7.1.3 R - Winding Resistance
WHERE USED
The measurement of the resistance of all windings should generally be the first group of tests carried out for any type of transformer. It checks that the wire is of the correct diameter, and has not been over-tensioned during winding.
The measurement also confirms that the connections between the test fixture and the transformer have been made properly. This is particularly important when Kelvin connections are required, perhaps for a test to follow, as a resistance measurement will confirm that both the power lead and sense lead are making good electrical contact.
To check that the resistance of a winding is correct, the tester applies a constant current (dc) to the selected winding. Both the current through and the voltage across the winding are measured; dividing the voltage by the current gives the value of the resistance.
SPECIFYING THE TEST LIMITS
Maximum Value - Specify limit as tightly as possible to ensure that correct wire has been used.
Minimum Value Not usually so critical - can be set to any value that ensures that there is no solder splash causing a short circuit between pins.
TEST CURRENT
The test current is set according to the value specified as maximum in the test limits:-
| Maximum
Resistance |
<0.1Ω |
<1Ω |
<10Ω |
<100Ω |
<1kΩ |
<10kΩ |
| Test current |
400mA |
200mA |
100mA |
30mA |
3mA |
300μA |
NOTE: Above 10kΩ, a constant voltage source of 8.0V is used.