TRANSFORMERS 171 



too hot, increase the air pressure, partially closing the 

 top damper; if the copper is too hot, increase the pres- 

 sure and partially close the lower damper. 



If the transformers are guaranteed for a maximum 

 rise of 35 deg. C. at normal load and 55 deg. rise after 

 50 per cent overload for two hours, the air should be 

 adjusted to give about 30 deg. rise on the copper and 35 

 deg. rise on the iron. These adjustments should be 

 carefully made during the first hours of the heat run. 



When properly adjusted the transformers should run 

 about four hours at a practically constant temperature. 

 Place the thermometers for measuring the room temper- 

 ature near the intake of the blower so as to get the 

 temperature of air delivered to the transformers. Read 

 all thermometers and take the resistance on one winding 

 of each transformer every hour. Iron temperatures 

 may be read while the transformers are under load, 

 since the frames are grounded. If the primary leads 

 are brought out at the top of the machine, the voltage 

 should be cut off when taking other readings ; if, however, 

 the transformers are bottom connected, the temperatures 

 may be read while the machines are under load. If 

 it can be avoided, do not change the position of ther- 

 mometers when taking readings. 



When ready to measure resistances, shut down the 

 blower, take off the load and measure the resistances 

 as rapidly as possible, so as not to allow the transformers 

 to cool off. One minute per transformer should be ample 

 time for these readings. The rise by resistance is 

 calculated as follows: 



t =Cold temperature of coil. 



= Hot temperature of coil. 



Rt =Cold resistance of coil. 



RT =Hot resistance of coil. 



T =(238 + 0^-238. 

 Kt 



T t =Rise in degrees C. 



During the heat run a careful inspection should be 

 made for loose laminations. If any transformers are 

 found that rattle or buzz, due to loose iron, they should 



