INTERNAL TEMPERATURES OF RELAY WINDINGS 



145 



The plotted points show the mean temperature of the layers between taps 

 plotted against their mean radius; the dashed boundary lines show the 

 inner and outer radii of the coil. The different curves correspond to the 

 different levels of steady state power input indicated. 



It is apparent in Fig. 2 that the central part of each curve is comparatively 

 flat, and that it would therefore be difficult to determine the radius of 

 maximum temperature directly. It may be noted in passing that the maxi- 

 mum temperature is not greatly in excess of the mean temperature. This 

 justifies the usual engineering practice of taking the mean temperature as a 

 criterion of whether the coil is overheated or not. 



Table I 

 Heat Distribution in 197 Switch Coils 



Total Heat Input 



(Watts) 



Q (watts per Cm.^) . . . 

 Max. Temperature T' 



(°C) 



Temperature Ti (°C) . . 

 Temperature T^ (°C) . . 

 K (Calories/°C/sec./ 



cm.) 



r' (Cm.) 



Heat to Core (Watts) . 

 Heat to Cover 



(Watts) 



(Per Cent.) 



Inner Surface Temper- 

 ature (°C) 



Outer Surface Temper- 



ature(°C) 



3.00 

 0.268 



0.69 X 10-3 



0.93 



1.39 



6.04 

 0.540 



105 



95 



102 



0.79 X 10-3 



0.95 



2.99 



3.05 

 51 



91 



99 



8.98 

 0.802 



139 

 124 

 132 



0.82 X 10-3 



0.94 



4.37 



4.61 

 51 



119 



130 



From each curve in Fig. 2 there was read the maximum temperature T 

 and the temperatures Ti and Ta at n = 0.622 cm. and ri = 1.156 cm. re- 

 spectively, corresponding to the inner and outer points plotted. These 

 temperatures are listed in Table I together with the corresponding values of 

 Q, the power input divided by the volume of the coil (11.2 cm^). From these 

 data, the radius r' of maximum temperature has been computed by the 



Q A 



procedure described above. With r' known, the quantity - was computed 



from equation (4) for the case r = n , T = Ti. The resulting values of / 

 and K are included in Table I. Using the value of r' thus determined, the 

 division of the heat between that going to the core and that going to the 

 cover was computed with the results shown in the table. The values of n 

 and ^2 used in the above computation are, as indicated in Fig. 1, internal 

 to the coil. Taking new values of n and rz corresponding to the core and 

 cover radii respectively, the temperatures at these surfaces were computed 



