300 



PRINCIPLES OF ELECTRICAL DESIGN 



radial length of pole greater than two or two and one-half 

 times the width of pole core (measured circumferentially) would 

 be a poor design, because the gain in winding space due to increase 

 of radial length would be largely neutralized by the greater amount 

 of flux per pole due to leakage. 



The current density in the field magnet windings of salient- 

 pole rotating-field alternating current generators is usually be- 

 tween 1,100 and 1,600 amperes per square inch. 



The required useful flux per pole being known, the flux to be 

 carried by pole core and yoke may be calculated if the leakage 

 factor is known. The calculation of the permeance of the air 



1000 



8000 



2000 3000 4000 5000 6000 7000 



Peripheral Speed of CSalient Pole) Rotor- Feet per Minute 



Fia. 123. Cooling coefficient for field windings of rotating-field alternators, 

 paths between poles is tedious and somewhat unsatisfactory. It 

 seems therefore best, in designs of normal types, to assume a 

 leakage factor based on measurements made on existing machines. 

 The leakage coefficient will be low in machines with large pole 

 pitch, and high in the case of slow-speed engine-driven genera- 

 tors with a large number of closely spaced poles. The following 

 approximate values may be used for estimating the flux in poles 

 and yoke ring. 



FACULTY OF APPLIED SCIENCE. 



Date... 



