Motor Insulation Systems (4)

4. Insulation Stresses

Several different physical phenomena can cause electrical insulation to deteriorate or fail. These include thermal, contamination, mechanical, vibration, voltage, carrier frequency and the method of winding the turns of insulation.

Thermal
The service life of an insulation system is generally determined by thermal stress. All insulation systems deteriorate over a period of time due to the effects of thermal stress. If the insulation always remains at its rated temperature, it should not fail during its rated service lifetime. If the insulation continually exceeds rated temperature, its lifetime will be shortened in proportion to the level and duration of the excess temperature. The insulation may last longer than the rated lifetime if its temperature remains below the rated level for significant periods of time.
Figure 4 shows the relationship of insulation life versus temperature normalized at 25°C (100% life). Increasing the temperature to 130°C decreases insulation life to 83% from nominal. Increasing the temperature further to 155°C (Class F insulation limit) or 180°C (Class H insulation limit) will further reduce insulation life.


Environmental
Contamination of the motor windings reduces dielectric strength dramatically, especially when fast rise time, high frequency voltages from IGBT inverters, are involved. A drip-proof motor that has successfully operated in a moist, sloppy pump pit may fail in short order when transferred from line power to inverter power. This is because contaminants such as oil, salt, acid, alkalies, grease, dirt, detergents, disinfectants, carbon black, chlorines or metal dust create conductive paths along the surface of the varnished windings, especially when combined with moisture from the surrounding environment. This facilitates high frequency surface tracking, which can effectively produce short circuits between otherwise insulated portions of the windings.

Mechanical
When a motor is line-started at full voltage, the powerful magnetic fields produced push and pull the stator coils back and forth, and large inrush currents generate rapid heating of the stator conductors, causing them to expand. The surrounding iron stator core heats up less, has a lower thermal expansion rate, and doesn’t move at the same rate as the copper coils it supports. As a result, the copper coils strain against the varnish that adheres them to the core, causing fractures where the coils exit the core. Each successive across-the-line motor start repeats the cycle of flexing and expanding, worsens these breaks, and increases the chance that a conductor will abrade it’s remaining insulation and short to ground.
Once the insulation has cracks, moisture and contaminants will find their way in, further reducing insulation integrity. Similar expansion rate differences are present in the motor’s rotor circuit, where the iron core expands slower than the copper conductors (used in large motors) it holds, and faster than aluminum conductors (used in smaller motors). Generally, however, the stator windings fail before the rotor does.
During inverter starts, however, motor voltage and frequency are slowly increased rather than applied at full value. Motor coils are not subjected to the excessive heating and flexing that occurs during line starts, thus extending motor life. Only when the inverter is bypassed, does the motor experience the starting stresses described above.

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