THE^INDUCTION MOTOR. 



The dependence of the primary current on the 

 secondary or rotor current is exactly similar to that 

 existing in the case of a static transformer. We may 

 illustrate the relation by referring to Fig. 188. 



In the case of the unloaded motor, the primary 

 current will be the magnetising current necessary to 

 produce the rotating field. This rotating field we may 

 write equal to Z v = k^ c t l where k^ is constant, and c t ^ 

 represents the primary ampere turns. The rotor current 

 which is produced when the motor is loaded gives rise to 

 a magnetic flux, perpendicular to Z^ having a value 

 Z 2 = & 2 C 2 1-2, as illustrated in Fig. 175 (page 351). The 

 ampere turns supplied to the motor will have a value suffi- 

 cient to maintain the resultant flux. Consequently, if Z is 

 the resultant flux, the primary current will be given by the 



expression , The varying relation, between the 



primary and secondary currents may be represented by 

 the diagram Fig. 188, in which the horizontal line 



FIG. 188. RELATION BETWEEN PRIMARY AND SECONDARY CURRENTS. 



represents the constant no-load ampere turns required to 

 produce the primary flux ; the vertical side represents a 

 series of values of the secondary ampere turns and the third 

 side represents the resultant ampere turns of the primary 

 winding. Fig. 189 shows the relations between primary 

 and secondary ampere turns given in Fig. 188 plotted in 



