Induction Motor. I^^l 



The resistance K of each of tho circuits formed l>v jnn.n.r a pair 

 ■of strips in series by two perfect conductors will U 



But R = 7t/iHIKl .•.^=4(7 



That is, if the ends of the conductinj.^ strips were joined hv per- 

 feet conductors, we would have , e(,ual to four times the reiirt- 

 ance ot the copper from end to end. The remainin^^ factor of five 

 IS due to the fact that the effective resistance of the paths joi.nnir 

 -the strips is not zero. 



The large increase in the value <.f k in ti.e experiments witl. the 

 Cradle dynamometer is probably due to the fact that in the«e 

 experiments the air gap between the copper and the pole pieces was 

 smaller than in the case of the i>ermanent ma-net. 



Another point, which must be determined, l^fore it is possible to 

 design a new apparatus, is the* extent to which the magnetic resist- 

 ance of a magnetic circuit can l>e inferred from the dimensions of 

 the apparatus. 



To do this, the " Immisch " motor was used, and the flux thread- 

 ing the iron core was measured for different diameters of the core, 

 .and different magneto-motive forces, with the following results : 



Averaire iipi r.eiigtli of Magnetic ., ^ .\tftK"eto 



of Sf !.:."' ,, !•• Kap ^ resistance of ^^Z.. """^ ^' '^"*- 



"^ (both sides). airjfap. Motne force. Resultant Flux. 



60.43 0.73 0.0121 3.47 x 10 J 0.0251 



6.01 X 10» 0.0301 



39.4 1.03 0.0261 3.70x10^ 00380 



6.61 X 10^ 0.0418 



28.8 1.87 0.0650 3.7J x 10^ 0.0095 



6.S1 x 10=* 0.0(;30 



The numbers in the last column givo the effective magnetic resist- 

 ance of the Circuit. 



Comparing these values with tlie values of the resistance as cnl- 

 •culated from the dimensions, it will be noticed that foi- small air 

 gaps, the effective resistance is larger than the calculated. an«l also 

 that the value increases with increasing flux density. This nu'ans 

 a higher magnetic leakage at higher flux densities. Wni when the 

 xiir gap is 2. cm. long, the calculated and effective magnetic ri^sist- 

 ances are practically the same, and also the valu*- «»f the effective 

 resistance is almost independent of the Mux <lensity. 



Certain dimensions in the new apparatus are hxed : — 



(i.) The external diameter of the rotor is limited to 30 cm. 

 (ii.) The internal diameter of the i>ole ^wkk^ must Iw not leM 



than 7.3 cm. 

 (iii.) The external diameter of the armature must not exceed 

 5.5 cm. 



