378 THE INDUCTION MOTOR. 



demagnetising action due to the rotor currents. Practi- 

 cally the only field which the stator windings can 

 maintain (Avith stationary short-circuited rotor) is a 

 leakage field between the teeth of the stator core, since 

 any flux entering the rotor is at once neutralised by the 

 rotor currents which it induces. The experiment repre- 

 sents, consequently, the condition of maximum leakage. 



In Fig. 184 the values of the currents shown in 

 Fig. 183 are plotted in another way, viz., as the ratio of 

 stator to rotor current. The partly-dotted straight line 

 passing through zero is the curve referred to, and 

 shows that the ratio of the currents is constant. This 

 curve is similar to the curve of ratio of primary 

 to secondary current in a transformer, and gives the 

 ratio of transformation of the induction motor when 

 considered as an alternating-current transformer. 



It is interesting to note that the ratio of the two currents 

 always approximates to this straight line, quite irrespec- 

 tive of the load on the motor, primary voltage, or resistance 

 in the rotor circuit, since these do not affect the ratio of 

 transformation of the two windings. 



As a result of this experiment it would always be 

 possible to calculate the value of the rotor current from- 

 a curve of stator currents obtained in a load test. This 

 is of special value, as it is not usual to measure the rotor 

 current when carrying out a test. The full-line curve in 

 Fig. 184 shows the ratio of primary to secondary currents 

 when the motor works under load (as obtained in the 

 next experiment), and shows that when fully loaded the 

 ratio of the currents approximates to the theoretical ratio 

 of transformation. This ratio would be maintained at all 

 loads if it were not for the considerable no-load current 

 of the motor when working at normal voltage. 



The special practical application of the two measure- 

 ments just described will be further alluded to later. 

 They are of importance, because in one case the motor 

 works (when near full voltage) practically without slip, 

 and without magnetic leakage. In the other case, both 

 slip and magnetic leakage have their maximum values. 



EXPERIMENT LI. LOAD-TEST OF AN INDUCTION MOTOR. 



DIAGRAM OF CONNECTIONS. 

 (Same as for Experiment L., Fig. 181, p. 370.) 



