8-B] 



CIRCLE DIAGRAM. 



289 



satisfactory, while for motors under i H.P. the results are of little 

 value, unless refinements* are introduced in the construction of the 

 diagram. It is, however, in testing large motors that a method of 

 testing without load, as by the circle diagram, is particularly desirable ; 

 small motors can be readily tested by brake or other load methods. 



33. Calculation of Secondary Resistance. The short-circuit watts 

 W s, are chiefly due to copper losses in the primary and secondary. 

 In reality various load-losses are included,! which cannot be sepa- 

 rately determined. These copper losses are (^-j- ^ 2 )/s 3 , per phase, 

 where ^/s z is the primary copper loss and RJs* is the secondary 

 copper loss. Here R 2 is the secondary resistance in terms of the 

 primary ( 16, i6a, Exp. 5-6; 2oa, Exp. 5~C) and is the quantity 

 to be determined. 



Per phase, we have 



hence, 



Copper loss= (# 1 + J R 2 )/ s a ; 

 R 1 -\-R 2 = copper loss-^-/s 8 . 



Since R t is known, R 2 is thus determined. 

 For a 3-phase motor, 



and, 



In the present test 



'^4-7^= 1/3(6,810-^56^') = 0.708 ohms; 



hence, 



2 = 0.708 0.255 = 0.453 ohms. 



34. Leakage Reactance. The leakage reactance X, of an induc- 

 tion motor, both primary and secondary in terms of the primary, can 



*(32b). See a comprehensive article by H. C Specht, Elec. World, 

 p. 388, Feb. 25, 1905, in which it is said the modifications introduced give a 

 diagram applicable to induction motors of all sizes, single-phase or poly- 

 phase. To correct for error due to primary resistance, Specht tips his 

 diagram slightly, by dropping A and raising O' a small amount. Such a 

 correction was pointed out by Heyland, p. 23 of the English translation. 



t ( 33a). This gives to R 2 a value somewhat greater than the value that 

 would be determined by direct current resistance measurement. 

 20 



