Measurement of DC resistance of transformer windings by current and voltage meter method

The current-voltage meter method
is also called the voltage drop method. The measurement principle of the voltage drop method is to pass a DC current through the winding to be measured, thereby generating a voltage drop on the resistance of the winding. The current passing through the winding and the voltage drop on the winding are measured. According to Ohm's law, the DC resistance of the winding can be calculated. The measurement wiring is shown in the figure.

Principle diagram of measuring DC resistance by current and voltage meter method

When measuring, the current loop should be connected first, and then the switch S2 should be closed after the current in the measuring loop is stable, and the voltmeter should be connected. When the measurement is completed, before cutting off the power supply, S2 should be cut off first, and then S1 should be cut off to prevent the induced electromotive force from damaging the voltmeter. The accuracy of the measuring instrument should not be lower than 0.5 level. The ammeter should use a small internal resistance, and the voltmeter should try to use a 4-digit high-precision digital multimeter with a large internal resistance. When the test uses a constant current source and the internal resistance of the digital multimeter is very large, generally speaking, the wiring of Figure 1-1 (b) can be used for measurement.
According to Ohm's law, the DC resistance value of the measured resistor can be calculated by formula (1-1).
RX=U/I (1-1)
RX——Measured resistance (Ω)
U——Voltage drop across the measured resistor (V);
I——Current passing through the measured resistor (A).
The wire of the ammeter should have a sufficient cross-section, be as short as possible, and have good contact to reduce the measurement error caused by the lead and contact resistance. When measuring a resistor with a large inductance, there should be enough charging time.