How to prevent damage to the insulation of the stator winding of an induction motor
About 80% of accidents with electric cars are related to damage to the stator winding... The high damageability of the winding is due to the harsh operating conditions and insufficient stability of the electrical properties of the insulating materials. V insulation damage can lead to a short circuit between the winding and the magnetic circuit, a short circuit between the turns of the coils or between the phase windings.
Causes of damage to the stator windings of asynchronous electric motors
The main cause of insulation damage is a sharp decrease in electrical strength under the influence of wetting the coil, contamination of the coil surface, impacts to the electric motor from metal shavings, metal and other conductive dust, the presence of vapors from various liquids in the cooling air, long-term operation of the electric motor at elevated winding temperature, natural aging insulation.
Winding damping can occur due to prolonged storage of an electric motor in a damp, unheated room.It has been found that the engine may become damp when the engine is idle for a long time. condition, especially when the ambient humidity is high or when water gets directly into the electric motor.
To prevent the coil from getting wet during storage of the electric motor, good ventilation of the warehouse and moderate heating in the cold season. During periods of extended engine shutdowns in wet and foggy weather, close the inlet and outlet air duct valves. In warm dry weather all valves should be open.
Dirty motor winding mainly due to using insufficient clean air for cooling. Along with cooling, the air in the electric motor can get coal and metal dust, soot, vapors and drops of various liquids. Due to the wear of brushes and slip rings, conductive dust is formed, which with the built-in slip rings settles on the motor windings.
Prevention of pollution can be achieved by careful maintenance of the electric motor and thorough cleaning of the cooling air. If necessary, periodically check the electric motor, clean it of dust and dirt and, if necessary, make small repairs to the insulation. With increased heating, as well as as a result of natural aging, the insulation significantly loses its mechanical strength, becomes brittle and hygroscopic.
When the machine works for a long time, the fastening of the grooved and front parts of the winding is weakened and due to vibrations, their insulation is destroyed... Winding insulation can be damaged: due to careless assembly and transportation of the electric motor, due to breakage of the fan or rotor belt, in the result on the grazing of the stator with the rotor.
Insulation resistance of the stator winding of asynchronous electric motors
The condition of the insulation can be judged by its resistance. The minimum insulation resistance depends on the voltage U, V, the electric motor and its power P, kW. Insulation resistance of the windings of the magnetic circuit and between them the windings with an open phase at the operating temperature of the electric motor must be at least 0.5 MOhm.
At temperatures below the operating temperature, this resistance must be doubled for every 20 °C (full or partial) difference between the operating temperature and that temperature for which it is specified.
Measurement of insulation resistance of electrical machines
Insulation resistance is usually measured with a special device - a megohmmeter. For windings of electrical machines with a rated voltage of up to 500 V, the voltage of the megohmmeter should be 500 V, for the windings of electrical machines with a rated voltage of more than 500 V, a megohmmeter voltage of 1000 V. If the measured insulation resistance of the winding is less than the calculated one, then clean and dry the coil if necessary.For this purpose, the electric motor is disassembled and remove dirt from the accessible winding surfaces with wooden scrapers and clean rags soaked in kerosene, gasoline or carbon tetrachloride.
Methods of drying asynchronous motors
Drying of protected machines can be done both disassembled and assembled, closed machines must be dried disassembled. Drying methods depend on the degree of moisture in the insulation and on the availability of heating sources. When drying with external heating, hot air or infrared rays are used. Hot air drying is carried out in drying ovens, boxes and chambers equipped with steam or electric heaters. Drying chambers and boxes must have two openings: at the bottom for cold air inlet and at the top for hot air outlet air and water vapor generated during drying.
The motor temperature must be increased gradually to avoid mechanical stress and swelling of the insulation. The air temperature must not exceed 120 °C for class A insulation and 150 °C for class B insulation.
At the beginning of drying, it is necessary to measure the temperature of the winding and the insulation resistance every 15-20 minutes, then the interval between measurements can be increased to one hour. The drying process is considered complete when the resistance value in the steady state. If the coil is slightly moistened, drying can be carried out due to the release of thermal energy directly to the parts of the electric motor.AC drying is most convenient when the stator winding is energized when the rotor is locked; while the phase rotor winding must be short-circuited. The current in the stator winding should not exceed the rated value.
Change in winding temperature and insulation resistance depending on drying time reduced voltage, then the connection scheme of the stator windings may not change, for single-phase voltage it is recommended to connect the phase windings in series. For drying energy losses in magnetic circuit and motor housing. To do this, with the rotor removed, the stator is laid with a temporary magnetizing coil covering the magnetic circuit and body. It is not necessary to distribute the magnetizing coil over the entire circle, it can be focused on the stator in the most convenient place. The number of turns in the coil and the current in it (cross-section of the wire) are chosen as follows so that the induction in the magnetic circuit is (0.8-1) T at the beginning of drying and (0.5-0.6) T in end of drying.
To change the induction, taps are made from the coil or the current is adjusted magnetizing coil.
Methods for determining the location of winding insulation failure
First of all, it is necessary to disconnect the phase windings and measure the insulation resistance of each phase winding of the magnetic circuit, or at least check the integrity of the insulation Determining the place of insulation failure with two voltmeters. Determination of a group of windings with damaged insulation by means of a test lamp. At This reveals a phase winding with damaged insulation.
Different methods can be used to determine the location of the fault: the method of measuring the voltage between the ends of the coil and the magnetic circuit, the method of determining the direction of the current in parts of the coil, the method of dividing the coil into parts and the method of «burning». In the first method of a phase winding with damaged insulation, a reduced AC or DC voltage is applied and voltmeters measure the voltage between the ends of the winding and the magnetic circuit. According to the ratio of these voltages, the position of the damaged winding relative to its ends can be estimated. This method does not provide sufficient accuracy at low resistance. coils.
The second method is that a constant voltage is applied to the voltage the ends of the phase winding combined in a common point and on the magnetic circuit. For the possibilities of regulation and limitation of the current in the circuit include the rheostat R. The directions of the currents in the two parts of the coil limited by the point of connection with the magnetic circuit will be opposite. If you successively touch two wires from the millivoltmeter at the ends of each group of coils, then the arrow of the millivoltmeter will deviate in one direction, while the wires from the millivoltmeter will not be connected to the ends of the group of coils with damaged insulation. At the ends of the following groups of coils, the deflection of the arrow will change to the opposite.
For a group of windings with damaged insulation, the deflection of the arrow will depend on which of the ends is closer to the location of the insulation failure; except In addition, the voltage at the ends of this group of coils will be less than at other groups of coils if the insulation is not close to the ends coil group. In the same way, an additional determination of the place is made. insulation failure inside the coil group.