172 COMMERCIAL ELECTRICAL TESTING 



be plainly tagged and a chalk mark made on the core 

 as near as possible to the point at which buzzing was 

 heard. The heat run and other tests should now be 

 finished, except the double and high potential tests, 

 which must always be taken after all repairs are made. 

 If a . motor-generator method can not be used, the 

 copper and iron heat runs may be taken separately. 



To make a short circuit heat run, short circuit the 

 secondary windings and apply normal current to the 

 primary. When this test is finished and the hot resist- 

 ances taken, open-circuit the primary, arranging the 

 primary leads so that there is no danger of any one 

 being injured, and apply normal voltage at proper fre- 

 quency to the secondary until the iron temperatures 

 are constant. Finish up the tests as if the heat run were 

 taken by the motor-generator method. The same 

 amount of air will be required and the heating will be 

 practically the same as though both iron and copper 

 were loaded at the same time. 



At the end of the heat run measure all resistances 

 carefully and read the thermometers. The same care 

 should be used as when taking cold resistances, and if 

 any set of readings indicates a doubtful increase of resist- 

 ance, the readings should be checked, using a different 

 set of meters. If the work is properly conducted, ten 

 minutes is ample time to take a complete set of resist- 

 ance readings on four transformers. A careful inspec- 

 tion of all soldered joints should be made to see that there 

 is no undue heating. 



When no overload is specified, the transformers may 

 be run for 20 minutes at 50 per cent overload current to 

 test soldered joints. This test follows that of hot resist- 

 ance. 



Overload Heat Run 



This test is ordinarily limited to two hours and is 

 taken as a continuation of the normal load heat run. 



Transformers are sometimes designed to run con- 

 tinuously at overloads, or may be guaranteed to operate 

 at a certain kilowatt output at some power-factor less 

 than unity. Overload heat runs should be very carefully 



