222 



THE MAGNETIC CIRCUIT 



[ART. 66 



winding, the distribution of the secondary currents is prac- 

 tically an image of the primary currents. Neglecting the mag- 

 netizing ampere-turns necessary for establishing the main or 

 useful flux, the secondary ampere-turns per pole per phase are 

 equal and opposite to the primary ampere-turns. A similar 

 assumption is also made in the preceding article, in the case of 

 the transformer. This assumption is not as accurate' in the 

 case of an induction machine, because here the magnetizing 

 current is proportionately much larger, due to the air-gap; 

 nevertheless, the assumption is sufficiently accurate for most 



FIG. 54. The slot and zig-zag leakage fluxes in an induction machine. 



practical purposes. Even if the magnetizing current is equal 

 to say 25 per cent of the full-load current, the difference between 

 the primary and the secondary ampere-turns should be less 

 than 10 per cent, because the magnetizing current is considerably 

 out of phase with the secondary current. 1 



With this assumption, the primary and the secondary current 

 belts shown in Fig. 54 may be considered as two sides of a narrow 

 fictitious coil which excites the leakage flux, causing it to pass 

 circumferentially along the active layer. 2 Neglecting the mutual 



1 See the circle diagram of an induction motor, for instance, in the author's 

 Experimental Electrical Engineering, Vol. 2, p. 167. 



2 Although the secondary frequency is different from the primary, with 

 respect to the revolving rotor it is the same as the primary frequency with 

 respect to the stator. Let s be the slip expressed as a fraction of the primary 

 frequency. Then the speed of the rotor is (1 s), and the frequency of the 

 secondary currents with respect to a fixed point on the stator is s + (1 s) = 1. 



