214 COMMERCIAL ELECTRICAL TESTING 



Cold Resistance 



Measure the cold resistance of the primary winding, 

 each half of the secondary, and the iron grids. To obtain 

 the resistance of each half of the secondary winding, turn 

 the switch to the extreme position and take readings, 

 showing the switch position by a sketch. Then throw 

 the switch into the other extreme position and measure 

 the other half of the secondary winding. 



Tap Voltage or Ratio 



When the switch contacts are accessible, full voltage 

 should be applied to the primary winding, reading the 

 voltage between the steps on the switch. This test will 

 show any wrong switch connections immediately. Cor- 

 rect connection can also be checked by a polarity test 

 on each step. If properly connected all steps will have 

 the same polarity; that is, the voltmeter deflections are 

 all in the same direction. 



If the contacts of the switch are inaccessible, the step 

 voltages may be taken as follows : Throw the switch to 

 the neutral position, apply full voltage to the primary, 

 and connect the voltmeter across the secondary. When 

 the switch is in the neutral position, no secondary voltage 

 will be obtained. Move the switch one step and read the 

 secondary voltage. Then move the switch to the next 

 step when the reading obtained should correspond to two 

 steps in series. Continue until the switch has reached the 

 extreme position. Bring the switch back to the neutral, 

 then test the steps on the other half. If the sections of 

 the secondary winding are properly connected to the dial, 

 the voltmeter readings should increase in equal incre- 

 ments. 



Core Loss 



Core loss may be determined from the primary but 

 is more satisfactorily determined from the secondary 

 winding. Throw the switch to the extreme position and 

 apply the rated boost or lower voltage, reading watts 

 input and exciting current at the proper frequency. 



