Introduction
Direct current (DC) winding resistance testing is one of the most fundamental yet critical diagnostic methods in transformer maintenance and quality assurance. By applying a controlled DC current through the transformer windings and measuring the resulting voltage drop, the test determines the winding resistance value according to Ohm’s law. This resistance reflects the condition of the winding conductors, joints, and tap changers—any deviation from the design value may indicate problems such as loose connections, shorted turns, or contact deterioration. Accurate winding resistance measurement is therefore essential for detecting faults early, ensuring transformer efficiency, and preventing costly outages.
In modern power systems, where transformers operate under high load and complex switching conditions, high-precision resistance testers are indispensable for both factory acceptance tests and field diagnostics. These instruments must provide stable DC output, rapid demagnetization, and high measurement accuracy even under inductive or high-current conditions. To meet these demands, several international manufacturers have developed advanced solutions with distinctive features and performance advantages. The following review introduces six representative models—the Megger DLRO100, Doble TRM-403, DV Power TWA40D, Raytech WR50-13, Kingrun JYR 40S/50S, and Hightest TRIORES-20—covering their test capabilities, measurement ranges, and unique benefits for transformer testing professionals.
1. Megger DLRO100 Series （Brand: Megger）
Test Current / Output Current: Up to ~100 A DC (specified as “100 A” or “100 A battery powered unit”).
Measurement Range: 0.1 µΩ to 1.999 Ω, with a resolution of 0.1 µΩ.
Main Advantages:
Price: Public listing around US $8,330 (model DLRO100EB).
Company & Product Links:
Company: https://www.megger.com
Product page: https://www.megger.com/en-us/products/dlro100-series-digital-low-resistance-micro-ohmmeters
Contact Email: https://app.mectronic.com/stkchk/stck0b26ff87f52c97d3cebcad3f63b3d73e
Recommendation:
This model is an excellent choice for users who require ultra-low resistance (µΩ level) measurements with high current output (~100 A) and enhanced safety (CAT IV), especially for field or mobile applications.
Test Current: Selectable from 1 A to 40 A (in 1 A steps).
Measurement Range: 1 µΩ to 500 Ω (TRM-403 model).
Main Advantages:
Supports three-phase simultaneous testing—no need for reconnection between phases.
Automatic demagnetization after each test, ideal for large power transformers.
Dynamic resistance analysis (DRA) for load tap changers (LTC), providing “make-before-break” graphical output.
Price: around US $5,800 （Estimated）
Company & Product Links:
Company: https://www.doble.com
Product page: https://www.doble.com/product/trm-403/
Contact Email: https://www.doble.com/contact/contact-us/
Recommendation:
For customers engaged in operation, maintenance, or diagnostics of large oil-immersed three-phase transformers with LTC, this model’s three-phase testing, demagnetization, and LTC analysis capabilities make it a premium solution.
Test Current:
For winding resistance testing: 5 mA to 30 A; for three-phase OLTC synchronized testing: up to 40 A DC.
Measurement Range: 0.1 µΩ to 10 kΩ.
Main Advantages:
Single connection for up to six windings, reducing reconnection time.
Performs three-phase resistance measurement and OLTC dynamic resistance (DVtest) simultaneously, analyzing transition time, open circuits, and mechanical/motor faults.
Built-in automatic demagnetization, high efficiency, and fast testing speed.
Price: around US $6,530 （Estimated）.
Company & Product Links:
Company: https://www.dv-power.com
Brochure: https://www.dv-power.com/product/winding-resistance-meters/winding-resistance-meter-twa-standard/
Contact Email: sales@dv-power.com
Recommendation:
In substation and large transformer testing applications where winding resistance and LTC diagnostics are both critical, the TWA40D provides exceptional dual-function performance and diagnostic depth, ideal for high-end users.
Test Current: Selectable from 25 mA to 50 A.
Measurement Range:
At high-current range (30–50 A): 0.00 µΩ to 1.3 Ω, accuracy ±0.1% Rdg ± 0.05 µΩ.
Extended range up to 100 kΩ.
Main Advantages:
Three-channel simultaneous measurement (for LV/HV/tertiary windings).
Advanced demagnetization and discharge circuits, suitable for large inductive loads.
Internal storage for over 10,000 test records, with USB and RS232 interfaces.
Price: Requires quotation.
Company & Product Links:
Company: https://www.raytech.ch
Product page: https://www.raytechusa.com/product/wr50-13/
Contact Email: welcome@raytech.ch
Recommendation:
Ideal for users needing up to 50 A test current and three-channel simultaneous testing. Compared with higher-end brands, Raytech offers strong performance at a competitive price point, making it an attractive option.
Test Current: Up to 40 A / 50 A (depending on model JYR-40S or JYR-50S).
Measurement Range: 0 to 20 kΩ.
Accuracy / Resolution: ±0.2% ± 0.1 µΩ; minimum resolution 0.1 µΩ.
Main Advantages:
Three-phase simultaneous testing, with automatic temperature compensation (for Cu/Al) and magnetization assistance.
Automatic phase balance calculation and phase filtering (including zero-sequence).
Automatically calculates the percentage of resistance deviation for 5", 15", and 30" resistors.
Touchscreen interface, USB data export, and optional Bluetooth connectivity for global users.
Price: Requires quotation
Company & Product Links:
Company: https://www.kritester.com
Product page: https://www.kritester.com/product/jyr40s50s-winding-resistance-tester.html
Contact Email: info@kritester.com
Recommendation:
As a leader in China's transformer testing market, Kingrun's JYR series products demonstrate the strong capabilities of Chinese manufacturers in this field, especially in high-current DC testing. With the widespread application of high-precision ARM CPU and integrated designs in its products, Kingrun's advantages in competing with world-renowned brands are becoming increasingly apparent.
Test Current / Parameters: Limited public data; described as a transformer winding resistance meter series.0.001A to 20 A
Measurement Range: 0.1 μΩ to 100,000 Ω
Main Advantages: Suitable as a budget-friendly or backup solution.
Price: US $7,200 （Estimated)
Company & Product Links:
Company: https://www.hightest.co.uk
Product page: https://www.hightest.co.uk/product/triores-20-20a-three-phase-winding-resistance-tester-with-built-in-printer/
Contact Email: sales@hightest.co.uk
Recommendation:
Can be used as a reference or supplementary model when evaluating budget-friendly solutions or spares.
For clients emphasizing ultra-low resistance, mobility, and maximum test current, choose Megger DLRO100.
For large three-phase transformers with LTC, Doble TRM-403 or DV Power TWA40D provide superior analysis and diagnostic functions.
For customers with cost sensitivity but requiring high specifications, Raytech WR50-13 or Kingrun JYR Series offer the best balance between performance and value.
Pricing Strategy Tip:
Request updated quotations from each manufacturer (including accessories, cables, calibration, transport, and taxes). Evaluate after-sales support, calibration certificates, and included accessories as added value when negotiating or comparing offers.
Other Related Articles:
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How should Winding Resistance be Tested Differently on CT and PT?
What is the Difference between DC Resistance and Insulation Resistance and How to Test Them?
8 Tips to Improve the Accuracy of DC Resistance Measurement
Why is the Tested Winding Resistance Always Inaccurate? You May Have Overlooked These 6 Key Points
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