RMO TT Series

RMO TT Series


On-load tap changer dynamic resistance measurement

  • Three resistance measurement channels
  • Four temperature measurement channels
  • Vibration measurement
  • Automatic resistance/temperature measurement for the Heat Run test
  • Rapid automatic demagnetization
  • Tap changer motor current monitoring channel
  • Automatic discharge circuit
  • Three models: RMO40TT, RMO60TT, RMO100TT


The Tap Changer Analyzer & Winding Ohmmeter of the RMO-TT series is designed for winding resistance measurement of inductive objects, and for a tap changer analysis. The RMO-TT

series is based on the state of the art technology, using the most advanced switch mode technology available today. The RMO-TT instruments are accurate (0,1%), powerful (up to

100 A) and still portable. The Instruments generate a true DC ripple free current with automatically regulated measurement and discharging circuit.

RMO-TT instruments can perform a simple, quick and reliable transformer on-load tap changer condition assessment. This series of instruments enable measurements of a winding resistance in every tap position, current ripple values (percentage of a current drop during a tap change) and transition times during the tap changes. Problems with a connection, contacts, and selector/diverter switch operation (tap change with interruption) can be detected with these measurements.


The list of the instrument application includes:

  • Three-channel winding resistance measurement, which enables simultaneous winding resistance measurement of up to three windings on a single phase.
  • Dynamic resistance measurement (DVtest) of on-load tap changers, performed on a single phase.
  • A measurement of on-load tap changer motor current by using a dedicated channel.
  • Measurement of on-load tap changer external tank vibrations using an accelerometer.
  • Heat Run test, which enables obtaining the Hot Spot temperature along with temperature/resistance graph during the cooling process.
  • A single-phase automatic transformer demagnetization.

Connecting RMO-TT to Transformer

The RMO-TT series has three separate resistance measurement channels, which enable simultaneous measurement of the primary, secondary and tertiary winding on the same phase. This significantly speeds up the measurement and reduces the total transformer testing time. At the same time, by saturating the magnetic core through the HV and LV windings the stabilization time and subsequently the total testing time is reduced even more.

Simultaneous Three-Channel Winding Resistance Measurement

The RMO-TT instruments inject the current with a voltage value as high as 55 V. This ensures that the magnetic core is saturated quickly and duration of the test is as short as possible. The three independent channels enable simultaneous testing of three windings in series – primary, secondary and tertiary windings, as presented in the figure above. There is enough memory within the RMO-TT instrument to store 1_000 measurements. All measurements are time and date stamped. The instrument is equipped with thermal and overcurrent protection. The RMO-TT has very high ability to cancel electrostatic and electromagnetic interference that exists in HV electric fields. It is achieved by a proprietary filtration solution applied to both, the hardware construction and the application software implementation.

On Load Tap Changers – Dynamic Resistance Measurement (DV test)

The RMO-TT can be used to measure the winding resistance of the individual taps of a power transformer without discharging between the tests. The unit also checks whether the on-load tap changer (OLTC) switches without an interruption. The moment a tap position is changed from one tap to another, the device detects a sudden, very short drop of the test current. These drops called "Ripple" should be consistent, where any drop out-of-line should be investigated. Tap changer malfunctions can be detected by analyzing the measurements of transition ripple, transition time, and visualizing DRM graphs. In addition, the tap changer motor current is recorded, and displayed on the same graph. The built-in tap changer control unit enables remote control of the tap changer operation from the instrument's keyboard.

On Load Tap Changers – Vibration Testing In addition to DVtest

The RMO-TT instruments can test the tap changer condition by measuring vibrations on an external tap changer tank. This non-intrusive test is performed using DV-Win PC software, and it enables detecting various mechanical problems and checking the timing of tap changer operation.

DV-Win Software

The DV-Win application software enables control and observation of the test process, as well as saving and analyzing the results on a PC. It provides a test report, arranged in a selectable form as an Excel spreadsheet, PDF, Word, or ASCII format. The software provides an OLTC (tap changer) condition assessment through analysis of the graphs representing dynamic resistance values during the tap changer transitions. Additionally, the DV-Win measures and calculates the OLTC transition time, the ripple and the winding resistance for each tap changing operation. 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.