Transformer Insulation and Temperature Rise

There are two aspects of transformer design related to temperature: Insulation Class and Temperature Rise. It is important to understand both aspects when choosing a transformer for a specific installation. Both insulation class and temperature rise assume a maximum ambient temperature of 40°C.

The most common insulation classes are as follows:

  • Class 220 (formerly “H”)
  • Class 180 (formerly “F”)
  • Class 150 (formerly “B”)
  • Class 105 (formerly “A”)

Each class designates the maximum temperature, in Celsius, that the insulation can withstand without damage (Class 220 can withstand 220°C, etc). This does not mean that the entire transformer is expected to reach these temperatures, rather, that the hottest point on the transformer may reach this temperature without damage.

Temperature Rise refers to the average rise in temperature in the transformer coils at full load. The expected temperature rise is a function of the current and impedance (efficiency) of the transformer. Standard rise temperatures are 150°C, 115°C, 80°C, and 50°C. These ratings have traditionally been paired with insulation classes as follows:

  • Class 220 <> 150°C
  • Class 180 <> 115°C
  • Class 150 <> 80°C
  • Class 105 <> 50°C

These pairings are based on a simple formula:

Insulation Rating = Max Ambient Temperature + Temperature Rise + Hot Spot

Therefore if we use the pairing for a Class 220 transformer we would have:

220°C = 40°C + 150°C + 30°C

So where does the hot spot number come from? Really, it is a bit of a fudge factor. Due to physical arrangement, material variation, etc., there will always be areas in a transformer coil that are warmer than others, so if I were to design a transformer with 220°C insulation and a 150°C temperature rise, my design must limit the hot spot temperature to no more than 30°C more than the rated temperature rise.

220°C – 40°C – 150°C = 30°C

While these traditional pairings make sense, many manufacturers have abandoned the lower insulation classes and manufacture all transformers with Class 220 insulation, leaving temperature rise as the only remaining thermal design consideration. In any case, it is always possible to specify a temperature rise that is well below the insulation class. Doing so will result in a transformer that runs cooler, lasts longer and is capable of handling overloads.

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