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Slot Resistance Thermometers SRT


Slot resistance thermometers – in short SRT - are electrical temperature sensors for measuring and / or regulation of the winding temperature of electrical motors or generators. They are mounted in the slot of the stator close to the copper windings.

Their design is similar to a long bar or sword and they include either a thin film measuring resistor (chip base ) or a resistance wire to measure the temperature. Depending on the application the construction is either rigid, or easily bendable or highly flexible SRT. 

In a chip-based temperature sensor the temperature detection is limited to a small local area because of the small sensitive area and the small size of the chip - usually only a few millimetres. Whereas slot resistance thermometers based on a resistance wire are measuring the temperature over almost the full active length of the sensor. 

Thus, the averaged temperature distribution is measured within the slot. EPHY-MESS provides an excellent solution by building wire based SRT in the form of a bifilar winding. The probe is immune to induced voltages caused by external electromagnetic fields.


Electrical equipment has to be protected against continuous thermal overheating in order to ensure safe operation and long life. Slot Resistance Thermometers are well suited to fulfil this task.The SRTs are built into the winding of electric motors and generators.

The thermometers are resistant against vacuum-impregnation and are isolated against high voltages. Therefore it is possible to integrate the SRT in the assembly of the machines during the production process. By using additional insulating materials, the dielectric strength can be further increased. The motor winding is impregnated together with the SRT - already integrated in the slots. Operating temperatures are possible up to 250°C, depending on the version.

All dimensions of the SRTs can generally be customized. This ensures a good matching with the sizes of the slots. The direct integration and installation of SRT into the slots allows continuous temperature control and therefore the use of motors at optimized operating condition close to the limit. Temperature control increases the optimal power efficiency.  All thermometers can be ordered in 3 - or 4-wire circuitry to avoid measurement errors due to long cables. The additional wires enable compensation of the cable resistances and increase the accuracy of the measurement.

To fulfil measuring redundancy, two independent thermometer wirings can be used. Certified configurations are available for use in hazardous areas.

Functional principle

The measurement winding is fed with the lowest possible measuring current. The bifilar winding prevents the occurrence of induced voltages, which could lead otherwise to measurement errors. The resistance of the measurement winding is temperature dependent. Their resistance increases linearly and becomes larger with the rise of the temperature.The temperature is determined by measuring the resistance of the wire. Slot resistance thermometers with bifilar measuring windings of Ni, Cu, or Pt-wire allow the temperature detection over the complete length of the detector whereas SRT with Chip sensors determine the temperature locally at the point of their installation in the slot. Wire based slot resistance thermometers therefore bring more precision to motor protection.


Available measurement methods

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