ISO ELEMENTS OF ELECTRICAL ENGINEERING. 



The temperature of a machine part, other than windings, is under- 

 stood to be its surface temperature as indicated by a thermometer 

 of which the bulb is laid flat against the part and covered by a 

 small wad of clean, dry cotton waste. A large wad of waste 

 would interfere too much with the free cooling of the part under 

 test. 



The heat run is made by operating the machine under full load 

 conditions for the specified time, or until a constant running tem- 

 perature has been reached. The machine is then shut down, and 

 the temperatures of the various parts are determined by ther- 

 mometers as above explained. Also the " hot " resistances of the 

 various windings are measured, from which the mean tempera- 

 tures of the windings may be calculated, the resistances at room 

 temperature having been measured beforehand. 



Important practical points concerning this heat test are given 

 in Art. 79. 



The commutator and brushes may be safely run at a much 

 higher temperature than the armature and field magnet windings. 

 This is because the mica insulation of the commutator is nearly 

 fireproof, while the cotton and varnish insulation of the armature 

 and field windings can not withstand a temperature beyond 100 C. 

 without charring. The bearings should be run at a compara- 

 tively low temperature to ensure satisfactory lubrication. The 

 allowable temperatures, as specified in the report of the Com- 

 mittee on Standardization of the American Institute of Electrical 

 Engineers,* are partly determined by the above considerations 

 and partly by differences in the methods used for determining 

 temperatures. Temperatures calculated from resistance meas- 

 urements are accurate mean temperatures, whereas temperatures 

 by thermometer refer to the surface layers, which are always 

 cooler than the interior. Thus a magnet winding may show a 

 temperature rise of 50 C. by a thermometer laid on its surface 

 when a temperature rise of 75 C. is obtained by calculation from 

 resistance measurements. 



* See Art. 79. 



