Withstand voltage is an important inspection step in the production, processing, and testing of motors . Reputable motor manufacturers will conduct withstand voltage tests in three stages: first, the electromagnetic wire will be tested at room temperature during the incoming inspection stage; second, the motor windings will be tested for inter-turn, inter-phase, and to ground withstand voltage before impregnation; and finally, the entire motor will be tested for withstand voltage during the factory testing stage.
Different manufacturers choose different test items, and different test equipment will also set different test methods. The winding inspection during the production process is quite comprehensive, and the insulation to ground, between turns and between phases must be checked. In particular, for three-phase motors with internal star connection, the phase-to-phase insulation must be checked before the star point connection. After the star point connection, only the insulation between turns and to ground can be checked.
During the inspection and testing of the entire machine, most of the newly deployed equipment undergo motor testing after the connecting pieces are assembled. The withstand voltage test can only check the insulation to ground, and the phase-to-phase insulation problem of the three-phase motor is not detected.
Inter-turn insulation testing principle
The inter-turn insulation test mechanism involves applying a high-voltage narrow pulse to both ends of the winding under test. This pulse energy generates a parallel self-excited oscillation between the winding and the matching capacitor. Due to the DC resistance of the winding, this resonance is a decaying wave that quickly approaches zero. By analyzing the difference between the oscillation waveform of the winding under test and the oscillation waveform of the standard winding, the quality of the winding under test can be determined, and whether there is an inter-turn short circuit or poor inter-turn insulation.
The traditional method for judging inter-turn insulation involves applying two oscillating waves, one from the standard winding and the other from the winding under test, to a two-wire oscilloscope and visually observing the difference in amplitude and frequency between the two waveforms. Based on experience, it is then judged whether the winding under test is qualified. The fundamental drawback of this method is the subjective arbitrariness of the judgment and the lack of quantitative indicators for assessment. This method also frequently causes disagreements and conflicts between manufacturers and inspectors.
With the development and popularization of computer technology, the inter-turn insulation test method has been greatly improved. A high-speed A/D system is used to digitize the simulated signal of the attenuated self-excited oscillation wave of the pulse voltage impact of the winding. Then, the computer analyzes, compares and calculates the waveform data, and judges the changes of various parameters.
There are many forms of parameters for waveform judgment, both domestically and internationally. These include methods such as the percentage method using the difference between the area of the tested winding's oscillation wave and the X-axis and the area of the standard winding's oscillation wave and the X-axis; the percentage method using the frequency difference between the two waves; the percentage method using the area difference between the two waves; the corona discharge method; and the bridge imbalance method. Currently, the most commonly used methods in China are the area difference percentage method and the frequency difference percentage method.
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