Insulation classes
Anyone specifying or using electric machines should have understanding of how the insulation is related to temperature. Three classes of insulation are in common use (with F being the most common):
- class B - with a maximum operating temperature of
- class F - with a maximum operating temperature of
- class H - with a maximum operating temperature of
Typically motors are designed for a maximum ambient temperature of 40 . The difference between the average winding temperature and any hot spot is limited and it is usual to allow a 10 margin for class B and F insulation and a 15 margin for class H.
| Insulation class | B | F | H |
|---|---|---|---|
| Maximum insulation temperature | |||
| Maximum temperature rise |
Note that electrical machines are designed for an overall temperature rise to a level that is below the maximum specified for the insulation materials. For example, using class-F insulation:
max ambient + max temperature rise = ,
which gives a thermal researve of . The larger the thermal researve, the longer the life expectancy of the insulation material.
When operating conteniously at the maximum rated temperaure of its class, the life expectancy of the insulation is about 10 years. Most machines do not operate at such extreme conditions because an additional safety margin is usually allowed between the calculated load torque requirements and the actual size of the machine chosen for the application. So life expectancy of a machine, which is correctly matched to its load and with suitable safety margins, can reasonably be taken between 15 and 25 years. It is common practice to design machines for class-B temperature rise but to actually use class-F insulation materials. This provides an extra thermal that will extend the life expectancy to more than 20 years. This also means that the machine could be used at higher ambient temperatures of up to or more, theoretically up to .
Running motors at a reduced temperature will also significantly extend the useful life. For example, a machine operating at will have an estimated life of
- only 300 hours with Class A insulation
- only 1800 hours with Class B insulation
- only 8500 hours with Class F insulation
- tens of thousands of hours with Class H insulation