Some motor windings may have good insulation performance under static conditions, but during operation, due to vibration and other reasons, the original weak points may break down due to the impact of current and voltage. Therefore, the end treatment process of motor windings is very critical, and assembly process control is a key element in preventing failures.
In addition to the inter-turn insulation control of the same-phase coils themselves, the insulation between different-phase coils and the insulation between the coils and related components such as the iron core, frame, and end cover are also involved at the ends of the three-phase motor windings.
Phase-to-phase insulation is crucial for end-insulation control. The shape, size, and binding method of the phase-to-phase insulation, the thickness and diameter of the insulating bushings for cross-phase coils, the welding quality when welding is involved, and the handling process, as well as the penetration and curing effect of the insulating varnish during impregnation, are all key aspects of quality control. Besides the compliance of the insulation characteristics of the winding core itself, the dimensional consistency during assembly, the protective effect of tooling and molds on the winding ends, and conductive foreign objects generated during component processing are all issues that must be addressed.
Some motor windings may have good insulation performance under static conditions, but during operation, due to vibration and other reasons, the original weak points may break down due to the impact of current and voltage. Therefore, the end treatment process of motor windings is very critical, and assembly process control is a key element in preventing failures.
Phase-to-phase insulation fault in motor windings
Phase-to-phase faults are a type of electrical fault unique to three-phase motor windings. They are electrical insulation problems that occur between phases of the winding. These problems can occur at the interlayer insulation locations of different phases in the same slot. More phase-to-phase problems occur at the ends of the windings, especially at the fixed ends of cross-phase and winding lead wires.
At the winding ends, phases are separated by slot insulation, and the cross-phase windings involved are insulated with insulating sleeves. However, during the winding binding and shaping process, the phase insulation may shift to varying degrees. The phase insulation hoses, especially at the points where the leads are welded to the main wire, may be damaged due to irregular weld points and other factors. These unavoidable factors are the main causes of phase-to-phase faults.
During the impregnation and baking process of the windings, the penetration of insulating varnish can compensate for some inherent manufacturing defects, but the original damaged parts remain weak points in the electrical insulation of the motor windings. This is especially true for 2-pole motor windings, where the relatively large winding span makes end shaping more difficult, leading to a higher probability of phase-to-phase faults. Given this characteristic, the treatment of phase-to-phase insulation during the manufacturing process of motor windings should be ensured using appropriate fixtures to reduce and eliminate phase-to-phase insulation damage.
Interlayer insulation of motor windings
In modern low-voltage electrical appliances, as long as high-strength enameled wire with stable quality is selected, and the uniformity of the wire turns is strictly controlled in the coil winding process, and the performance indicators such as the pinhole density, elasticity, breakdown voltage and softening breakdown of the insulation layer of the electromagnetic wire itself are controlled, the insulation layer of the enameled wire can generally meet the requirements of interlayer insulation, and there is no need to add an interlayer insulation pad.
However, due to the voltage gradient between layers, and considering the complex factors that the coil may be subjected to during operation, such as thermal stress, mechanical stress, electromagnetic force, and solvent vapor pressure, the interlayer insulation capacity of the coil may decrease. To improve the operating stability and service life of the coil, many coils still require the selection of appropriate thin-film insulating materials as interlayer insulating pads.
Selection of interlayer insulation materials
● For coils that require impregnation with insulating varnish, it is generally advisable to use an insulating fiber film with strong absorption of varnish as interlayer insulation to enhance the penetration of the impregnation into the deep layers of the coil and increase the adsorption of solid components of the impregnation varnish between the coil turns (varnish load).
● For coils with high interlayer voltage gradients that are not treated with insulating varnish impregnation, various insulating impregnated fiber products such as varnished cloth and varnished silk can be selected. For interlayer insulation pads in large and medium-sized low-voltage electrical coils, various electrical films and their composite products, such as polyester film and polyester film insulating paper composite foil, can also be used. These film-constructed coil interlayer pads have high dielectric properties and heat resistance, but due to their relatively large thickness, the fill factor of the coil winding is correspondingly reduced. They are generally suitable for interlayer insulation pads in large and medium-sized low-voltage electrical coils.
Interlayer insulation and phase-to-phase insulation of motors
In a double-layer winding, there are upper and lower coils of different phases within the slot. They bear line voltage, so the interlayer insulation material and structure are the same as the slot insulation. The width of the interlayer insulation should be sufficient to enclose the coils, separating the upper and lower coils, and its length should be 40-70 mm longer than the core length.
When the coil ends are in the same phase, no additional phase-to-phase insulation is required. However, for higher power or larger coil sizes in 2-pole motors, fiberglass ribbon (commonly known as "tip wrapping") is wrapped around the nose of each coil to enhance insulation. Between coil groups of different phases, triangular end phase-to-phase insulation is inserted to separate adjacent coils of different phases. These coils bear line voltage, so their insulation material is the same as that of slot insulation.
For low-voltage motors, to prevent arcing, the minimum distance between the windings and the ground is 10 millimeters.
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