Transformer winding resistance is a critical diagnostic tool for assessing the health of a transformer.
Testing it helps detect loose connections, shorted turns, and tap changer faults.
Regular resistance checks ensure electrical reliability, prevent costly failures, and support long-term performance in both power distribution and industrial applications.
What Is Transformer Winding Resistance?
Transformer winding resistance refers to the natural opposition to current flow within the coils (windings) of a transformer.
These coils are typically made from copper or aluminum wire, and their resistance depends on material type, conductor length, cross-sectional area, and temperature.
Even though the resistance is usually very low, it plays a crucial role in the performance and diagnostics of transformers.
Why Is Winding Resistance Testing Important?
Detects internal faults like shorted turns or open circuits
Identifies issues with loose connections and deteriorating contacts
Verifies the condition and operation of tap changers
Supports preventive maintenance by spotting early signs of damage
Helps ensure long-term reliability and reduce unexpected outages
Equipment Used for Winding Resistance Testing
Digital Low-Resistance Ohmmeters (DLRO): Measure very low resistances with high accuracy
Micro-ohmmeters with Kelvin Bridge: Minimize lead resistance errors using four-wire measurements
Built-in Safety Features: Include automatic discharge, voltage monitoring, and ground detection for operator protection
Understanding and regularly checking winding resistance helps maintain a transformer’s efficiency and extend its operational life.
How to Perform a Winding Resistance Test
1. Ensure Transformer Is De-Energized and Grounded
Before starting, make sure the transformer is completely de-energized and securely grounded.
This step is crucial for safety and for preventing inaccurate readings caused by stray voltages.
2. Connect Meter Leads per Configuration
Attach the test equipment leads based on the transformer’s wiring setup (Delta, Wye, etc.).
Always follow the manufacturer’s guide and ensure tight, clean connections to prevent lead resistance from affecting results.
3. Inject Stable DC Current
Using a micro-ohmmeter or DLRO, apply a stable DC current through the winding.
Avoid AC or fluctuating signals, as they can cause inaccurate readings or core saturation.
4. Wait for Reading to Stabilize (Inductance Delay)
Large windings cause initial resistance values to drift due to inductance.
Wait until the reading stabilizes before recording—this may take seconds to minutes depending on transformer size.
5. Record and Compare Each Winding’s Resistance
Measure and document resistance values for all phases.
Compare each result to adjacent windings to detect abnormalities like shorted turns or poor connections.
6. Use Temperature Correction for Accurate Analysis
Resistance varies with temperature, so correct values using the formula:
RC = RM × (CF + CT) / (CF + WT)
Where:
RC = Corrected resistance
RM = Measured resistance
CF = 234.5 (Copper) or 225 (Aluminum)
CT = Corrected temperature (usually 75°C)
WT = Winding temperature during test
Test Setups by Transformer Configuration
Each transformer configuration requires a specific test setup to ensure accurate resistance readings:
Single-phase: Measure resistance across H1-H2 for the primary and X1-X2 for the secondary.
Three-phase delta: Test phase-to-phase—H1-H2, H2-H3, and H3-H1.
Wye: Measure from each phase to neutral—X1-X0, X2-X0, and X3-X0.
Dual windings: For time efficiency, you can test both primary and secondary windings simultaneously.
On-load tap changers: Keep the test current flowing while switching taps to check continuity and avoid repeated charging delays.
Off-load tap changers: Fully discharge the transformer between each tap position change to prevent inaccurate readings or equipment damage.
Common Causes of Abnormal Resistance Readings
Abnormal winding resistance values can indicate hidden transformer issues, such as:
Loose or corroded connections that increase resistance and create hot spots
Shorted turns or internal winding damage caused by overloading or insulation failure
Faulty tap changer contacts, especially in on-load configurations
Poor factory assembly or aging insulation that degrades conductivity over time
Temperature deviation, since resistance rises with heat—uncorrected readings may falsely suggest problems
Identifying these issues early through resistance testing helps prevent transformer failure and ensures safe, efficient operation.
Conclusion
Winding resistance testing is a simple yet powerful way to spot early issues in transformers and keep systems running reliably.
Need support with transformer diagnostics or equipment?
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FAQs
What should be the winding resistance of a transformer?
Winding resistance typically ranges from a few milliohms to several ohms, depending on the transformer’s size and design. In 3-phase transformers, phase-to-phase values should differ by no more than 2–3%.
What is the acceptable value of winding resistance?
A difference of less than 2% between phases or compared to previous test records is generally acceptable.
Some technicians also apply a 1.02 multiplier to the lowest phase value as a threshold.
What should winding resistance be for three-phase motors?
Acceptable readings are between 0.3 to 2 ohms. A reading of 0 may indicate a short, while values above 2 ohms or infinite may suggest an open circuit.
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