GENERAL THEORY OF THE INDUCTION MOTOR. 299 



simple circuits shown in Fig. 257 represents the action of an 

 actual single-phase induction motor, and the combination of 

 simple circuits shown in Fig. 258 represents the action on the 

 assumption that the magnetizing current in the stator winding is 

 constant* 



The circle diagram may be applied to the single-phase induc- 

 tion motor on the basis of Fig. 258 as follows : Measure the 

 magnetizing current and the power input of the motor at zero 

 load. This determines the vector OM, Fig. 242. Measure the 

 stator current and power input at stand-still, thus determining the 



Supply main 



Fig. 258. 



vector OP' , Fig. 242. Draw the circle exactly as in the case 

 of the polyphase motor. Measure the stator resistance R' '. 

 The core loss at zero load is equal to OM p x E f minus R f x OM z 

 (see Fig. 242). The core loss is assumed to be the same at all 

 loads. Then OQ' x E f minus core loss minus R' x OP is 

 the power delivered to the rotor at stand-still, and this power 

 divided by I 1 ( = MP ) gives the equivalent resistance, R", of 

 the rotor. 



To calculate the performance curves of the single-phase induc- 

 tion motor from the circle diagram, choose a series of positions 

 of the point P, Fig. 242, and for each position make the fol- 

 lowing calculations : E' x OQ is the total power input. The 

 rotor current I" (= I') is equal to MP, and the rotor RP 



* This assumption is not nearly so accurate in the single-phase motor as it is in the 

 polyphase motor because of the great reduction of the cross-flux <J> C with decrease of 

 speed. 



