170 ALTERNATING CURRENTS 



readings connecting w and n, to plot the results on squared paper, 

 and select two well-defined points on the curve for the purpose of 

 finding E and H. 



A practically constant value of B with varying frequency is easily 

 secured by keeping the exciting current of the alternator used in the 

 experiment constant, and running the machine at different speeds, so 

 as to obtain different frequencies. 



93. Heating Test of Transformer 



An important test which applies to all classes of electrical 

 machinery is the heating test. This is carried out for the purpose 

 of ascertaining the temperature to which the windings of a trans- 

 former will ultimately rise when the transformer is kept fully loaded. 

 The importance of this test is due to the fact that when the 

 temperature exceeds a certain limit, the insulation of the windings 

 undergoes rapid deterioration, and frequent renewals will be 

 necessary. The safe temperature limit for the windings may be 

 taken as 90 C. Since the temperature of the surroundings may at 

 times be as high as 35, it follows that 55 C. must be regarded as 

 the maximum permissible rise of temperature for any portion of the 

 windings. But since the temperature of the coil is not uniform, it is 

 advisable to fix the limit for the mean temperature rise of the coil at 

 50 C. 



The most reliable method of determining the temperature of a coil 

 is the increase of resistance method. The resistance is measured 

 before commencing the test, and is then periodically measured as the 

 test proceeds. The temperature coefficient of copper may be taken 

 as 0'004, so that if r c = resistance of coil, measured at a known 

 temperature say t just before the commencement of the test, and 

 T Q = resistance of the same coil at C., then 



....... 



If after a few hours' run the resistance is found to have reached 

 the practically steady value r, the corresponding mean temperature 

 of the coil is given by 



t = 250^ 

 r Q 



and since r Q is known, being given by (1), t may be calculated, and 

 so the rise of temperature found by subtracting from t the temperature 

 of the surroundings at the end of the test. 



