2.2 R - Winding Resistance
R1, R2, and R3 represent the resistance of the copper wire used to wind
the transformer.
When current flows in the windings the resistance causes losses in the
windings (I2R losses), and generates heat.
In addition the winding resistances cause a voltage drop in the
windings when current flows, causing the output voltage to fall with
increasing load. (This effect is known as 'Regulation').
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