TRANSFORMERS 193 



take readings of watts, volts and speed while varying the 

 frequency. Plot watts loss as ordinates and cycles per 

 second as abscissae, and project the curve back to the 

 zero line. At zero frequency, the total loss will represent 

 the true ohmic loss or PR. (See Fig. 64.) 



In very exceptional cases, short circuit tests are taken 

 on transformers to determine their behavior in case they 

 are accidentally short circuited in service. This test is 

 obtained by connecting one winding of the transformer 

 to the power source, which should be of four or five times 

 the capacity of the transformer, and short circuiting 

 the other windings. The tendency of the ends of the 

 coils to flare out due to the excessive magnetic repul- 

 sion is the important point in this test. The test 

 should be short, as the current is from 15 to 30 times 

 normal. 



