Insulation is an essential part of all motors. Sufficient insulation must be provided to ensure live conductors within the motor are insulated from each other and from the motor frame, which is normally earthed. Different materials combine to form an insu­lation system, which varies according to the nature and condi­tion of the component to be insulated. Components include mo­tor windings, leads, terminals, slip rings, commutators, brushes and numerous auxiliary devices.

By their nature, insulation materials cannot withstand tempera­tures as high as most other parts within motors and conse­quently most performance aspects are usually limited by the in­sulation system. As elevated temperatures also degrade the materials used, the life of most motors is determined by the in­sulation system. Most motor failures occur because of an insu­lation related problem, whether this is due to excessive temper­atures, vibration damage, supply voltage transients, contamination or simply expiry of the expected insulation life.

This Section gives background information on the classification of insulation systems. Manufacturers normally decide the sys­tem materials and how they are combined and processed to give a reliable insulation system. However, in some cases there are alternative generic systems, which may be specified by the purchaser.

It is also important for the purchaser to understand the supply system and whether there could be any abnormal conditions that could affect the insulation integrity. The higher the supply system voltage, the more important it becomes that the insula­tion system and its manufacturer’s testing programme are properly specified.

Temperature classification

Insulation materials and insulation systems are classified ac­cording to the maximum temperature at which they can satis­factorily operate. Insulation has been progressively improved to Class E such that modern motors operate at higher tempera­tures then those manufactured 50 years ago.

The lettering does not follow an alphabetical progression due to the insertion of additional improved grades with the passing years.

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