Slot resistance thermometer
Slot resistance thermometers (NWT) are electrical temperature sensors used to measure and/or control the winding temperature of electrical motors and generators. They are installed in the slot of the stator, directly on the copper windings.
Bifilar NWTs
At a glance
- Low thermal capacity
- Instantaneous thermal coupling
- Outstanding electrical insulation
- Good thermal conductivity, taking voltage load in long-term operation into account
- Bifilar wire winding prevents induced voltages and therefore measuring errors
- Resistant to impact, pressure and vibration
- Insensitive to impregnation agents and to curing and drying processes
Chip NWT
Technical information on slot resistance thermometers
General
Slot resistance thermometers (NWT) are electrical temperature sensors used to measure and/or control the winding temperature of electrical machines such as motors and generators. They are installed in the slot of the stator, directly on the copper windings.
Either a platinum thin film measuring resistor (chip) or a platinum resistance wire (bifilar) is used to measure the temperature. Depending on the application, the versions are constructed rigid, slightly flexible or highly flexible.
With almost any dimensions possible, the thermometer is guaranteed to fit just about any slot size. The shortest slot resistance thermometers are just a few centimetres long; the longest versions ever installed are around 8 metres long.
Temperature sensors with chip measuring resistor record the temperature only in a very small local area, as the temperature sensitive length of the thin film measuring resistors used is usually just a few millimetres. In contrast, slot resistance thermometers with platinum wire measure the temperature along almost the entire active length of the sensor, thus recording the temperature distribution within the slot as a whole. EPHYMESS offers wire-wound NWTs exclusively with bifilar winding. This compensates the sensor against induced voltages from external electromagnetic fields.
Use
To ensure safe operation and a long life expectancy, electrical machines need to be protected against constant thermal overload. Slot resistance thermometers are perfectly tailored to this application and are installed in the winding slots of electrical motors and generators.
Resistant to vacuum impregnation and insulated against high voltage, the thermometers can be installed directly during machine production. Using additional insulating materials allows the electrical strength to be increased even further. The motor winding is then impregnated together with the slot thermometers installed. Depending on the design, the usage temperature can be up to 250°C.
Installing the thermometers directly in the slot is the only way to operate the motors close to their thermal stress limit and thus to achieve high, almost optimum, efficiency. In order to prevent measurement errors resulting from excessively long supply lines, all thermometers are also available in 3 or 4 wire circuits. This compensates the cable resistances and increases measuring accuracy.
Where measuring circuit redundancy is required, the slot thermometers can also be equipped with two measuring windings that are independent of one another. Certified special configurations also make them suitable for use in explosive areas.
Operating principle
In bifilar slot resistance thermometers, an insulated measuring wire is wound around a carrier housing to form a bifilar measuring winding. A platinum resistance wire with a nominal value of 100Ω (Pt100 as per DIN EN 60751) is commonly used for this. Copper or nickel can also be used for the measuring wire as an alternative. Here, temperature is measured along the entire temperature sensitive length of the sensor.
In chip slot resistance thermometers, a platinum thin film measuring resistor of type M-OK is used instead of a platinum wire. The temperature sensitive length is thus reduced to the active measuring length of the thin film sensor (point measurement). This is absolutely sufficient for many measuring purposes, as the requirements usually relate to the construction (slot) rather than the active measuring length.
The sensor is fed with the lowest possible measuring current. The resistance of the measuring winding depends on the temperature and rises linearly – the higher the temperature, the greater the resistance. It is therefore possible to determine the temperature by measuring the resistance.