62 . PRINCIPLES OF ELECTRICAL DESIGN 



The mean length of turn is approximately 4(w + t), or 4 X 

 3.75 = 15 in.; and the potential difference across the two coils 

 is 110 volts; thus, 



15 X 9,600 

 W = - 11Q - = 1,310 



Referring to the wire table on page 34, the wire of cross-section 

 nearest to the required value is No. 18 B. & S. gage, because 

 No. 19 will be too small to provide the necessary excitation. 

 This larger wire will provide a factor of safety, and it may be used 

 if the watts lost and the temperature rise are not excessive. 



Calculation of Temperature Rise. The space factor for No. 

 18 B.& S. D.C.C. wire, as taken off the curve of Fig. 15, is 0.54. 

 The cross-section of the copper in the coil is therefore 7 X 

 1.75 X 0.54 = 6.62, and the number of turns per coil will be 

 6.62/0.001276 = 5,180. Other required values are: 



Length of wire in one coil = 5,180 X 7^ = 6,500 ft. 



iz 



Resistance at 60C. = 6.5 X 7.42 = 48.2 ohms. 



55 



Current = 75-^ = 1.14 amp. 



obo.Z 



Total 7 2 R loss = 1.14 X 110 = 125 watts. 



Outer surface of both coils = 2X7 X 4 X 5.5 =308 

 Inner surface of both coils = 2X7 X4X2 =112 

 End surfaces of both coils = 4 X 1.75 X 4 X 3.75 = 105 



Total cooling surface -. = 525sq. in. 



The rise of temperature, by formula (30) Art. 11 taking 

 fc = 180, is 



I OK 



T = 180 X ~ = 43C. 



which is only slightly in excess of the specified temperature rise 

 (40C.). Another layer or two"* of winding would bring the 

 temperature down to the required limit; or, if preferred, the 

 length of the coil may be increased by a small amount without 

 appreciably adding to the reluctance of the magnetic circuit. 



It should be mentioned that the design of magnet as shown in 

 Fig. 21, is probably larger than would be necessary to fulfil 

 practical requirements, because it is not likely that the full 

 pressure of 110 volts would be maintained across the terminals 



