Motor insulation
All electrical devices which have windings must be good insulated to protect them against line to line and line to ground short circuit, each motor insulation has a class to define it.
Motor insulation increasing with varnish
Electrical motor insulation as well as generator insulation could be increased using insulation varnish which is a liquid with insulating properties.
While the wires of the winding is insulated, its a good practice to increase it with insulating varnish
Motor insulation class and allowed temperature
What is insulation class?
Motor temperature should not exceed a limitation which is defined by NEMA as insulation class
The motor surface temperature is 30C lower than winding temperature, and its not applicable any more to check motor temperature by hand testing, this is because motors can run at 125C surface temperature
For example: a motor with class F insulation class can run at 155C , while motor surface will be 155-30 = 125C
- Class A motor insulation: In this class insulation is done using paper, silk and cotton, max allowed temperature is 105C
- Class B insulation: In this insulation class material used are like glass, mica and fiber, max temperature is 130C
- Class F : In this class insulation the maximum allowed temperature is 155C
- Class H : The maximum allowed temperature for this insulation class is 180C
How to measure insulation resistance?
To perform insulation resistance for a motor you need a high resistance range ohmmeter, DC voltage of 500 to 1000V is applied between winding and motor body the reading should be in mega-ohm (1MOhm and larger is accepted).
