order to produce a two-pole 

 rotating field, the angular space-degrees between the phase belts 



1234 



(2) (3) (4 ) 



FIG. 226. Rotating field produced by 3-phase currents in a 4-pole, induction- 

 motor winding. 



of the winding must be the same as the electrical time-degrees between 

 their respective currents. If the machine has p poles, the angular 

 space-degrees between phase belts is 2/p times the electrical 

 time-degrees between their respective currents. For example, 

 in a six-pole, three-phase machine, the successive phase belts 

 start 40 from each other, that is, % X 120 or 40. In the 

 ordinary drum windings, however, (see Chap. V, pages 108 and 

 109, Figs. Ill and 112), the coil-sides lap back so that in the 

 above example the reversed phase belts would be 20 apart. The 

 currents in such adjacent belts are 60 apart, as shown in Fig. 226. 

 To reverse the direction of rotation of a two-phase, rotating field, 

 reverse the leads of either phase ; to reverse the direction of rota- 

 tion of a three-phase rotating field, interchange any two leads. 



