Electric Motor Winding Insulation

If a motor is subjected to an ambient temperature higher than 40 o c it must normally be de rated or a higher insulation class of material used.

Electric motor winding insulation.

Dip and bake epoxy varnishing trickle varnishing vacuum pressure impregnation and. The electrical insulation system for wires used in generators electric motors transformers and other wire wound electrical components is divided into different classes by temperature and temperature rise. A if the insulation resistance of a new cleaned or repaired motor that has been stored for some time is less then 10 mω the reason might be that the windings are humid and. There are four insulation classes in use namely.

Regular checks of motor insulation resistance the key to prolonging the life of any electrical device is periodic checks maintenance the insulation resistance of stored and active motors should be checked regularly. The megger will help to detect areas of deteriorating insulation where the windings may soon short. The highest voltage is normally seen by the first turn of the winding. Most repair shops will use a megger or a ground insulation tester.

If the value returned is. This insulation protects the windings from contamination shorting electrically and also make the windings more mechanically rigid. A b f and h. Standard motor capabilities established by the national electrical manufacturers association nema and expressed in the mg i standard part 30 indicate that standard nema type b motors can withstand 1000 v peak at a minimum rise time of 2 μs 500 v μs.

There are four types of varnishing methods that we will describe below. Electrical insulating material product categories electrical insulating material coated cloth products 26 insulating compounds 23 insulation paper and film 193 rigid laminate 44 sleeving tubing 190 standoff insulators 22 surge rope 12 varnish resin 462 wedges 80. The electrical insulation system is sometimes referred to as insulation class or thermal classification the different classes are defined by nema underwriters laboratories ul and iec. When a bad motor winding cannot be found by a physical inspection many times an ohmmeter may show that there is a defective winding.

Classes b and f are the most common in many applications. All four classes identify the allowable temperature rise from an ambient temperature of 40 c 104 f. After a motor is rewound the windings must be further insulated with resin or varnish. The correct insulation of the winding of a motor is determined by both the temperature rise in the motor and the temperature of the ambient air.