CHAPTER III. 



The Short-Circuit Current and the Leakage Factor 



29. An invaluable aid in the design of polyphase motors is af- 

 forded by the short-circuit current characteristic. In a motor hav- 

 ing a squirrel-cage armature, the starting current under different 

 voltages is identical with the short-circuit characteristic. If the re- 

 sistances of the armature and of the field were known, the power 

 factor of the feeding current could be calculated; thus it would not 

 even be necessary to use a wattmeter unless very accurate measure- 

 ments were required. Theoretically, then, the short-circuit char- 

 acteristic is sufficient for the determination of the leakage factor; in 

 practice, however, it will, in the majority of cases, be unadvisable to 

 depend upon the short-circuit curve, on account of the corrections 

 which become necessary. If the total resistance of the motor is 

 small, the lag of the current amounts to nearly a quarter of a period ; 

 the inductance of a motor at standstill (short-circuit) should always 

 be as small as possible; therefore, a very large current will go 

 through the motor at full voltage. Now, the leakage factor is always 

 more or less dependent upon the strength of the currents which 

 cause the leakage, hence the leakage factor at starting with only a 

 small resistance in the armature may be very different from as a rule, 

 smaller than the leakage factor upon which the diagram is con- 

 structed. In fact, we do not work the motor in that quadrant of the 

 semi-circle, which corresponds to the short-circuit characteristic. If, 

 therefore, I shall not make much use of the short-circuit curve for 

 the determination of the absolute value of the leakage factor, I shall 

 all the more avail myself of it for comparative purposes where it does 

 not so much matter whether the absolute value is correct or not, but 

 where the relative value is of importance. Such questions as the in- 

 fluence of a closed or open slot, of the number of slots, of the air- 



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