170 COMMERCIAL ELECTRICAL TESTING 



Shop transformers should always be interposed 

 between the primaries of the transformers in test and 

 the alternators to prevent the breaking down of the 

 armature and to avoid high potentials on the switch- 

 boards. "Step" the voltage either down or up, or 

 down and up again, depending upon circumstances; 

 but always have transformers between the alternator 

 and the primaries of the transformers in test. There 

 should be some resistance left in the field of the alter- 

 nator so that as the alternator fields and the winding 

 of the transformers heat up, the load can be kept normal. 



Place spirit thermometers in the top of each trans- 

 former to read the temperature of the air escaping from 

 the coils. Two thermometers should be used for the 

 primary and two for the secondary windings, placing 

 them about one inch above and just over the ducts 

 between the coils. Also, place two thermometers on the 

 top and one near the bottom, and two thermometers to 

 read the temperature of the air escaping from the iron. 

 The transformers can now be loaded. With the alter- 

 nator running at proper speed, the total exciting current 

 of the transformers should be read and the secondary 

 voltage can be checked. 



Air blast transformers may run at full load for 50 

 minutes without air, in order to heat them up and thus 

 shorten the heat run. Some transformers can not be 

 operated for more than 20 minutes without air and they 

 must be carefully watched to see that they do not get 

 too hot. After the air blast is put on, it is usually neces- 

 sary to keep the iron damper closed for some time to 

 allow the core to heat up, as the copper heats much faster 

 than the iron. The amount and pressure of air required 

 depends on the guarantees as to temperature and to 

 some extent on the voltage of the transformers. The 

 large amount of insulation on the coils of high voltage 

 transformers tends to retard radiation. 



If the transformers are guaranteed for a maximum 

 temperature rise of 40 deg. C. at normal load, and 55 deg. 

 C. rise after a 25 per cent overload for two hours, the 

 air should be adjusted to give about 35 deg. rise on the 

 copper and 40 deg. rise on the iron. If the iron seems 



