Today, Ms. Can will analyze the importance of composite materials and insulating hoses, which are not commonly used in three-phase motors but are very critical, and explain the relationship between insulating materials and phase-to-phase faults.
What is phase-to-phase in an electric motor?
Phase-to-phase insulation refers to a short circuit or breakdown between the phases of the three-phase windings of a faulty motor. This can be determined using a megohmmeter or a withstand voltage tester. During the manufacturing process of the stator and rotor windings of a three-phase asynchronous motor, reputable companies must inspect the phase-to-phase insulation of each winding. For stators or wound rotors with internal star connections, the phase-to-phase insulation check must be completed before the star connection.
Inter-turn short circuits (Ms. has already posted a dedicated tweet on this topic) are tested using an inter-turn tester, while phase-to-phase short circuits or breakdown faults are usually tested using a withstand voltage tester. Inter-turn short circuits include short circuits between coils in different phase groups, short circuits between coils in the same phase group, and short circuits between turns in the same coil.
Characteristics of the location of phase-to-phase faults
Phase-to-phase short circuit faults typically occur between layers at the winding ends and between upper and lower layers of coils within the slot. Causes of phase-to-phase short circuits include non-compliance of phase-to-phase insulation dimensions, insulation damage or defects, misalignment of interlayer spacers, or damage during winding.
It is particularly important to emphasize the size control of the phase-to-phase insulation at the winding ends. In order to pursue aesthetics and the processability of binding, the size of the phase-to-phase insulation should be as small as possible. However, these pursuits may lead to insufficient creepage distance between phases. This problem is not reflected when the motor is unloaded, but it will be exposed once the motor is under load.
Damage to the insulation of the winding connection wires or lead wire bushings can also cause phase-to-phase short circuits. This is a typical problem at the winding ends. The wires between phases are insulated by insulating bushings. If the bushing length is insufficient or the bushing is damaged, the most direct consequence is a phase-to-phase insulation failure.
It is particularly important to note that companies in northern regions often use 2740 flexible tubing for motors with insulation ratings of F and below. This tubing is prone to breakage at low temperatures, whereas 2760 insulating tubing avoids this problem. Given the balance between quality and cost, manufacturers are advised to consider both motor manufacturing requirements and actual usage needs when making their selection of insulation materials.
When impregnation is inadequate and the wire turns fail to form a solid whole, a large amount of external dust will accumulate in the gaps between the turns. Under the action of electromagnetic force, the wires vibrate and rub against each other, and the dust trapped in the gaps acts as an "abrasive." Over time, this wears down the wire insulation, causing phase-to-phase or turn-to-turn short circuits. Impregnation is a special and critical process in motor manufacturing, playing a vital role in controlling phase-to-phase insulation faults.
Key requirements for phase-to-phase insulation control
In summary, it is not difficult to see that the key to controlling phase-to-phase insulation faults lies in the composite materials and insulating bushings involved. For a motor, the proportion of these two materials is relatively small, but once a problem occurs, it is fatal. Selecting materials that meet the requirements is the primary condition for controlling and solving this problem.
During the manufacturing process, the dimensions and shape of the phase-to-phase insulation, as well as the dimensional control of the interlayer strips, are crucial factors. Additionally, the protection of the insulation material during the wiring process is essential. Furthermore, the curing effect of the windings after impregnation and drying is also critical.
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