Magnetic Wire Quality Control and Winding Interturn Fault Analysis

Electromagnetic enameled wire is the main raw material of motor windings, which must meet the requirements of conductor shape and specification, long-term work at high temperature, strong vibration and centrifugal force at high speed, corona and breakdown resistance under high pressure, and chemical corrosion resistance under special atmosphere; the manufacturing process requires the coil to be able to withstand tension, bending and wear during winding and embedding, as well as expansion and erosion during soaking and drying. Avoiding the use of unqualified electromagnetic enameled wire in motor windings is the primary task of electromagnetic wire detection and control. Therefore, the incoming inspection of electromagnetic enameled wire must clarify key monitoring indicators and acceptance criteria. In each production process and transportation process, attention should be paid to strengthening protection, avoiding the winding sequence of electromagnetic enameled wires with hidden quality hazards, and reducing the probability of inter-turn failure.

1. Main indicators of electromagnetic enameled wire detection and control.

The main indicators that need to be controlled for electromagnetic enameled wire are the external dimensions, mechanical properties, heat resistance, electrical properties, film continuity and chemical resistance of electromagnetic enameled wire.

1.1 Check and control of the external dimensions of electromagnetic enameled wires.

The size inspection of electromagnetic enameled wire mainly includes the allowable tolerance of wire diameter, the maximum outer diameter and the minimum film thickness. The allowable tolerance of wire diameter refers to the allowable deviation between the actual value of the bare wire diameter and the specified value. The smaller the tolerance, the more beneficial it is to the calculation of resistance, turns and other values during the design process. The maximum outer diameter is the maximum allowable diameter of the wire, and the excess of the maximum outer diameter will increase the coil size and increase the slot filling rate. The minimum film thickness is the minimum film thickness allowed for the wire. If the film is too thin, it is easy to produce pinholes, resulting in a decrease in pressure resistance, scratch resistance, chemical resistance, and aging resistance, resulting in serious potential quality problems of motor windings. Table 1 shows the allowable ranges of various parameters for enameled wires of different diameters.

1.2 Testing and control of mechanical properties of electromagnetic enameled wires.

In the process of winding and embedding the motor coil, mechanical forces such as tension, friction, bending force during winding, friction during embedding, and inter-turn friction caused by vibration during motor operation all act on the winding. If the core wire or paint film of the enameled wire fails to meet the requirements, the mechanical force will bring different degrees of damage to the enameled wire. The enameled wire wound into a coil is mainly affected by two external forces. One is tension. If the elasticity of the paint film is insufficient, the elongation of the paint film after being stressed cannot keep up with the elongation of the conductor, and the paint film will crack. Bending force. After bending, the outer edge paint film is elongated, and the inner edge paint film is compressed. If the elasticity is not good, the paint film will crack. The thin wire is easy to stretch and is tested by the elongation method; the thick wire is greatly affected by bending, so it is tested by the winding method. The scratch resistance of the paint film of electromagnetic enameled wire is expressed by the number of scratch resistance or breaking force of the paint film under a certain external force. During the operation of coil winding, embedded wire and electrical products, there will be pressure or friction on the enameled wire. Therefore, the mechanical performance test of electromagnetic enameled wire is an important test item.

1.3 Enameled wire heat resistance test.

The heat resistance of enameled wire directly affects the design and use of electrical equipment. The temperature rise of electrical equipment is limited by insulating materials such as enameled wire. Using enameled wire and supporting materials with high heat resistance can obtain better reliability under the condition of unchanged structure, or achieve the energy-saving effect of reducing the volume, weight and consumption of non-ferrous metals and other materials under the condition of constant power. The heat resistance test of enameled wire mainly includes thermal aging test, thermal shock test and thermal softening breakdown test. Thermal shock test is to study the heat resistance of enameled wire film under mechanical stress. Heat-resistant softening breakdown test: In the coil, the lower enameled wire is subjected to the tension of the upper enameled wire. If the enameled wire is in the drying process of pre-drying or dipping or running at high temperature, the enameled film is softened by heat and gradually squeezed thin under pressure, and the coil may short-circuit between turns.

Testing and control of electrical properties of enameled wires.

The current in the enameled wire circulates along the coil core wire to generate electromagnetic induction, which makes the motor winding work. If the dielectric strength of the enameled film fails to meet the requirements, the coil will be short-circuited. The test standard for dielectric strength is the minimum breakdown voltage of electromagnetic enameled wire, and the minimum breakdown voltage of electromagnetic enameled wire of different diameters is shown in Table 2.

Breakdown voltage means that under the action of a strong electric field, the bound electrons in the polymer obtain energy from the electric field, become free electrons, and move in the direction of the electric field. When free electrons collide with other bound electrons in motion, they are excited into free electrons, which increases the conductivity of the polymer. When the electric field strength exceeds a certain value, a hole connecting the two electrodes is formed in the polymer, so that the increased current passes through, and the polymer loses its insulating properties and is broken down. The strength of the voltage applied at this time is called the breakdown voltage. When the voltage breaks down, the heat will cause thermal damage to the polymer, and the polymer will melt and burn. The main factors affecting the breakdown voltage are film thickness and roundness, degree of curing, and external impurities in the paint. In the electrical performance test of enameled wire, the main factors affecting the breakdown voltage test data are twist, voltage application time, boost speed, film elongation and pressure temperature.

1.5 Continuity and chemical resistance of paint film.

For small and medium-sized motors, the number of turns of electromagnetic enameled wires with scattered windings is large, the windings are tight, and there are many contact surfaces between wires. If there are many pinholes in the enameled wire film, the pinholes overlap between the two turns, which will cause a short circuit between the wires. The chemical resistance of enameled wire includes acid resistance, alkali resistance, salt spray resistance, moisture resistance, oil resistance, solvent resistance, refrigerant resistance, and radiation resistance, which are generally controlled by electromagnetic wire manufacturers. During the coil impregnation process, the enameled wire must undergo thermal shock and softening breakdown thermal performance tests, and will also be eroded by solvents such as toluene, xylene, petroleum solvents and oil in the impregnation paint. Different paint films have different tolerances to different solvents. When impregnating, the solvent impregnation paint suitable for the enameled wire should be selected according to the characteristics of the enameled wire.

2. Hazard and detection of enameled wire pinholes.

The continuity of the paint film is an important key performance of the electromagnetic enameled wire. The number of pinholes in the paint film must be within the qualified range under a certain length. A professional paint film continuity tester can be used to check the number of pinholes. The fewer pinholes, the better the integrity of the electromagnetic wire film. Pinhole defects can be detected using the principle of high voltage discharge, and the electromagnetic wire is in semi-envelope contact with the high-pressure concave wheel. When the film thickness is insufficient or there are serious bare copper defects, the instrument will record the specific defect number.

The reasons for pinholes in the paint film can be attributed to two types: one is that the copper material is not suitable, and the molded wire core has many burrs, which may be caused by the raw material itself or the wire drawing process; secondly, uneven painting, impurities in the paint, The quality of the paint and the drying method after painting can easily lead to the failure of the motor between turns, which affects the service life of the motor.

At present, the on-line detection system for enameled wire film continuity used by most electromagnetic wire manufacturers is a non-contact detection method based on DC high-voltage electric field sensors. Its basic principle is that a DC high-voltage circuit generates a DC high voltage, which is connected to the cylindrical metal conductor in the high-voltage electric field sensor, and generates a high-voltage electric field inside the cylindrical metal conductor. The enameled wire passes through the high-voltage electric field at high speed under the drive of the take-up wheel. When the electric field strength is large enough, corona will be generated on the surface of the enameled wire. If the enameled wire with defective enameled film passes through, a leakage current caused by partial discharge will form between the metal conductor, the enameled wire and the take-up wheel. By dynamically measuring the change of leakage current of enameled wire, the defect location and severity of enameled wire can be accurately measured and recorded.

When detecting with this method, the enameled wire and the sensor have no contact during the whole process, and no additional damage will be caused to the enameled wire. It is very suitable for online detection of enameled wire running at high speed on the production line. The metal conductor in the high-voltage electric field sensor is approximately cylindrical, and the electric field generated is basically the same in all directions, so the detection ability in all directions is basically the same, and the detection sensitivity is high. The electromagnetic wire detection in motor manufacturing usually adopts the paint film continuity tester, which can only be detected in the semi-envelope state, and its ability to find defects is weak. However, due to the randomness of sampling, the performance compliance of the electromagnetic wire can be qualitatively judged.

3. The effect of insufficient paint on the performance of the motor.

For motor products, the immersion drying process is the key control link, which is mainly controlled by process equipment and process management. The immersion drying of motor windings is to fill all the gaps in the windings with insulating paint. After drying, the windings become a solid whole to ensure that the insulation, heat dissipation and vibration performance of the motor meet the requirements and reach the comprehensive performance level of the motor. When disassembling the motor with winding failure, it is found that the windings are loose, which has a great adverse effect on the performance of the motor.

In order to obtain a fully effective paint coating on the motor windings, attention should be paid to the materials, process parameters and process control involved in the soaking and drying process. The viscosity of the paint is very critical during the dipping process. If the paint is too thin, there is not enough paint to fill the gaps in the windings, and the paint is too thick to enter the windings, especially the core grooves. Dipping paint with VPI vacuum pressure equipment can effectively improve the dipping effect. In the actual production process, the state of the dipping equipment and the viscosity of the paint must be dynamically monitored. In the process of winding insulation treatment, most of the impregnation and drying processes are carried out separately. Pre-drying before impregnation can remove moisture and air from the windings. After impregnation, if the baking temperature is low, the paint that has entered the windings will be lost, and the insulation treatment effect will not be good; if the temperature is too high, it will cause the aging and failure of materials such as electromagnetic wires and insulation. For most drying processes, the windings are in a static state, which will cause uneven distribution of insulating paint relative to the windings, and the curing time of insulating paint is too long, which will increase the uneven impact on the insulation treatment. In order to improve the insulation treatment effect of the winding, the baking equipment can increase the rotation treatment function to ensure that the winding is in a rotating state during the drying process, thereby effectively improving the distribution uniformity of the insulating paint in the winding.

According to the winding processing technology, the filling amount of insulating paint should be increased as much as possible during the impregnation and drying process to ensure the impregnation and drying effect. The selection of equipment, the performance parameters of insulating paint, process parameters and the degree of process execution are the focus of production and processing process control.

4. Conclusion.

During the motor manufacturing process, windings, burial, wiring, transportation and assembly may cause damage to the electromagnetic wire insulating layer, resulting in inter-turn failures in motor testing or actual operation. In addition to the damage factors caused by production and processing to the electromagnetic wire, the quality performance of the electromagnetic wire itself is also very critical. The adhesion of the electromagnetic wire insulating layer, the uniformity of the insulating layer and the smoothness of the conductor directly affect the reliability of the motor winding. For the formed winding, the mechanical strength of the electromagnetic wire insulating layer is required to be high during the coil drawing process. Poor quality electromagnetic wires often have damage to the insulating layer, resulting in serious failure of the inter-turn insulation. The selection of the electromagnetic wire during the design process, the satisfaction of the winding process, and the protection of the insulating layer of the electromagnetic wire during the processing are the key elements of the fault control of the electromagnetic wire drop. The part of the electromagnetic wire damaged during the manufacturing process is a quality hazard during the operation of the motor. During the motor start-up process, the first coil in the winding directly connected to the power supply suffers the greatest impact. During the embedding process of the motor winding, the coil is often deformed the most seriously. Through the inspection of the actual fault cases, it is found that the inter-turn failure of the motor winding without strengthening measures mostly occurs in the first turn. Special insulation measures can effectively reduce the inter-turn failure of the winding.