1. Principle of Operation
Winding resistance testers apply a regulated DC current through a winding and measure the voltage drop (V) across it. Using Ohm's Law:
R=VIR = \frac{V}{I}R=IV
Where:
RRR = resistance (ohms)
VVV = voltage drop (volts)
III = current injected (amps)
This method is non-intrusive and ideal for low-resistance measurements in transformers, motors, generators, and reactors.
2. Types of Winding Resistance Testers
| Type | Features | Use Case |
|---|---|---|
| Manual DC Testers | Basic measurement with analog or digital output | Small motors, coils |
| Micro-ohmmeters | High-precision, low-resistance meters | LV windings, CT/PT |
| Transformer Test Sets | Multi-phase, high-current, auto temperature correction, demagnetization | HV transformers, tap changers |
| Automated Test Systems | Integrated with DGA, insulation, TTR tests | Utility-level testing and diagnostics |
3. Detailed Test Procedure
Preparation:
De-energize equipment.
Discharge stored energy.
Verify safety grounding.
Disconnect CTs/PTs if needed.
Execution:
Select appropriate test current (typically 1 A–25 A; some testers go up to 100 A).
Connect Kelvin-type four-wire leads:
2 leads for current injection
2 leads for voltage sensing
Start the test and allow reading to stabilize (especially important for large transformers).
Record values, phase-by-phase and per tap.
Demagnetization:
Done automatically or manually post-test.
Reduces residual flux to prevent abnormal inrush current upon re-energizing the transformer.
4. Temperature Correction
Since copper/aluminum resistance varies with temperature:
RT=RT0×[1+α(T−T0)]R_T = R_{T_0} \times [1 + \alpha (T - T_0)]RT=RT0×[1+α(T−T0)]
RTR_TRT: measured resistance at test temp TTT
RT0R_{T_0}RT0: corrected resistance at reference temp (usually 20°C or 75°C)
α\alphaα: temperature coefficient (Cu ≈ 0.00393/°C; Al ≈ 0.00403/°C)
Many modern testers auto-correct using internal temperature sensors or external RTDs.
5. Interpretation of Results
| Condition | Observation | Possible Cause |
|---|---|---|
| One phase significantly higher | High resistance connection, bad joint | Loose crimp, corroded lug |
| One phase significantly lower | Shorted turn or winding | Insulation breakdown |
| All values high | Bad contact, small test current | Lead or connection issue |
| Unstable readings | Incomplete saturation, test current too low | Try increasing current or wait |
| Variation from previous records | Aging, thermal damage | Trending needed |
6. Sources of Error
Improper lead connections (especially using 2-wire instead of 4-wire).
Test current too low (affects accuracy due to noise).
Not waiting for thermal stabilization.
External magnetic influences (e.g., nearby energized equipment).
Poor demagnetization affects follow-up tests.
7. Standards and Guidelines
| Standard | Description |
|---|---|
| IEEE C57.12.90 | Standard test code for liquid-filled transformers |
| IEC 60076-1 | Transformer general specifications |
| NETA ATS/MTS | Acceptance and maintenance testing standards |
| IEEE 62-1995 | Guide for diagnostic field testing of electric power apparatus |
8. Key Specifications to Compare in Testers
Current output range (e.g., 1 mA to 100 A)
Resistance range (e.g., 0.1 µΩ to 10 kΩ)
Resolution (e.g., 0.1 µΩ)
Temperature compensation
Phase-by-phase testing support
Data storage and PC interface
Demagnetization function
Battery vs mains powered
9. Examples of Winding Resistance Testers
| Brand | Model | Features |
|---|---|---|
| Megger | MTO300 | Three-phase, automatic demag, temp comp |
| DV Power | RMO100H | Up to 100 A, fast stabilization, dual channel |
| OMICRON | DIRANA + CP SB1 | Integrated diagnostics, smart analytics |
| Raytech | WR50-12 | Fast, high-accuracy, rugged |
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