Temperature Rise and Insulation System Ratings

Dr. Transformer:   Welcome back to Power Magnetics Inc. Institute!  Temperature rise and insulation for transformers, reactors and inductors is today’s hot topic.  Your burning questions as a design engineer and these answers can hopefully increase the reliability of your products in the field.  And some of those are deployed in pretty extreme environments, like our Trap Reactors and Current Limiting Reactors and Furnace Transformers.  You can find them in steel mills, deserts of the Middle East or the frozen tundra of the North Slopes!  On with the instruction…..

When we are talking to design engineers we notice there is a lot confusion about temperature rise, ambient temperature and insulation system temperature rating, and how they interact.  Hopefully we can simplify, amplify, elucidate and illuminate this topic for you today!

Docta Reacta:  Oh, please!  Get on with it in plain English!

Dr. Transformer:  Ok, we can at least make it clearer!

ANSI standards define temperature rise as the average rise as determined by change in D.C. resistance of the windings. To that is added the ambient temp and the hot spot allowance (see drawing).  The assumed ambient temperature (as again defined in the standards) is 40°C average 50°C max.  The hot spot allowance is 30°C. (The Europeans use 20°C, but what do they know?)

Typical standard dry type transformers are rated 150°C rise, with 220°C insulation system.  150°C rise + 40°C ambient + 30°C hot spot allowance = 220°C ultimate temperature, which agrees with the insulation system rating.  That will work okay as long as the load is at or below nominal, and the ambient is not too hot (most transformer failures occur during the hottest days of summer).


Insulation Systems Diagram

Insulation Systems Diagram


Another approach is to build in safety factor. As is our standard practice with Trap Reactors, Load Bank Reactors, Current Limiting Reactors (CLR) and other special magnetics, we do this: start off with a 115°C Rise unit add a 60°C ambient and a 30°C allowance = 205°C ultimate temperature. With a 220°C Insulation system, this gives a safety factor of 15°C.  It doesn’t sound like much, but that should more than double the expected 40,000 hour life of the unit.  That doubling effect holds true, roughly for every 10°C you can lower the ambient.

It is important to understand that there is no automatic relationship between insulation system rating and temperature rise.  A given insulation system and ambient will dictate a maximum allowable temperature rise (ref: 220° Insulation system and 150°C Rise above) but the designer can specify that or anything lower.

Final word: The old letter system of insulation rating has been supplanted with the actual temperature rating. ‘A’ – 105°C, Class ‘B’ – 150°C, Class ‘F’ -180°C, Class ‘H’ – 220°C.

So there you have it, ladies and gentlemen, the hot topic for today!

We welcome your comments and questions to this post.  And if you have any suggestions about other topics you would like us to cover about our transformers, reactors and inductors, please let us know via a comment or the contact us page!

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  • Diego


    Do not quite understand what you mention. I give an example, if I need to design a transformer with temperature rise 150C and insulation class 220C . I need insulation class H?