The windings are the most critical part of a motor ; improper processing can lead to fatal electrical and mechanical failures. Reputable motor manufacturers will use necessary instruments and tooling to conduct essential inspections of these components.
The inspection of the windings involves two aspects: visual inspection and electrical quality compliance. These two inspections are conducted before motor assembly and after the entire machine is assembled, respectively. The pre-assembly inspection is performed after the windings are wired but before varnishing. Motor manufacturers typically refer to this as the electrical semi-finished product test. The visual inspection is largely the same, but different manufacturers use different methods for checking electrical performance. For example, some use a comprehensive testing instrument, some use a bridge circuit + turn-to-turn tester + withstand voltage tester, while others use low-voltage current measurement. We will discuss and share more on this topic at a later time; today we will only cover the visual and electrical inspection items.
1. Visual inspection of the winding
After the windings are assembled, the end extension lengths should be consistent, and the end extension heights must meet the drawing requirements. The highest point of the rotor winding end and the outer clamp must be 2-5 mm lower than the outer diameter of the rotor core. The windings must maintain a certain insulation distance from the frame, shaft, wind deflector, and other parts. Connecting wires and leads must be securely tied, and there should be no blockage in the cooling air path. All insulation should be properly lined as specified. The slot wedges should not be loose or protrude from the slot opening. The slot opening insulation should be wrapped and pressed under the slot wedges, and there should be no breakage at the slot exit. All insulation materials should be free of looseness and protrusions to prevent noise caused by wind during motor operation (such problems are relatively common in motor production, especially for windings operated manually, where human factors play a greater role. Without effective testing and control tools, these issues can only be discovered during motor testing).
After impregnation, the paint film should be smooth and flat, without wrinkles, bubbles, or raised lumps. The film should feel elastic when pressed with a finger, but without fine cracks or any undried insulating varnish seeping out from any breaks in the film (this issue mainly involves the impregnation and drying processes; Ms. Can has also encountered this). When spraying a topcoat at the ends, the coating should be complete and of uniform color.
2. Three-phase winding current balance test
After the three-phase AC windings are wired and before impregnation, a three-phase current balance test should be performed to check the symmetry of the three-phase windings. Compared with measuring the DC resistance of the windings, this test is more likely to detect errors in the number of turns or wiring. For example, when a low-voltage three-phase symmetrical voltage (usually 3-10% of the rated voltage) is applied to the test winding at the power frequency, if the difference between the maximum or minimum value of the three-phase current and the average value does not exceed ±3% of the average value, and the value is close to that of a motor of the same model and specification, then it is considered qualified. If the three-phase current is unbalanced, the power supply connector should be changed and the test repeated to analyze whether there is a problem with the power supply wiring. Then record the values of the three-phase current and three-phase voltage. Pay attention to whether there is a serious inter-turn short circuit in the winding (local overheating of the coil, or even smoke), and check whether the winding wiring meets the requirements of the drawing.
When using this method for testing, the following criteria shall be used for judgment:
(1) During the test, no magnetic objects are allowed to be placed on the test winding and its core, so as to avoid increasing the reactance of some coils and artificially causing the three-phase current imbalance.
(2) If the three-phase current of the tested winding is balanced, but the current under the same applied voltage differs from that of the same model by more than ±3%, it may be due to incorrect connection of the number of parallel circuits or pole pairs, or incorrect star or delta connection (these errors are much greater than ±3%), or the number of turns and pitch of the coil used do not match the drawings.
(3) To determine the cause of the three-phase current imbalance in the stator winding, the wiring of the winding is usually checked first. Check whether the start and end of each phase winding are connected incorrectly, whether the polarity of each coil or pole phase group is reversed, whether the number of slots per pole and each phase is equal or grouped according to a certain rule, whether there are any missing connections, broken wires or poor welding of the coils, and whether one phase winding is connected to another phase. Then check whether there is a phase-to-phase short circuit or two places where the insulation to ground is broken.
(4) To determine the cause of the imbalance of the three-phase current of the rotor winding, first check whether there is a short circuit in the parallel head; then check whether the connection of the three-phase winding is correct (whether the position of the output head is correct, whether the pitch is bent incorrectly), and finally open the connection line of the neutral point of the three-phase winding to check whether there is a short circuit between phases or two grounding.
(5) If the rotor winding has been untied, it is not easy to check the winding pitch by appearance. The neutral point connection in the three-phase winding can be loosened, and then the parallel connection of each phase winding can be checked with a lamp (or multimeter). If the number of parallel connections of each phase is equal to the number of slots of each pole and each phase of the winding, and each pole phase group is evenly distributed on the circumference, it indicates that the connection is correct.
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