208 COMMERCIAL ELECTRICAL TESTING 



Core Loss 



For low potential three-phase regulators, core loss is 

 measured in the usual way by applying normal potential 

 to the primary winding. For regulators the primary 

 voltage of which exceeds 1100 volts, core loss should be 

 measured on the secondary winding. 



For six-phase two-circuit primary regulators, one set 

 of core loss readings on lines 1-3-5 and another on 2-4-6 

 should be taken. Either set should give the correct 

 core loss. For six-phase diametrically connected regu- 

 lators, core loss may be determined by applying six-phase 

 voltage, reading the core loss in each phase and taking 

 the sum of these losses. It may also be taken by con- 

 necting the primaries in delta, reversing one primary 

 coil to maintain the proper distribution of magnetic flux. 

 Apply the rated primary voltage and determine the core 

 loss by the two wattmeter method. Another method of 

 determining core loss is by connecting the primaries 

 in Y and applying 1.73 times the rated potential. One 

 coil must be reversed for the Y connection, as is done for 

 the delta connection. 



In making a core loss test, record the voltage, exciting 

 current and wattmeter readings. The test must be made 

 at the proper frequency and the generator supplying 

 the loss must operate at normal voltage. The magnet- 

 izing current will vary from 20 to 40 per cent, depending 

 upon the air gap. A curve should be taken beginning 

 at 50 per cent normal voltage and increasing to at least 

 125 per cent normal voltage. Whenever possible, 

 neither potential nor current transformers should be 

 used with the wattmeter, in consequence of the very 

 low power-factor. During the core loss tests the arma- 

 ture should be in the maximum boost position. A curve 

 should also be taken by holding normal voltage and 

 varying the position of the armature. 



Impedance 



Impedance is usually measured by short circuiting 

 the secondary and applying sufficient voltage to the 

 primary winding to give full load current. The imped- 



