642 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



input will be zero. Manufacturing tolerances which produce dissimilari- 

 ties in the mndings and cores may cause a small output with zero dc 

 input. 



When the dc leakage current flows through windings b and b', a bias 

 flux is established in each of the cores. This causes the shape of the flux 

 versus time curve to be changed as illustrated in Fig. 11 (e). Since the 

 ac input will saturate the cores, both half cycles of the wave will be 

 flat topped, but the flat portion of one-haK cycle will be increased and 

 the other decreased. Also the steeper parts of the curve will be brought 

 closer together on one-half cycle and further apart on the other. 



As illustrated in Fig. 11 (e) the dc bias will have a different effect 

 upon the flux in each of the two cores. In one core the bias aids the 

 positive half cycle of the input current, and in the other the negative 

 half cycle. The result is that the core reaches saturation more quickly 

 and, therefore the flux curve is flatter on the half cycle which is aided by 

 the bias. 



This change in the shapes of the flux time curves produces correspond- 

 ing changes in the shapes of the voltage-time curves of the output 

 windings b and b'. These are shown in Fig. 11 (f). The peaks of the 

 voltage curves occur at the points of maximum rate of change of the 

 flux curves, and of course the flat portions of the voltage curves (near 

 zero) are lengthened or shortened depending upon the flatness of the 

 flux curve tops. This skewing of the two voltage curves prevents the 

 output voltage cancellation which was obtained with no dc, and gives 



MAGNETTOR 



DC 



■^ 



DC 



4/i.F 



/S'c 



2000 -CYCLE 

 BAND PASS 



FILTER AMPLIFIER 



[> 



DETECTOR 



CONNECTED 



TO LINE 

 ^X 



rH5!5- 



AC 



INTERFERENCE 



FILTER 



I 

 I 

 I 

 I 



I 

 ■* 



ADJUSTED FOR 

 DIFFERENT TEST 

 RANGES BY TEST 

 CONTROL CIRCUIT 



> 



..X-J 



Fig. 10 — Block (liagnim of line insulation test circuit. 



