50 PRINCIPLES OF ELECTRICAL DESIGN 



Various proportions can be tried, also different values of the 

 magnetic density in the air gap. The pull per square inch de- 

 pends upon B 2 ; but, by forcing the density up to high values, the 

 ampere-turns required become excessive, and the weight and cost 

 of the copper coils, prohibitive. More will be learned by trying 

 various proportions and roughly estimating the cost, than by a 

 lengthy discussion of the manner in which the various dimensions 

 are dependent upon each other. It will probably be found that 

 the most economical initial density will not exceed 11,000 

 gausses; and (by formula 16, Art. 8) the pull, in pounds per 

 square inch, is 



(11,000) 2 _ 7ft 

 1,730,000 " 



thus, with the usual cylindrical core, 



TT D 2 



total force, in pounds = F = 70 r- 



= 55 D 2 

 whence 



D = 0.135 VF (31) 



The magnet can now be sketched approximately to scale, and the 

 necessary ampere-turns computed, all as previously explained in 

 Art. 4. Although Fig. 18 shows a very short air gap, the same 

 methods apply to the calculation of magnets with longer air gap, 

 provided this is not so great as to cause excessive magnetic 

 leakage. A practical rule which determines the minimum length 

 of the winding space is that this length, h, should never be less 

 than twice the air-gap length, I. 



15. Magnetic Clutch. The design of a magnetic clutch to 

 transmit power between a shaft and pulley or any piece of rotat- 

 ing machinery, is generally similar to that of the circular type 

 of lifting magnet. Fig. 19 shows a common type of magnetic 

 clutch with conical bearing surfaces, although the conical shape 

 is not essential, and the wedge action of the cone-shaped rings is 

 not relied upon to increase the pressure between the surfaces in 

 contact. When the two iron surfaces are held together by the 

 action of the exciting coil, the flux density over the area between 

 the two annular pole faces must be such as to produce a force 

 that will prevent slipping between these faces. A factor of 

 safety of 2.5 to 3 is generally allowed. 



FACULTY OF APPLIED SCiLNCE. 



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