The preventive test of motor insulation in motor maintenance is discussed

When measuring the insulation resistance of motor windings, it should be carried out respectively in the actual cold state and the hot state. The inspection test is carried out in the actual cold state.

According to the rated voltage of the motor, megohm meter is selected. If the rated voltage of the motor is below 500V, 500V megohm meter is selected; if the rated voltage of the motor is between 500-3000V, 1000V megohm meter is selected. Rated voltage above 3000V, choose 2500V megohm meter. When measuring insulation resistance of embedded or thermometers, a megohm meter not higher than 250V should be used.

When the beginning end of each phase winding of the motor is drawn out of the housing, the insulation resistance of each phase winding to the housing and between them should be measured separately. If the interwindings are connected inside the motor and only three outgoing ends are drawn, the insulation resistance of all the windings to the housing is measured. Insulation resistance of stator windings and rotor windings should be measured separately for the wound rotor motor. After measurement, the winding should be discharged to the ground.

1, insulation resistance test

Insulation resistance test is the simplest and most basic method to check the insulation state of electrical equipment. This test is carried out before other tests. By adding DC voltage to ground and between windings, the body resistance and surface resistance can be measured to reflect the defects of insulation materials and insulation structure, as well as the moisture absorption and dirt of insulation, which is a non-damage test.

Megohm meters are commonly used in the field to measure insulation resistance. There are mainly two specifications: 500V and 1000V. Due to the influence of dielectric absorption current, the indicating value of megohm meter gradually increases with time. The speed of detection is usually 120r/min. The value or stable value of 60s after the voltage is applied can be read as the insulation resistance value of engineering. The insulation resistance of the winding of the split camera shall not be less than 10 Mω.

2, DC high voltage leakage current test

Dc leakage current is the conductance current, which is composed of two parts, one is the current generated along the insulation surface, the other is the ion current in the insulation body. At the initial stage when DC voltage is applied to the winding, the leakage current also includes capacitor charge-discharge current and absorption current. Capacitor charge-discharge current is the charge-discharge current formed by the capacitor under alternating electric field generated by the shape, geometric size and property of the insulator, and the absorption current is the current generated by the repeated polarization of the medium under alternating electric field.

The purpose of DC leakage current test is to analyze the leakage current test results effectively and find out insulation problems. According to the field test, the following conclusions are drawn: when the leakage current swings violently, the insulation has fracture defects. Most of this situation appears in the notch or the place where the end is close to the ground. In addition, it may be the wax tube of the lead line is damaged. When the leakage current increases with time, it indicates that the insulation has high resistance defects and delamination, relaxation or moisture entering the insulation. Leakage current rises disproportionately. Under the same phase adjacent test voltage, if the leakage current exceeds 20% disproportionately along with the voltage, the insulation will be damp or dirty.

The leakage current increases rapidly under a certain voltage value (that is, the electronic activity increases), indicating that the insulation damage is serious and there is a risk of breakdown. If the discharge current difference of each phase is too large, the defect part may be far away from the end, or there is dirt.

3, AC voltage test

Ac voltage test, known as insulation strength test and dielectric strength test, is the most direct method to identify the insulation strength of electrical equipment, and is the best simulation of the insulation of AC motor during normal operation. It is of decisive significance to judge whether the motor can run normally, and it is also an important means to ensure the insulation level of the motor and avoid insulation failure. It can simulate the actual operation of the motor more truly than the DC voltage test and examine the ability of insulating materials to withstand high voltage alternating electric field.

According to the breakdown principle, some physical and chemical changes will occur in the interior of insulating materials in the high voltage alternating electric field, which can effectively discover the local free defects and the weakness of insulation aging. It is a destructive test that can effectively expose the insulation defects of the generator groove and the defects at the notch because the voltage is mainly divided by the capacitor under AC voltage. Each test will cause damage to the insulation and accumulate. Some hidden dangers and weaknesses inside insulation materials can be exposed from the AC voltage test, so in the AC voltage test, each test voltage needs to be reduced to 70%-80% of the previous test voltage. In the insulation test of AC motor, only when the insulation resistance test and DC voltage test and DC leakage current test qualified before the AC test. The site requires the motor to withstand the voltage test of 2000V power frequency AC for 1 minute, and there is no flashover and breakdown phenomenon of motor maintenance