Winding resistance and tap changer diagnostics, with the OLTC control built in
The RMO-TD series combines the winding ohmmeter and the on-load tap changer analyser in one device. Its three models — RMO25TD, RMO40TD and RMO60TD — inject up to 60 A with a 55 V output voltage that saturates the core quickly, measure three windings in series at once and record the dynamic resistance of the tap changer with 0.1 ms sampling, without discharging the transformer between taps.
Advantages
DVtest: the tap changer, X-rayed
The dynamic resistance measurement catches every brief current drop during the tap change; analysing the ripple, the transition time and the tap changer motor current exposes faulty contacts and selectors.
Tap changer control from the device
The built-in tap changer control unit switches the OLTC position from the instrument itself, and with the PC software the whole test — measure, change tap, measure again — runs automatically.
Three channels and a shorter test
The three voltage channels measure primary, secondary and tertiary windings connected in series, and saturating the core through the winding with the most turns cuts the stabilisation time.
Demagnetization and Heat Run test
After the DC measurement the device demagnetizes the core with alternating current of decreasing magnitude, and the Heat Run function extrapolates resistance and temperature to the switch-off instant.
Applications
- Winding resistance of power and distribution transformers and autotransformers.
- Dynamic resistance analysis of on-load tap changers of every type.
- Generators, motors and other inductive objects; also busbars and non-inductive joints.
- Heat Run test with hot-spot extrapolation.
- Automatic single-phase demagnetization of the core after the test.
About the product
The measuring range covers 0.1 µΩ to 100 kΩ with a typical accuracy of ±(0.1% of reading + 0.1% of full scale) up to 2 kΩ, and a Pt100 channel logs temperature from -50 to +180 °C. The memory stores 1000 measurements (5000 optional) and the included PC software drives the sequence over USB, plots the DRM graphs and exports reports to Excel, PDF or Word.
Accessories: current and sense cables with TTA clamps, tap changer control cable, 30/300 A current clamp, temperature probes up to 20 m, verification shunt, optional built-in thermal printer, Bluetooth and transport cases.
Technical specifications
| Models | RMO25TD, RMO40TD and RMO60TD |
| Test current | 5 mA – 60 A DC depending on model |
| Output voltage | Up to 55 V DC |
| Measuring range | 0.1 µΩ – 100 kΩ |
| Typical accuracy | ±(0.1% rdg + 0.1% FS) up to 2 kΩ |
| DVtest (DRM) | 0.1 ms sampling |
| Protection | IP67 with the lid closed |
| Dimensions and weight | 478 × 194 × 389 mm; 13 – 13.2 kg |
Frequently asked questions
What exactly does the DVtest measure?
The dynamic resistance of the tap changer during the transition: the device records the current at 0.1 ms, computes the ripple and time of each tap change and overlays the motor current on the same graph.
Must the transformer be discharged between taps?
No. The measurements of all tap positions are chained without interrupting the test current, and the device warns if it detects an interruption during switching.
Why does the 55 V output voltage matter?
Because it speeds up the saturation of the magnetic core: the sooner the current stabilises, the sooner a reliable reading is obtained and the shorter the whole test.
How is the core demagnetized after the test?
The device internally reverses the polarity of the controlled DC current, applying decreasing magnitudes down to zero following an automatic program.
What differs between the three models?
The maximum current (25, 40 or 60 A) and the output power (700, 900 or 1000 W); the case, the accuracy and the analysis functions are the same.
