Maintenance of windings of electric motors
Causes of damage to motor windings
During the operation of electrical machines, the insulation of the windings is gradually destroyed as a result of its heating, the impact of mechanical forces from vibrations, dynamic forces during start-up and transient processes, centrifugal forces during rotation, the influence of moisture and corrosive environments, contamination of various dust.
Irreversible changes in the structure and chemical composition of insulation are called aging, the process of deterioration of insulation properties as a result of aging is called wear.
The main cause of insulation failure on low voltage machines is temperature effects. With the thermal expansion of insulating materials, their structure weakens and internal mechanical stresses arise. Thermal aging of the insulation makes it vulnerable to mechanical loads.
With a loss of mechanical strength and elasticity, the insulation is unable to withstand normal vibration or impact conditions, moisture penetration and uneven thermal expansion of copper, steel and insulating materials.Shrinkage of the insulation from the thermal effect leads to a weakening of the fastenings of coils, wedges, channel seals and other fastening structural parts, which contributes to the failure of the winding at relatively weak mechanical influences. In the initial period of operation, the impregnating varnish cements the coil well, but due to the thermal aging of the varnish, the carburization deteriorates and the effect of vibrations becomes more noticeable.
During operation, the coil can be contaminated with dust from the surrounding air, oil from bearings, coal dust during brush operation. In the working rooms of metallurgical and coal plants, rolling, coking and other workshops, the dust is so fine and light that it gets into the machine, in places where it seems impossible to get it. It forms conductive bridges that can cause overlap or change in the enclosure.
Maintenance of windings of electric motors
The external surface of the machine and the accessible internal parts during maintenance are cleaned of dust with a dry cloth, a hair brush or a vacuum cleaner.
During the current repair of the coils, the machine is disassembled. The coils are checked, blown with dry compressed air and, if necessary, wiped with napkins soaked in gasoline. During the examination, they check the reliability of the fastening of the front parts, wedges and bandages. Eliminate the malfunctions found. Weakened or broken dressings on the ends of the stator windings of round wire are cut off and replaced with new ones made of glass or mylar cords or strips.
If the coating of the coil is in an unsatisfactory condition, then the coil is dried and covered with a layer of enamel.It is not recommended to cover the coil with a thick layer of enamel, because the thickened layer worsens the cooling of the machine. The quality of the performed repair is checked by measuring the insulation resistance before and after the repair.
Shorted windings of asynchronous motors during maintenance, as a rule, are not repaired, but only checked. If malfunctions are found, the rotors are sent for overhaul.