5-B] TEST BY LOSSES. 177 



54. Insulation and Temperature Tests. These tests are of com- 

 mercial importance but need no full discussion here. The Standardi- 

 zation Rules specify fully the conditions under which they are to 

 be made; details of the tests are described in the usual handbooks. 



55. Insulation. The insulation is tested between each winding 

 and all other parts. The applied voltage is increased gradually, so 

 as to avoid any excessive momentary strain. This is usually done 

 by some means of primary control in a special testing transformer. 

 Various companies make testing transformers for obtaining high 

 potential for this test and furnish detailed instructions for their use. 

 The voltage is preferably measured by means of a spark gap with 

 a high protective resistance in series with it. The test consists in 

 seeing that the apparatus withstands a specified over-voltage for a 

 specified time without breaking down. 



56. Heat Runs. These are made under full-load voltage and 

 full-load current for a specified time, temperatures being found by 

 thermometers and resistance measurements. The heat run could be 

 made by actually loading the transformer, but is usually made by 

 some kind of opposition or pumping back method, of which there are 

 several. No load is then required and no power, except enough to 

 supply the losses. 



A common form of opposition run employs two similar trans- 

 formers: the two secondaries (low potential side) are connected in 

 parallel to source A, of normal frequency and normal voltage, which 

 supplies the core loss; the two primaries are connected in series, 

 opposed to each other, and are then connected to source B, which 

 supplies the normal full-load current. (Source B requires a voltage 

 equal to twice the impedance voltage of one transformer and can be 

 of any frequency, i. e., it may or may not be the same frequency as 

 A.} All windings now have full-load current and normal voltage. 

 Instruments in A will give, if desired, the core loss and exciting 

 current; instruments in B will give copper loss and impedance 

 voltage. (The two transformers need not be identical.) 



Instead of connecting source B in the high potential side, a com- 

 mon modification is to connect the high potential windings of the two 

 transformers directly in. opposition and to insert source B in series 

 with the low potential winding of one of the transformers. This has 

 13 



