During the manufacturing process of motors, it is necessary to conduct withstand voltage tests on their windings. Many motor manufacturers conduct withstand voltage tests on electrical semi-finished products and finished products before they leave the factory. Different test conditions will apply different peak voltages.
Withstand voltage testing is the most effective and direct method for assessing the insulation strength of a motor. During operation, the insulation of a motor is subjected to the effects of electric field, temperature and mechanical vibration over a long period of time, which will gradually deteriorate, including overall deterioration and local deterioration, eventually leading to insulation defects.
The standard stipulates that the insulation structure of high-voltage electrical appliances, current transformers, bushings, and insulating terminals, if mainly composed of ceramic and liquid materials, only requires a 1-minute withstand voltage test; if mainly composed of organic solid materials, a 5-minute withstand voltage test is required. For motor products, the standard specifies a 1-minute withstand voltage time, and there are even shorter requirements.
Many motor manufacturers use AC withstand voltage tests. This test has a cumulative effect, but it is closer to the actual operating state of the motor and can expose defects in the winding insulation in advance. It can also effectively find weak points in the insulation. Therefore, withstand voltage tests are an effective test to determine whether a motor can be put into operation.
AC withstand voltage test voltage value
In AC withstand voltage testing, the most critical issue is correctly selecting the peak value of the test voltage. On the one hand, it is necessary to maintain the insulation level; on the other hand, it is also necessary to consider the insulation degradation caused by excessively high test voltage. Ms. Can believes that the evaluation and control should be approached from the following aspects:
● The value, duration, and number of overvoltages that may occur on the test specimen. If the probability of the test specimen experiencing a high overvoltage is extremely small, it is not necessary to use an excessively high test voltage; if the test specimen may experience a high voltage, the value of the test voltage should be appropriately increased. In other words, the selection of the applied voltage should be combined with the motor's operating conditions.
● The insulation level and condition adopted in the motor design. During operation, various conditions cause the insulation of a motor to gradually deteriorate, leading to a decrease in insulation performance. Therefore, the degree of insulation damage and the number of years of operation should be considered when determining the test voltage. For example, the test voltage of a motor in operation should be 75-90% of the factory voltage, and repeated testing is not recommended.
Factors affecting insulation breakdown voltage
●Moisture will reduce the breakdown strength of insulation. For materials that do not easily absorb moisture, the breakdown voltage drops to about half of its dry value after absorbing moisture. For materials that easily absorb moisture, the withstand voltage after absorbing moisture may be only one percent of its dry value.
● The duration of voltage application has a direct effect on insulation breakdown. Higher voltage results in a shorter breakdown time, while lower voltage results in a longer breakdown time.
●Temperature has a significant impact on insulation breakdown. As temperature rises, dielectric loss increases, and poor heat dissipation leads to thermal breakdown. In other words, rising temperature can cause the breakdown voltage to drop.
● The power supply frequency is closely related to the dielectric loss. Since the dielectric loss increases with the increase of the power supply frequency, the breakdown voltage is also directly related to the power supply frequency.
● The duration of voltage application has a direct effect on insulation breakdown. Higher applied voltage will result in breakdown, and the breakdown voltage is also directly related to the power supply frequency.
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