200 POWER-HOUSE DESIGN 



plished by hot air. This air can be forced through the exciter by 

 a blower, or boxing and barriers arranged to cause the hot air to 

 circulate through the machine by natural draft. This hot air 

 may be obtained from a steam radiator, a hot-air furnace, electric 

 heater or a bank of incandescent lamps. In any case the air 

 should not be higher than 80 C. When a blower is used a cheese- 

 cloth screen over wire mesh is advisable on the blower intake to 

 reduce the amount of dust blown into the windings. The exciter 

 may then be brought slowly up to voltage as soon as the insulation 

 to ground tests satisfactorily. 



The large A.C. generators should be brought slowly up to 

 speed, and a short-circuit heat run put oh as soon as the bearings 

 are in satisfactory condition. This short-circuit current should 

 be the full load current only on maximum rated machines. Ma- 

 chines with an overload guarantee may be run at the overload 

 current. The short-circuit run should be continued until the 

 proper insulation resistance is reached. It is advisable, however, 

 to run twenty-four hours on short circuit, even when the insula- 

 tion tests satisfactorily. 



After the short-circuit run, the machine should be brought up 

 to normal voltage and run for several hours before going on the 

 load. This is to heat the iron thoroughly. On very large gen- 

 erators it is advisable to continue the drying for twenty-four hours 

 as follows: Two hours 10 per cent over voltage, then two hours 

 full load current on short circuit, the drying continuing by alter- 

 nating open and short circuit every two hours. 



For transformer drying see section on " Transformers. " 



Insulation Resistance. Insulation resistance may be obtained 

 by the following method when a megger or bridge is not available : 



Connect one side of a direct-current source of power to the 

 windings to be tested; connect the other side of the direct-current 

 circuit to a portable voltmeter and then read the voltage when the 

 free side of the meter is connected to the other side of the circuit 

 where it is attached to the windings. Call this reading V. Then 

 connect to the frame of the machine, being careful to get a good 

 contact; call this reading Fi. Then 



where R = the cold resistance of the insulation, and Ri= the resist- 



