155
Appendix
12.9.5 Four-Wire Connection for Transformer Winding Resistance
Measurement
Since the winding resistances are very small, the test set is connected in 4-wire
configuration. It has to be observed that the contact resistances of the
connection clamps do not falsify the measuring result (Figure 12-13).
Figure 12-13 Four-wire resistance measurement
A constant current source is used to feed a direct current into the winding. The
test current should be at least 1% of the rated current to bring the core into
saturation. On the other hand it should not exceed 15% of the rated current to
avoid temperature rise during measurement. A relatively high no-load voltage
enables a quick saturation of the core and a final value with only minor
fluctuations is reached. Therefore, in most cases the charging time per tap is
distinctly less than 30 seconds. By pressing one button on the test system, all
values are logged and a test report is automatically generated (Figure 12-14).
The resistance values are automatically corrected to the reference temperature
by automatic calculation utilizing the following formula:
Measuring
lines for I
Measuring
lines for U
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