Winding Resistance and Low-Resistance Measurement
Winding Resistance and Low-Resistance Measurement
This domain tests DC resistance measurement of transformer, motor, and switchgear current paths using stable current, four-wire connections, and temperature correction.
Core rules
- Inject a stable DC current and measure the voltage drop; R = V/I. Keep test current low enough to avoid heating the winding.
- Use four-wire Kelvin connections: current leads outside, voltage leads directly on the test points.
- Allow readings to stabilize; large inductive windings take time.
- Correct to a reference temperature. For copper: R2 = R1 x (234.5 + T2)/(234.5 + T1). For aluminum use 228.1.
- Compare phases and historical values; a transformer phase imbalance is typically investigated above about 2%.
Worked example
A copper transformer winding measures 0.100 ohm at 25 C. Correct to 75 C: R75 = 0.100 x (234.5 + 75)/(234.5 + 25) = 0.100 x 309.5/259.5 = 0.119 ohm. The reasoning: resistance rises with temperature, so a cool winding will read lower than the 75 C reference; comparisons to nameplate or previous tests must use the same base temperature.
Traps
- Using two-wire leads and including lead resistance.
- Reading before the inductive circuit stabilizes.
- Using the aluminum constant for copper.
- Testing a winding while it is still cooling from load.