The two ends of the internal lead wires of the motor are connected to the motor winding main wire and the terminals in the motor junction box, respectively. Although they account for a small proportion of the total material of the motor, their electrical performance reliability is crucial. Once a problem occurs, even if it is caused by the use of a frequency converter, resulting in a fault condition that never occurred under the original power frequency conditions, the suspicion of manufacturing quality cannot be shaken off. For motor manufacturers, it is essential to pay close attention to the quality requirements of motor lead wires. On the one hand, the safety factor should be selected to be higher; on the other hand, necessary quality control should be implemented, and products suitable for the motor's operating conditions should be selected to prevent unnecessary problems from occurring.
The quality and technical requirements that the lead wires should meet
● The lead conductor should be annealed copper wire. The conductor surface should be smooth, free of oil and contaminants, and free of burrs, sharp edges, and protrusions that could damage the insulation layer. Each individual wire may be tin-plated or unplated. Tin plating is intended to prevent oxidation of the lead wire due to temperature and humidity factors during energized operation, which would affect the conductivity of the lead wire.
● Requirements for the diameter of single filaments in conductors. The standard specifies the maximum diameter in order to ensure the shaping and flexibility of the lead wire through diameter control.
● Resistance control. The standard specifies the maximum resistance value of the lead wire at 20℃. The purpose is to control the purity of the lead wire conductor material to ensure the degree of heat generation during operation and avoid burnout of the lead wire or damage to the motor performance.
● Shielding requirements. For cables with voltage ratings of 6kV and above, the conductors should have a non-metallic shielding layer. The shielding layer should be made of semiconductor material, and its surface should be smooth, without obvious protrusions, sharp corners, particles, or damage marks. The shielding layer should be tightly bonded to the lead insulation layer.
● Insulation layer thickness and covering requirements. The insulation should tightly cover the conductor isolation layer and be easily peelable without damaging the conductor. The insulation layer should be smooth and uniform, with the thinnest point thickness not less than 90% of the nominal value, with a lower tolerance of 0.1 mm. For cables with a semiconductor shielding layer, the shielding layer should not be included in the thickness measurement.
● Insulating sheath performance requirements. The sheath thickness should be uniform, the surface smooth, and the minimum thickness should not be less than 85% of the nominal thickness, with a lower deviation of 0.1mm.
Cable testing methods and techniques
● Resistance testing: Using a resistance tester or a double-arm bridge, a 1m sample is taken to test and calculate the resistance at 20℃. See formula (1) for details:
In the formula:
R20—Resistance per kilometer at 20℃, in Ω/km;
The measured resistance value of a cable of length L at Rx—t℃, in Ω:
L—Measurement length of the sample, in meters.
t—Conductor temperature at the time of measurement (based on ambient temperature), in °C
● Power Frequency Voltage Test: The cable shall withstand a power frequency withstand voltage test at a specified voltage (as specified in the standard). Take a sample of at least 5m from the test cable, remove the insulation from one end of the sample (approximately 10mm), being careful not to damage other insulation during removal. Immerse the sample in a room temperature water bath, ensuring that both ends of the sample extend at least 200mm above the water surface, and guarantee that no surface flashover discharge occurs under the specified test voltage. After immersing the sample in water for 1 hour, apply the specified voltage using a power frequency withstand voltage tester and maintain it for 5 minutes without breakdown.
To meet this requirement, better lead wire manufacturers will add necessary monitoring instruments during the production process, and monitor the process through spark tests and insulation thickness tests.
●Structural inspection. Inspection shall be conducted according to standard specifications, involving wire type and diameter. For round sheathed cables, the difference in outer diameter measured at any two points on the same cross-section shall not exceed 15% of the upper limit of the average outer diameter.
● Bending test. The mandrel should not show any visible cracks or other damage after being bent according to the specifications, and should withstand the immersion voltage test specified in the standard.
● Thermal effect and solvent resistance tests. These two tests are for cable manufacturers; motor manufacturers generally do not conduct these tests.
● Impregnation resistance test. Since different motor manufacturers use different impregnation varnishes and have varying drying process parameters, it is recommended that motor manufacturers conduct this test. Once the test is passed, cable suppliers should not be frequently changed. Cable manufacturers should promptly communicate with motor manufacturers after changes in materials or processing techniques to prevent non-compliance.
● Length, appearance, and markings. These requirements are specified in the standards. Cables with a nominal cross-section not exceeding 35mm² should be delivered in lengths of no less than 50m, and in principle, no breaks should occur. Most manufacturers adhere to 100m per package. The standard stipulates that the cable should bear continuous markings including the manufacturer's name, cable model, and voltage rating. The markings should be clear and able to withstand at least 10 light rubs with water-soaked degreased cotton.
As a motor manufacturer, the selection of leads should be controlled through the procurement channels, rather than through complex incoming inspections. However, as a crucial component of motor products, we cannot ignore the quality control of leads.
What types of cables require irradiation, and what are the advantages of irradiated cables?
Irradiated cross-linked wires and cables have a wide range of applications, currently mainly used in power, communications, electronics, chemical industry, vehicles and ships, aerospace, military industry, oil extraction, subways and household appliances.
Irradiating wires and cables with electron accelerators is a radiation processing technique. This technology integrates electronic technology, high-energy nuclear physics, vacuum technology, computer technology, radiation chemistry technology, and wire and cable manufacturing technology, making it a model of advanced technology. The high-energy electron beam produced by the electron accelerator acts on the interior of the polymer, causing changes in its molecular structure. The original linear macromolecules are transformed into an insoluble and infusible three-dimensional network structure, thus giving the material special heat resistance, chemical resistance, radiation resistance, high flame retardancy, and high strength.
The main features of its products are:
1. Excellent heat resistance: Irradiation cross-linking can significantly improve the heat resistance of cables. For example, after irradiation cross-linking, the long-term allowable operating temperature of polyethylene material can be increased from 60~70℃ to 90~150℃, and the short-circuit temperature can be increased from 160℃ to 250℃.
2. Increased current carrying capacity of cables: Irradiated cross-linked wires and cables have a current carrying capacity per unit conductor cross-section that is about 20% higher than that of ordinary wires and cables.
3. It has excellent insulation and electrical properties.
4. It has high mechanical strength, good aging resistance, chemical stability, and resistance to environmental stress cracking.
5. Improve flame retardant properties.
6. High safety: PVC cables produce toxic gases that are harmful to the human body when burned, and their service life is extended to 40 years.
