TRANSFORMERS 173 



watched, particularly those of short duration. Special 

 attention should be given to the length of the run, as 

 the temperatures often rise very rapidly. At the finish 

 of the heat run, record all temperatures and measure 

 all resistances. The same air pressure should be used 

 for the overload as for the normal load. 



Insulation Test Double Potential Test 



In this test, as well as in the core loss and impedance 

 test, the alternator supplying the voltage should be 

 operated at as near normal voltage as possible, so as to 

 avoid distortion of the wave form. Double potential is 

 applied to test the insulation between turns and between 

 sections of the coils. Since it is impossible to obtain 

 double voltage on a transformer at normal frequency, 

 due to high density in the iron, the frequency must be 

 increased. Apply twice the normal voltage for one 

 minute, followed by one and one-half times normal 

 voltage for five minutes. The last test is taken in order 

 to discover any short circuits that might develop during 

 the double potential test, and yet not become apparent 

 in the short time that the double potential is applied. 

 The primary bushings should be cleaned before the test 

 and the transformer guarded to prevent accidents from 

 the high voltage circuits. Any buzzing or leakage of 

 current should be noted. 



In applying and taking off the high potential, vary the 

 alternator field gradually; that is, do not open the field 

 switch with a jerk, for if this is done trouble may occur. 

 As soon as this test is taken, make the proper comments 

 on the test sheet. 



In case a transformer breaks down, the defective coil 

 should be located and plainly marked. Then, in disas- 

 sembling the machine, the coil can be easily found and 

 the cause of the defect ascertained, thus preventing a 

 repetition of the breakdown. 



Air Readings 



A method at present used is to read the velocity of. 

 the air through a standard orifice by means of an air 

 meter. Knowing the velocity and the area of the orifice, 



