Insulation classes (IEC 60085)
The insulation class (thermal class) says how hot the winding insulation may get and still reach its expected life. The classes are defined in IEC 60085; IEC 60034-1 sets how far a motor winding may rise above the cooling air at rated load. Both assume a coolant (ambient air) of at most 40 °C.
Thermal classes and temperature rise
| Class | Thermal class | Allowed rise at 40 °C | Hot-spot margin |
|---|---|---|---|
| A | 105 °C | – | – |
| E | 120 °C | – | – |
| B | 130 °C | 80 K | 10 K |
| F | 155 °C | 105 K | 10 K |
| H | 180 °C | 125 K | 15 K |
The rise is the average winding temperature, measured from the change in winding resistance. The hottest spot inside the winding runs hotter than that average; the margin is what is left between ambient + rise and the class limit. The rise limits shown are the usual ones for indirectly cooled windings, listed here for classes B, F and H only; IEC 60034-1 has separate limits for some machine types and sizes. Higher thermal classes exist too: 200 (N), 220 (R) and 250.
"Class F insulation, class B rise"
Many motors are built with class F insulation (155 °C) but designed so the winding only rises as much as class B allows (80 K). At 40 °C ambient that puts the average winding at 120 °C, 35 K below the class F limit instead of 10 K. That spare margin is used for:
- longer insulation life, since insulation ages faster the hotter it runs;
- ambient temperatures above 40 °C or installation at altitude;
- some overload, or extra heating from a frequency inverter.
Converting the temperatures: temperature converter. A temperature rise in kelvin is the same number in °C.
Summarised from the standard named above. For a design decision, check the motor maker's datasheet and the current edition of the standard. More datasheet reference pages.