CHAPTER XIV 

 REGULATION AND EFFICIENCY OF ALTERNATORS 



103. The Magnetic Circuit. Except for the fact that the field 

 magnets usually rotate, the design of the complete magnetic cir- 

 cuit of an alternating-current generator differs little from that of 

 a D.C. dynamo. Given the ampere-turns required per pole, and 

 the voltage of the continuous-current circuit from which the ex- 

 citing current is obtained (usually about 125 volts), the procedure 

 for calculating the size of wire required is the same as would be 

 followed in designing any other shunt coils (see Art. 10, Chap. II 

 and Art. 58, Chap IX). When estimating the voltage per pole 

 across the field winding, a suitable allowance must be made for 

 the pressure absorbed by the rheostat in series with the field 

 windings. The exact amount of excitation required under any 

 given condition of loading can, of course, be determined only 

 after the complete magnetic circuit has been designed. 



A higher current density may be allowed in the copper of rotat- 

 ing field coils than in stationary coils against which air is thrown 

 by the rotation of the armature, because the cooling is more ef- 

 fective, the difference being especially noticeable at the higher 

 peripheral speeds. In the absence of reliable data on any par- 

 ticular type and size of machine, the curve of Fig. 123 may be 

 used for selecting a suitable cooling coefficient. The cooling sur- 

 face considered includes, as before, the inside surface near the 

 pole core and the two ends, in addition to the outside surface of 

 the coil. It is to be understood that the cooling coefficient ob- 

 tained from Fig. 123 is approximate only, being an average of 

 many tests on different sizes and shapes of coils on rotating field 

 magnets. 



In determining the amount by which the pole must project 

 from the yoke ring, it is well to allow about 1 in. of radial length 

 of winding space for every 1,500 ampere-turns per pole required 

 at full load (i.e., estimated maximum excitation). An effort 

 should be made to keep the radial projection of the poles as small 

 as possible in order to prevent excessive magnetic leakage. A 



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