Transformer winding rise

[cccelcj]

I have a transformer with a 460v delta primary and a 460v y secondary. It is an isolation transformer. It is 220kva 3 phase, winding rise 150 deg c, 5.20% impedance @ 170deg c.
What does the winding rise mean?

Thanks.

 

[davidbeach]

It means that at full load, in a 40 deg C ambient, the winding will run at 190 deg C.

 

[cccelcj]

How can this temperature be measured? Can I use a thermometer or thermcouple placed within close proximity to the windings?

 

[davidbeach]

Not really. That is the conductor temperature inside the insulation. The transformer manufacturers have thermal models of their transformers that allow a thermal probe in one location to "measure" the temperature in another location. From the description of your transformer, it is likely a small, dry-type, transformer without any thermal monitoring. Don't overload it, and don't expose it to an ambient temperature in excess of 40 deg C and you won't need to worry about the winding temperature rise.

Winding temperature becomes more of an issue with larger transformers, particularly those that use cooling systems to allow more capacity than the self cooled rating of the transformer.

 

[electricpete]

You can in theory determine the average temperature of the winding by making two resistance measurements.
1 - with transformer secured at known ambinet temperature.
2 - With transformer operating at steady temperature, deenergize and immediately measure resistance.

To the extent the winding has not cooled down, you can use the ratio of the two resistances to infer the temperature based on copper temperature coefficient of resistance.

To get fancier you can log resistance vs time after deenergization and use that info to project back to resistance (temperature) at the exact time of deenergization. The exponential decay of temperature rise after deenergization produces a straight line on a plot of log scale for temperature and linear scale for time.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[mc5w]

Also, according to General Electric's installation manual for dry ventilated transformers, the case metal will be about 90 degrees Celsius, which is hot enough to cause 2nd degree burns.

Actually, anything over 124 degrees Fahrenheit will cause an instant skin burn. Slightly lower temperatures will cause a first degree burn after prolonged contact - not recommended even for masochists.

If you are running this transformer at over about 60% load it should be fenced off or otherwise be barriered against direct skin contact.

Most dry transformers that supply say 120 volt outlets in say a retail store carry no more than 1/3 of full load. Hence, the directive from Department of Energy that dry transformers be designed to produce optimal efficiency at 1/3 load.

 

[electricpete]

I wanted to change one thing on my post, but no edit feature ;-)

Measuring resistance to determine average winding temperature is not just a theory, it has been widely practiced for years by transformer and motor manufacturers.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[stevenal]

Pete,

Don't forget that scientific "theory" refers to something well established, and is not the same as "hypothesis." Sorry to rag on you, but this distinction seems to be getting lost these days.