Measuring the DC resistance of a transformer winding or a motor coil is the fastest way to find a defect no visual inspection will reveal. The test is almost trivially simple: push a known direct current through the winding, measure the voltage drop, and divide. What separates a defensible measurement from a misleading number is everything around those three actions — how the leads are attached, how long you wait, what temperature the winding is at, and what you compare the result against.
This guide covers the full procedure for both transformer windings and motor coils, including the parts most write-ups skip: when a four-wire meter is genuinely necessary, how to correct readings to a reference temperature, and how to judge whether a result means pass or investigate. It is aimed at commissioning checks and routine transformer testing on dry-type and low-voltage units, not only utility-scale power transformers.
Winding resistance measures the conductor path directly. Anything that adds resistance, removes a parallel path, or shorts turns together changes that number predictably. Three findings account for most of the value of a winding test.
A short between adjacent turns creates a closed loop that circulates current, overheats locally, and removes conductor length from the circuit. The affected phase reads slightly lower — often by a fraction of a percent, which is why phase and baseline comparison both matter. In a small transformer, one shorted turn can raise losses enough to destroy insulation within hours.
Tap contacts, terminal blocks, and bolted joints all sit in series with the winding. A pitted contact or oxidised terminal shows up as a higher reading on one tap position or one phase. This is the most common real-world finding and the easiest to fix — which is what makes a winding test worth running rather than merely diagnostic. A resistance that jumps at one tap and recovers at the next points almost directly at the tap changer.
Large windings use multiple parallel strands; if one breaks, the winding still conducts through less copper and resistance rises — invisible to a ratio or insulation test. And even when nothing is wrong, the first reading has value: corrected to a common temperature, it becomes the baseline every later test is compared against. For how the winding and core are arranged, see this explanation of Transformatorenkonstruktion und Eisenkernstruktur.
Generic advice over-engineers this. A four-terminal (Kelvin) connection separates the current-carrying leads from the voltage-sensing leads, so lead resistance never enters the measurement. It is mandatory when winding resistance is in the same order of magnitude as lead resistance — on utility power transformers, where a phase may read a few milliohms.
Small dry-type units are different: a 1 kVA winding commonly reads from half an ohm to several ohms, while clean leads contribute only 0.05 to 0.1 Ω. Null the lead resistance with the meter’s relative function, keep clips tight, and verify the null before each phase.
Current must be high enough for a stable voltage drop and low enough that winding heating does not shift resistance mid-measurement.
| Winding resistance | Typical test current | Practical note |
|---|---|---|
| Below 1 Ω | 10 A or more | Four-wire mandatory; settling may take minutes |
| 1 – 10 Ω | 1 A to 5 A | Common range for small dry-type transformers |
| 10 – 100 Ω | 0.1 A to 1 A | Control and instrument transformer windings |
| Above 100 Ω | Below 0.1 A | Two-wire acceptable with lead nulling |
Higher current gives a stronger signal but heats the winding faster. On small units, staying at 1 A to 5 A is the better trade. For how much current a winding is rated to carry in service, see why transformers are rated in kVA.
The physics is identical, but access is the problem. On a three-phase motor, the internal connection determines what you can measure without opening the machine.
Motor coils use heavier conductor than a transformer of similar frame size, so readings are lower and four-wire measurement becomes more relevant, while high inductance means longer settling. Phase imbalance is judged more loosely — many programmes accept about 5%, though the manufacturer’s tolerance governs. This is also not a substitute for insulation resistance testing, which measures the path to ground.
Copper resistance rises with temperature at roughly 0.39% per degree Celsius, so a reading taken on a warm winding can easily be 15% higher than the same winding measured cold. Comparing an uncorrected value against a factory figure measured at another temperature produces a false fault — or hides a real one. Use the standard copper relationship:
Rref = Rmeasured × (234.5 + Tref) ÷ (234.5 + Tmeasured)
For aluminium windings, substitute 225 for 234.5. A winding measuring 2.40 Ω at 32 °C, corrected to a 75 °C reference:
The correction adds 16% — far larger than the tolerance you are testing against. Measure temperature as close to the winding as possible.
| Vergleich | Commonly applied limit | What a failure suggests |
|---|---|---|
| Deviation from factory or baseline | Within ±2% after correction | Deformation, strand breakage, joint degradation |
| Phase-to-phase unbalance | Within 2% on larger units; up to 4% on smaller | A single-phase fault — tap contact, terminal, or shorted turn |
| Consistency across tap positions | Monotonic, predictable change | A discontinuity points at the tap changer |
| Stability over time | Stable within seconds of settling | Creeping values indicate a test-circuit problem, not the winding |
Compare like with like: temperature-corrected against temperature-corrected, same tap position, same terminal pairs. Where the manufacturer supplies factory data, that data governs over generic limits.
DC resistance measurement earns its place because it is the only routine test that looks directly at the conductor path. Ratio tests confirm turns; insulation tests confirm isolation. Neither will tell you that a tap contact has pitted or that a parallel strand has broken. A temperature-corrected reading, compared phase to phase and against a recorded baseline, catches those problems while the equipment is still on the bench — and a dry-type or Steuertransformator with a clean history is far easier to diagnose later, because you know what normal looks like for that winding.
If you are specifying new equipment and want factory resistance data supplied with the test certificate, browse the step-up and step-down transformer range, or look at a specific example such as the STU series 100 VA to 2 kVA to see the ratings and connection details you would be working with.
There is no universal value — it depends on rating, voltage class, and conductor size. A small 1 kVA dry-type unit may read from half an ohm to several ohms, while a utility power transformer phase can read a few milliohms. The meaningful reference is the manufacturer’s factory value or your own baseline for that winding.
Yes, for higher-resistance windings such as small control transformers — provided the meter resolves to 1 mΩ, you null lead resistance using the relative function, and the reading is stable. Below roughly 1 Ω, use a four-wire winding ohmmeter.
Either the winding is still heating from the test current, or there is a poor connection in the test circuit. Reduce the current and retry, and check that the clips are clean and tight.
Until the display stops changing. On small transformer windings that is often under a second; on large power transformer windings it can take several minutes. A value captured before the transient decays is not the DC resistance.
Many maintenance programmes accept a phase-to-phase spread of about 5% on motor stator windings after temperature correction — looser than the 2% to 4% typically applied to transformer windings. The motor manufacturer’s tolerance always takes precedence.
