Turns ratio testing — covered in our companion article — verifies that a transformer's windings have the correct number of turns relative to each other. DC winding resistance testing checks something related but genuinely different: whether the physical copper path through each winding has the low, consistent resistance it should.
What Winding Resistance Testing Catches
A DC winding resistance test applies a stable DC current through a winding and measures the resulting voltage drop, calculating resistance directly. This is specifically sensitive to fault types that don't necessarily affect turns ratio:
Loose or degraded connections. A connection that's loosened over time or corroded shows up as elevated resistance, even though the winding itself is completely intact.
Broken strands within a winding. A partially broken strand reduces effective conductor cross-section and raises resistance without creating a complete open circuit.
Tap changer contact wear or misalignment. Worn or pitted contacts show up as elevated resistance at the affected tap positions.
Shorted turns. A winding with shorted turns also typically shows reduced resistance, providing a useful cross-check with turns ratio testing.
Why a Standard Ohmmeter Doesn't Work
Transformer winding resistance values are typically very low — often well under an ohm. At these levels, test lead and contact resistance becomes significant, which is why winding resistance testing uses dedicated micro-ohmmeters (DLROs) employing four-wire (Kelvin) measurement, isolating the actual winding resistance from the test setup's own lead resistance.
Why the Test Takes Time
Transformer windings are highly inductive. When DC current is first applied, it doesn't reach steady-state instantly — an accurate reading can only be taken once current has stabilised, which can take seconds to minutes per measurement for large windings. Higher test current drives the winding to saturation faster, shortening test duration.
Temperature Correction
Winding resistance varies with temperature, and meaningful comparison between tests requires correcting measured resistance to a common reference temperature, using the winding's actual temperature measured directly via a probe.
A Complete Transformer Electrical Test Program
| Test | What it verifies | What it can miss |
|---|---|---|
| Turns ratio (TTR) | Correct turns count, shorted turns, tap changer ratio | Connection resistance problems that don't affect ratio |
| DC winding resistance | Physical conductor and connection integrity | Turns count errors that don't significantly affect resistance |
| Insulation resistance / PI / DAR | Insulation condition between windings and to ground | Conductor and connection integrity |
| Dielectric withstand (hipot) | Insulation's ability to withstand overvoltage stress | Normal-operation conductor issues |