RMO TW Series

RMO TW Series


  • Three resistance measurement channels

  • One temperature measuring channel

  • Accuracy: (0.1% rdg + 0.1% F.S.)

  • Light

  • On-load tap changer verification

  • Automatic resistance measurement for the Heat Run test

  • Automatic and fast demagnetization

  • Automatic discharge circuit

  • Resistance measurement of transformers, rotating machines and non-inductive objects

  • Four models: RMO10TW, RMO20TW, RMO30TW, RMO50TW


The RMO-TW instrument can perform a simple, fast and reliable verification of a transformer on-load tap changer. This instrument allows measurements of a winding resistance at each tapping position of an on-load tap changer without discharging between tests. Problems with OLTC switching, such as interruption, can be detected with these measurements. The results are displayed on a backlit LCD screen.


The list of applications of the instrument includes:

  • Three-channel winding resistance measurement, which allows simultaneous measurement of winding resistance of up to three windings in a single phase.

  • Interruption detection during on-load tap-changer (OLTC) operation, performed in a single phase.

  • Heat Run test, which allows to obtain the hot spot temperature together with the resistance graph during the cooling process.

Connection of the RMO-TW to a transformer

The RMO-TW has three separate resistance measurement channels, which allow simultaneous measurement of the primary, secondary and tertiary winding in the same phase, as shown in the figure above. This significantly speeds up the measurement and reduces the total transformer test time. At the same time, by saturating the magnetic core through the HV and LV windings, the stabilisation time and subsequently the total test time are further reduced.

Simultaneous measurement of three-channel winding resistance

primary, secondary and tertiary windings. Inside the RMO-TW instrument there is enough memory to store 5000 measurements. All measurements are time and date stamped. The instrument is equipped with thermal and overcurrent protection. The RMO-TW has a very high capacity to cancel the electrostatic and electromagnetic interference that exists in high voltage (HV) electric fields. This is achieved by means of a patented filtering solution applied to both the construction of the hardware and the implementation of the application software. A special mode is provided for resistance measurement at multiple de-energized tap changer (DETC) positions.

Multiple tap changer position resistance test

The winding resistance of all tap changer positions in one phase can be measured as part of a single test, without any discharge between measurements. Different test modes are provided for on-load tap-changers (OLTC) and de-energized tap-changers (DETC). The instrument detects interruptions during operation of on-load tap changers and produces a warning message for the operator.

Automatic transformer demagnetization

After a DC current test, such as a winding resistance measurement, a magnetic power core or a measuring transformer can be magnetized. In addition, when a service transformer is disconnected, a certain amount of magnetic flux may be present and trapped in the core. Remaining magnetism can cause various problems such as incorrect measurements on a transformer, or a burst current on starting a power transformer, or incorrect operation of protective relays due to magnetized CT cores. To eliminate this source of possible problems, demagnetization should be performed. When the discharge process is complete, the RMO-TW can perform a fully automatic demagnetization. Demagnetization of the magnetic core of a transformer requires an applied alternating current with a decreasing magnitude up to zero. The RMO-TW provides this alternating current by internally changing the polarity of a controlled DC current.

DV-Win Software

The DV-Win application software allows you to control and observe the test process, as well as to save and analyze the results on a PC. It provides a test report, organized selectively as an Excel, PDF or Word spreadsheet. The software provides an additional OLTC (tap changer) verification option by recording the test current during the transition. The standard interface is USB (RS232 is optional).

If you want to acquire Transformer winding resistance meters with quality, you can also consult prices of Transformer winding resistance meters.

If you have any doubts or need advice from our engineers, don't hesitate to contact us and we will be pleased to help you. You can also call us at (+34) 982 20 99 20.