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Ratiometers are special devices for measuring the transformation ratio of transformers.

Ratiometers of this type are often referred to as transformer turns ratio testers (TTR for short). They are central diagnostic instruments for the maintenance, commissioning and quality assurance of transformers.

Below you will find the functionality and special aspects of ratiometers for transformers, their use and the advantages of these devices:

How ratiometers work:

Ratiometers measure the ratio of the number of turns between two windings of a transformer. The underlying idea is that the voltage in an ideal transformer is directly proportional to the number of turns. Therefore, the turns ratio can be determined by measuring the voltage.

Typical procedure:

1. Feeding in a known alternating voltage:
   The device feeds a defined, stable test frequency, usually 50 Hz or 60 Hz, into the primary winding of the transformer. The voltage is usually low, e.g. 8-100 V, in order to test the transformer in no-load operation without loading it.
2. Measurement of the secondary voltage:
   The generated voltage at the secondary winding is measured. As the transformer operates at no-load, the measured voltage is proportional to the turns ratio.
3. Calculating the transformation ratio:
   The device automatically calculates the turns ratio from the ratio of the applied primary voltage to the measured secondary voltage. Modern devices also compensate for phase shifts and impedance deviations.
4. Comparison with target values:
   The measured values are compared with the technical data of the transformer in order to detect production deviations, ageing or winding damage.

Special aspects when using ratiometers:

1. Phase relationship: many modern ratiometers also display the phase relationship between the primary and secondary sides - an important indication of winding twists or incorrect connections.
2. Automatic winding switching: Devices with automatic switching of the windings enable fast measurements, even with three-phase transformers, without manual reconnection.
3. Temperature and frequency corrections: High-quality ratiometers take account of ambient conditions in order to provide the most precise measurement results possible.

Use of ratiometers:

Ratiometers are used in several areas.

1. Commissioning new transformers:
When first used, a check is made to ensure that the turns ratio matches the specified design data.
Deviations could indicate manufacturing defects, incorrect winding connections or transportation damage.

2. Regular maintenance and diagnostics:
Periodic inspection helps to detect damage such as winding faults, partial winding breaks or signs of ageing at an early stage before major failures occur.

3. Quality assurance in production:
In the production of power transformers or small distribution transformers, the ratiometer is a standard testing tool to check manufacturing tolerances.

4. Troubleshooting in the field:
In the event of faults in grid operation, the ratiometer is used to quickly check whether the transformation ratio is still correct - an important aid in isolating the causes of faults.

Advantages of ratiometers:

• Fast, non-destructive testing.
• High accuracy even with complex winding configurations.
• Early detection of manufacturing or ageing faults.
• Simple procedure, can also be used in the field.

Ratiometers play a central role in the commissioning, maintenance and production of transformers. They help to detect deviations from the target value at an early stage and thus ensure the operational safety and reliability of electrical power supply systems.

Here you will find the most important aspects to look out for when purchasing ratiometers.

When purchasing ratiometers - i.e. devices for measuring the transmission ratio (turns ratio) - a number of technical, qualitative and practical aspects are important. These devices are essential for the maintenance, commissioning and troubleshooting of transformers, which is why they must be selected carefully.

• A key selection criterion is the accuracy with which the device can measure the transformation ratio. High precision is particularly important for transformers with a high voltage ratio or asymmetrical windings, e.g. voltage transformers. Pay attention to:
• A low measurement deviation, e.g. in the range of ±0.1 % or better.
• A high display resolution in order to detect even small deviations.
• The ability to reliably measure even small transformers with a low number of turns.
• A good ratiometer should generate a sufficiently stable and defined AC test voltage. Different voltage ranges are useful depending on the application:
• Low voltage supply e.g. 8 V or 80 V AC for safety and non-destructive testing.
• Adjustable voltage for different transformer sizes
• Compatibility with typical mains frequencies 50 Hz / 60 Hz or optionally with other frequencies
• Transformers are available in a wide variety of designs: Single-phase, three-phase, with taps, autotransformative, star-connected or triangular-connected. The ratiometer should therefore:
• Support single-phase and three-phase measurements
• Provide automatic phase switching, also for three-phase operation without reclamping.
• Be able to work with any winding ratios, e.g. 0.8:1 to 10,000:1.
• Recognize and automatically test taps. This is ideal for OLTC (on-load tap changer) tests.
• For a complete diagnosis, it is important that the device also detects the phase position between the primary and secondary voltage in addition to the turns ratio. This function:
• Enables the detection of incorrect winding connections or phase twists
• Supports the identification of the correct vector group
• Saves time when connecting new transformers
• A modern ratiometer should be intuitive to use, even under difficult conditions, e.g. out in the field. Useful features are:
• A color display with clear menu navigation.
• A touchscreen or robust button operation.
• Automated measurement sequences for complex devices or OLTCs (on-load tap-changers).
• Storage and export of measurement data via USB or wirelessly, e.g. WLAN or Bluetooth.
• PC software or app for evaluation and documentation.
• As transformers work with high energies, the ratiometer must also be protected in the event of unintentional connection to voltage or earth potential. Pay attention to:
• overvoltage protection at all inputs.
• Isolated or electrically isolated measurement inputs.
• Warning functions in the event of incorrect connection.
• CE conformity and compliance with international standards, e.g. IEC 61010.
• In the field or in power plants, measuring devices must be robust and transportable. Important aspects:
• Impact-resistant housing with IP protection, e.g. IP54 or better.
• Low weight for mobile use.
• Fixed, shielded measuring cables with high-quality connectors.
• internal battery operation or mains operation with charging function.
• Professional users need reliable service:
• Such as regular calibration by the manufacturer or certified laboratories.
• Technical support in the event of a fault.
• Good availability of spare parts and long-term service.
• Regular firmware and software updates.
• A professional device should:
• Be able to automatically generate test reports e.g. PDF or CSV.
• Provide standard-compliant results, e.g. in accordance with IEEE C57.12 or IEC 60076.
• Provide support for test reports and automatic comparative tests.

When purchasing a ratiometer, it is not only measurement accuracy that is important, but also versatility, robustness and user-friendliness. Look for automatic functions, support for different winding configurations, safety features and good software integration. Only then can the device be used reliably in demanding environments - from production and maintenance to fault diagnosis. A sound purchasing decision pays off in the long term thanks to precise measurement results, time savings and reliability.

Ratiometers can be purchased from various suppliers. Comprehensive information about manufacturers and suppliers of Ratiometers from all over the world can be found on Exportpages.