236 PRINCIPLES OF ELECTRICAL DESIGN 



On account of the conditions under which they have to operate, 

 dynamo machines when used as motors are more often totally 

 enclosed than when used as generators. In the case of the larger 

 units forced ventilation would then be resorted to, but the 

 smaller sizes may be self-cooling. The temperature rise is then 

 largely equalized throughout the machine, and somewhat higher 

 surface temperatures are allowable than in the case of open-type 

 machines. A temperature rise of 60, by thermometer, is allow- 

 able inside the machine but this means that the temperature 

 rise of the enclosing case must be considerably less than this, say 

 30 or 35C. 



In the absence of data on the particular type of enclosed motor 

 under consideration, a cooling coefficient of 0.007 to 0.009 may 

 be used. This figure denotes the number of watts that can be 

 radiated per degree Centigrade rise of temperature from every 

 square inch of the entire external surface of the enclosed motor. 



