Thermal protection devices fror stator windings 6

[petronila]

Dear All,

I would like to receive your inputs about how calculate/select this types of devices (thermistors, RTDs,thermocouples, etc.) based on insulation class of the stator windings.

Thanks and Regards

Petronila

 

[edison123]

Thermistors - Used exclusively in mush windings (LV motors) since they are small and cheap. They only act as on/off switch for the particular temperature they are designed for (usually from 100 to 150 deg C). Can't be used to track winding temperatures and they fail often.

RTD's - Used exclusively in form wound windings. They are flat and rectangular fitted in between top and bottom layers of coils/bars. Can measure temperatures from 0 to 200 deg C and are normally connected to a temperature scanner+controller which can track the winding temperatures 24x7 with individual alarm and trip points. Failure rate is minimal and last the life of the windings. A minimum of 3 (one per phase) and up to 12 nos. are used based on the machine diameter. I have seen water cooled generator stator windings having RTD's for all slots to detect loss of water through the bars.

Thermocouples - Rarely used in electric machines since their normal range is over 200 Deg C.

RTD's are more accurate than thermocouple at temperatures less than 200 deg C, are more sensitive and react faster to any temperature change than thermocouples and have a smaller drift when compared to thermocouples. Hence, their wide spread use in electrical machines.


Muthu

http://www.edison.co.in

 

[Gr8blu]

petronila: Your device has to satisfy two criteria - be able to protect the machine component (in this case, winding) based on its insulation rating AND be able to reliably differentiate temperature based on the application (note that some applications restrict allowable temperatures to well below the actual insulation class limit).

edison123 has listed the available devices, with their respective pros and cons quite clearly. I would add that there are a few basic resistance temperature detectors (RTDs) which have varying ranges and capabilities based on the base metal used. They are:
1) copper element which reads 10.0 ohms at 20 C. Works great for temperatures in the -10 C to + 50 C range.
2) platinum element which reads 100 ohm at 0 C. Works great for temperatures in the -50 C to +250 C range.
3) nickel element which reads 120 ohm at 0 C. Works great for temperatures in the -50 C to + 180 C range.

Thermistors and RTDs are not intended to be in direct contact with live conductors. Thermocouples can - and often are (provided the data acquisition is done with that in mind).

Converting energy to motion for more than half a century

 

[RRaghunath]

Gr8blu,
Now I can see why Platinum RTDs are so popular. Thanks for sharing the info.

 

[electricpete]

Good info from edison and Gr8blu

Also, an article from EASA (a motor trade organization) on the subject can be found here:
Temperature Detectors: Choosing The Correct Device To Fit Application


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