52 



PRINCIPLES OF ELECTRICAL DESIGN 



values should be fairly high, but they will depend upon whether 

 the magnetic circuit is of cast iron or steel. The unknown 

 quantities are then R and A, and equation (32) can be put in the 

 form 



33,000 X (hp.) X k X 1,730,000 



R X A = 



91 X 10 8 X (hp.) X k 

 NcB z 



(33) 



where B is in gausses, and the coefficient of friction, c (for dry 

 surfaces), may be taken from the accompanying table. If the 

 surfaces are lubricated with oil or grease, the friction coefficient 

 may be lowered as much as 40 per cent. One reason for using a 

 large factor of safety is to allow for the possibility of dirt or oil 

 getting between the surfaces in contact. 



TABLE GIVING APPROXIMATE VALUES OF THE COEFFICIENT OF FRICTION, c 



The formula (33) permits of either quantity R or A being 

 calculated when one of them is known or assumed. It is the 

 business of the designer to determine usually by trial the 

 dimensions which will give the best results. So far as cost is 

 concerned, a large diameter may show a saving in materials; but 

 the labor cost not omitting the cost of patterns when but few 

 castings are required should also be considered. 



At times when slipping occurs, as in a magnetic brake, or 

 when throwing in a magnetic clutch while there is some relative 

 movement between the two parts, there is a powerful retarding, 

 or driving, action as the case may be, due not to the direct 

 magnetic pull between the surfaces in contact, but to the fact that 

 eddy currents are produced in the polar faces on account of 

 the cutting of the magnetic lines. This cutting of flux is similar 

 to what occurs in the unipolar, or so-called homopolar, type of 

 D.C. generator, where the currents are confined to certain paths 

 and collected by means of sliding brushes. 



