The windings are the heart of the motor. To ensure the normal operation of the motor, in addition to the correctness of the winding parameters, the insulation performance of the windings must also be guaranteed. This mainly includes the motor's insulation to ground, phase-to-phase insulation (for three-phase motors), and turn-to-turn insulation. We can check the insulation to ground and phase-to-phase insulation using a voltage tester, and many motor repair shops use insulation resistance testers. However, checking the turn-to-turn insulation of the windings is more difficult, especially for motor repair shops. They can only perform the final test after the motor is fully assembled, and some can only verify the effect during actual operation.
During the assembly process, excessive force applied to the windings, especially when the slot fill factor is high, can damage the inter-turn insulation, leading to inter-turn short-circuit faults. For inter-turn insulation testing after winding assembly, directly applying voltage to the winding under test is generally the most convenient method. However, at the power frequency (50Hz), the reactance of the winding is not high, and increasing the test voltage may generate an unacceptably large current. Therefore, only by increasing the frequency of the test power supply or inserting an iron core into the winding to increase its reactance can the inter-turn insulation reach the specified test voltage.
With the development of the times, the inter-turn insulation tester is widely used in the inter-turn insulation inspection of three-phase motor windings. However, in situations where there is no inter-turn insulation tester or the tester is not suitable, how can the inter-turn insulation inspection of the windings be carried out? Ms. will share several testing methods with everyone.
Piercing the Iron Heart Method
Insert the unwound rotor core (an open-circuit wound rotor) into the stator core cavity, and apply 1.3 times the rated voltage to the three-phase winding terminals for 3 minutes. If there is a short circuit between turns, the shorted turns will generate a large short-circuit current, causing localized overheating, discoloration, and even smoke in the coil; it will also cause three-phase current imbalance and abnormal electromagnetic noise. To ensure that the test current does not exceed the rated current, the stator and rotor cores must be aligned, and the gap should not be too large (approximately 0.5 mm). This method is simple and can be performed together with the three-phase current balance test. However, the core assembly and disassembly are cumbersome, and it is only suitable for small-capacity motors. For products with more stable production and using high-strength enameled wire, where short circuits between turns are less frequent, this test can be performed after final assembly.
Medium frequency power supply method
To reduce the load current and internal voltage drop of the intermediate frequency generator (IFG), a capacitor is connected in parallel with the tested winding to bring the load power factor close to capacitive 0.9. During the test, one phase of the tested three-phase winding is connected to the output terminal of the IFFG. The generator's excitation current is gradually increased until the output voltage reaches the specified value, maintained for approximately 10 seconds, and the current value is recorded. Then, the excitation current is reduced and disconnected. The other two phases are tested in the same manner. If the maximum and minimum current values of the three-phase windings exceed 3% of the average value, or if the windings locally overheat or even smoke, it indicates an inter-turn short circuit. Because the two non-tested phase windings also have induced voltages during the test, their terminals must not be short-circuited.
For medium-frequency generators with a rated frequency of 1000~2500Hz, the iron core can be omitted from the winding under test, and the test voltage is generally twice the rated voltage.
Short-circuit detector method
The short-circuit detector method uses an induced voltage in the coil to check for inter-turn short circuits. A short-circuit detector is an open-core iron core made of stacked silicon steel sheets. One end of the core is made to fit the inner diameter of the stator; the other end is made to fit the outer diameter of the rotor. In use, it is placed across the slot of the coil being tested, a 36-volt AC power supply is connected, and a thin iron sheet is placed across the slot of the other turn of the coil being tested. If there is an inter-turn short circuit, the induced short-circuit current will cause the iron sheet to vibrate and produce noise. While this method is simple and easy to implement, its low sensitivity means it can only detect short circuits when they are severe. To improve the sensitivity of short-circuit detectors, in recent years, many organizations have developed electronic inter-turn short-circuit detection instruments, also known as inter-turn testers.
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