Motors form the heart of the industrial processes. As the processes are run continuously, motors are also subject to long running hours. They are subject to electrical, mechanical as well as thermal stresses as below:
Electrical: High breakdown voltages/ long running hours/ Partial discharges / Leakage currents
Thermal: Deformation due to temp. rise / De laminating due to aging.
Mechanical: High forces and torques under tough mechanical conditions
To withstand the difficult operating conditions, motor insulation should be strong to ensure long lasting performance. Various insulation classes in practice as per IEC are as below:
Insulation class | Maximum attainable temp | Permissible operating temp | Insulators used |
A | 105 | 100 | Varnished Cloth/Oil Impregnated Paper |
E | 120 | 110 | Enamels with Polyvinyl base/ Molding powder with Phenol-formaldehyde |
B | 130 | 120 | Mica/Glass with bitumen/shellac/bakelite |
F | 155 | 145 | Glassfibre/Mica with Poly amides or Polyurethane |
H | 180 | 165 | Mica/Asbestos/Glassfibre with Silicone resins/elastomers |
The common types of insulation classes in use are B for small motors and F for medium and large motors. The general industrial practice is to use F class insulation with temp rise restricted to class B.
Insulation operating temperature needs to be adhered to strictly as any temp. rise will result in deterioration of life . The following chart will give us an idea about the same.
Hope this gives us a basic idea about motor insulations. Various methods like Synthetic resin casting, Vacuum Pressure Impregnation are used to make provide better insulating properties.
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