THE INDUCTION MOTOR 



335 



seen from Fig. 219 which shows the direction of the fields 

 given in Fig. 218. 



Action of any Polyphase Winding. A three-phase 

 winding, supplied with three-phase power would give a 

 rotating field in exactly the same manner as has just been 

 described for the two-phase winding. In fact, any poly- 

 phase winding connected with a suitable polyphase power 

 supply generates a rotating magnetic field, which remains 

 constant in strength as it rotates. 



91. Speed of Rotation of the Field. By referring tc 

 Figs. 218 and 219, it is seen that, in one cycle of current, 

 the magnetic field travels from pole 1 all the way around 

 the stator and back to pole 1. If we had represented a 



Direction of magnetic field for each 90bf a cycle. 



FIG. 219. Direction of Magnetic Field at Successive Instants. 



stator having two pairs of poles per phase, an analysis of 

 the rotating field would have shown that it moves half 

 way around the stator in the time required for one cycle of 

 the current. It is seen that the magnetic field travels over 

 one pair of poles per phase for one cycle of current^ and this 

 irrespective of the number of poles or phases. A two- 

 phase motor having four poles per phase requires two 

 cycles of current for a complete revolution of the magnetic 

 field; this holds true whether the motor is two-phase, three- 

 phase, or any number of phases. 



Calculation of the Speed of the Field. When it is stated 

 that an induction motor has four poles, four poles per phase 

 is always meant. A four-pole, three-phase motor, for 

 example, would actually have twelve coils (or sets of coils). 



