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The comprehensive upgrade of electrification, high-speed and high-voltage of new energy vehicles has pushed the automotive drive motors from the traditional industrial motor system to form an independent automotive motor technology system. Enameled wire, as the core material of motor winding, is the key carrier for carrying current, realizing electromechanical energy conversion, and guaranteeing the safety of electrical insulation, and its material performance, insulation structure, and process suitability directly determine the power density, operating efficiency, thermal stability, and full life cycle reliability of automotive motors.
Compared with industrial general-purpose enameled wire, automotive motor enameled wire needs to be adapted to the vehicle high-voltage platform, high-frequency inverter drive, ultra-high-speed rotation, wide temperature fluctuations, continuous vibration and shock, and complex humidity and heat corrosion, and other extreme composite conditions, which puts forward strict automotive requirements for heat resistance, corona resistance, mechanical vibration resistance, aging resistance, and voltage and insulation resistance.
The working environment of automotive drive motors is characterized by multi-load coupling, random working conditions and extreme environment, with electrical stress, thermal stress, mechanical stress and chemical corrosion stress superimposed over a long period of time, which is the core difference between automotive enameled wires and industrial enameled wires. Clearly define the requirements of automotive extreme working conditions, is the premise of enameled wire selection, structural design and process optimization.
New energy vehicles all use inverter controllers to drive motors, IGBT, SiC power devices, high-speed switching, will output high-frequency PWM pulse voltage. Pulse voltage has the characteristics of steep edge, instantaneous overshoot and high frequency repetition, which will generate instantaneous overvoltage in the winding turn-to-turn, layer-to-layer, and stator-rotor gap, with the peak value far exceeding the rated motor operating voltage. Especially in the 800V high-voltage platform models, instantaneous pulse over-voltage up to 1200V or more, it is very easy to trigger air ionization, resulting in local corona discharge phenomenon.
Corona effect will continue to bombard the insulating layer of enameled wire, resulting in molecular chain breakage of insulating resin, varnish film pulverization, microporous expansion, long-term operation will occur after the insulation voltage drop, local leakage, turn-to-turn short-circuit and other irreversible failure. At the same time, the ozone and nitrogen oxides generated by corona will corrode the insulation layer and winding structure, accelerating the aging of the whole machine. Therefore, high-frequency pulse resistance, local corona resistance, and high PDIV (Partial Discharge Initiation Voltage) have become the core electrical indexes of automotive enameled wires, which are also the key criteria distinguishing them from ordinary industrial enameled wires.
The operating temperature range of automotive motors is extremely wide, and the low-temperature starting conditions of the whole vehicle can reach -40℃, and under high load conditions such as high-speed climbing and full-load acceleration, the temperature of local hot spots in the windings can break through 180℃-220℃, and there are frequent alternating hot and cold and sudden changes in temperature conditions. Poor heat dissipation in the confined space of the rotor, long-term high-speed operation of the heat accumulation is serious, the winding insulation layer continues to be in a high-temperature thermal aging state.
Ordinary 130 ℃, 155 ℃ grade insulated enameled wire in this condition will quickly appear paint film embrittlement, cracking, peeling, insulation performance greatly attenuated. Vehicle-grade enameled wire is generally required to achieve H-class (180 ℃) and above heat-resistant grade, the high-end models use 200 ℃, 220 ℃ ultra-high heat-resistant system, with excellent resistance to thermal aging, thermal shock, temperature change performance, can be a long time in high temperature working conditions and stable work.
The maximum speed of automotive motors generally exceeds 16,000rpm, and some high-performance models can reach over 20,000rpm. The rotor winding rotates with the rotor at high speed, and is continuously subjected to huge centrifugal force; at the same time, the bumps, shocks and road vibration during vehicle driving, superimposed on the electromagnetic vibration brought by the motor’s alternating torque, puts the winding in a multi-dimensional state of alternating mechanical stress in the long term.
This working condition puts forward very high mechanical requirements for enameled wire: strong adhesion of paint film, friction resistance, tensile resistance, bending resistance, and it is not easy for the paint film to fall off, crack, and wear after winding. Once the enameled wire is damaged, it will directly cause turn-to-turn short circuit and short circuit to ground, resulting in motor scrap. Therefore, automotive enameled wires must have high strength enamel film, excellent flexibility and structural stability, and be suitable for fully automated precision winding, embedding, and shaping mass production processes.
Vehicle motors are mostly oil-cooled, water-cooled cooling structure, winding long-term contact with cooling oil, antifreeze vapor, while the vehicle operating environment is humid, salt spray, dust and other complex media. Ordinary enameled wire insulation layer oil resistance, hydrolysis resistance, corrosion resistance is poor, long-term contact with the cooling medium will appear paint film swelling, softening, peeling, resulting in insulation failure. Automotive-grade enameled wire needs to have excellent resistance to refrigerant, oil, heat and moisture, hydrolysis resistance, suitable for the long-term operation of the motor closed cooling system needs.
Based on the different power levels, voltage platforms, speed ranges, stator and rotor working conditions of automotive motors, the industry has formed a complete enameled wire product matrix, mainly including polyurethane enameled wire, polyesterimide enameled wire, polyamideimide composite enameled wire, polyimide special enameled wire, corona-modified enameled wire and flat enameled wire, with a clear gradient of performance for each type of wire and clear adaptability scenarios.
Polyurethane enameled wire is heat-resistant grade B (130℃)/F (155℃), the core advantage is the excellent direct weldability, welding without scraping the paint film, can be directly high temperature dip welding, convenient processing, high mass production efficiency, low cost. However, its heat resistance, voltage resistance, mechanical abrasion resistance and aging resistance are weak, and it can’t withstand high-frequency, high-pressure and high-temperature continuous working conditions.
In the field of new energy vehicles, polyurethane enameled wire is not applicable to the main drive motor, and is only used for low-power auxiliary motors, window motors, wiper motors, air-conditioning blower motors and other low-voltage, low-temperature, low-load auxiliary parts, which is a low-end automotive supporting wire.
Polyesterimide enameled wire has a heat-resistant grade of 180°C (Class H) and is the basic main wire for automotive motors. It has stable insulation performance, good mechanical strength, up to standard temperature resistance grade, outstanding cost performance, good hydrolysis resistance and aging resistance, and can meet the needs of conventional 400V platform, low to medium speed, and conventional working conditions of the main drive motor.
Polyesterimide enameled wire is the standard wire for mass-produced economical new energy motors, and is widely used in the stator windings of household passenger cars and entry-level hybrid models. However, its shortcomings lie in the corona resistance performance is general, in the 800V high pressure, ultra-high speed and high frequency conditions, partial discharge aging faster, can not be adapted to high-end high-voltage platform models, generally not used in rotor windings and high-pressure high-frequency working conditions motor.
Polyamideimide (PAI) composite enameled wire is a high-end main wire for automotive use, using a double-layer composite insulation structure: the bottom layer of polyesterimide to ensure adhesion and processing performance, the surface layer of polyamideimide to provide ultra-high heat resistance, abrasion resistance, resistance to refrigerant corrosion, the overall heat-resistant grade exceeded 200 ℃ -220 ℃.
The overall heat-resistant grade exceeds 200℃-220℃. The comprehensive performance of the wire is extremely balanced, with five core advantages: ultra-high temperature resistance, strong mechanical wear and vibration resistance, excellent refrigerant oil resistance, high dielectric strength, and long thermal life. It can be perfectly adapted to the rotor confined high temperature accumulation temperature, high-speed centrifugal vibration, oil-cooled immersion, high-frequency pulse impact and other harsh conditions, is currently the first choice for automotive rotor windings, 800V high-voltage platform motors, high-speed and high-performance motors of the conventional high-end wire. The majority of mainstream new energy passenger car main drive motor rotor winding, are used PEI + PAI double-layer composite enameled wire.
Polyimide enameled wire is currently the highest level of heat-resistant special insulation wire, long-term temperature resistance of up to 220 ℃ -260 ℃, at the same time has the ultimate radiation resistance, high and low temperature impact resistance, chemical corrosion resistance, ultra-long thermal life performance, dielectric strength and insulation stability of the industry’s top.
Due to the extremely high cost and processing difficulty, the wire is not used in ordinary mass-production passenger cars, and is only suitable for aerospace-grade automotive equipment, special high-temperature motors, extreme working conditions test prototypes, military vehicle motors and other high-end special scenarios, belonging to the top special solutions in the automotive enameled wire system.
Corona-resistant enameled wire is a functional wire specially developed for high-frequency and high-voltage PWM working conditions. It is doped with nano-alumina, silicon dioxide and other inorganic modified fillers in the polyesterimide and polyamideimide resin system, which reconfigures the microstructure of the insulating layer to effectively inhibit partial discharge, block corona particles from bombarding, and slow down the aging of the insulation.
Ordinary enameled wire in high-frequency pulse conditions corona-resistant life of only a few hundred hours, while the modified corona-resistant enameled wire can reach more than 10,000 hours, the PDIV value is greatly improved, can effectively solve the 800V high-voltage platform, SiC high-frequency frequency converter motors insulation aging problems, is the current high-end high-voltage models of the immediate needs of the configuration, mainly used in high-end passenger car stator, rotor drive motor stator, rotor windings throughout the domain.
Flat enameled copper wire is the core upgrade program for high power density motors. Compared with the traditional round wire, flat wire has regular cross-section and close arrangement, and the slot fullness rate can be increased from 40%-45% to 65%-70% for round wire, which can significantly increase the conductor cross-sectional area in the same slot space, reduce the DC resistance of the windings, decrease the copper loss, and increase the current-carrying capacity and power density. Power density.
Automotive flat wire commonly used PEI + PAI composite temperature insulation, corona resistant modified system, suitable for high-speed, high-pressure, high-efficiency working conditions, is currently more than 200kW high-power, high speed, high-pressure platform main drive motor mainstream standard. Its shortcomings lie in the complexity of the molding process, high investment in equipment, shaping difficulty, the uniformity of the paint film on the wire, bending resistance to cracking, dimensional accuracy requirements are extremely high.
Automotive motor stator and rotor working conditions are very different, the stator is a static part, mainly subject to electrical stress and conventional temperature rise; rotor for high-speed rotating parts, superimposed on the high-temperature confinement temperature, centrifugal vibration, alternating mechanical stress, the degree of severity of the working conditions is much higher than the stator. Therefore, the stator and rotor winding must be differentiated selection strategy, can not be unified wire type.
Characteristics of stator winding: good heat dissipation, no high-speed centrifugal load, low vibration, mainly subject to high-frequency impulse voltage, industrial frequency temperature rise, conventional hot and humid environment. The core of selection is based on “high voltage resistance, corona resistance, low loss, cost-effective and balanced”.
400V conventional platform, low-power household models: stator preferred 180 ℃ polyester imide enameled wire or ordinary composite enameled wire, to meet the durability requirements of conventional working conditions, cost-controllable, mass production and mature.
800V high-voltage platform, medium and high power models: the stator must adopt corona-resistant modified PEI+PAI composite enameled wire to eliminate insulation aging caused by high-frequency impulse partial discharge, and enhance the reliability of the motor’s full range of operation.
High power density, ultra-high-speed mass production motors: the stator fully adopts corona-resistant flat wire program, which improves slot fullness, reduces loss, strengthens heat dissipation, and supports the expansion of the motor’s high-power output and high-efficiency range.
Characteristics of rotor winding: poor heat dissipation, high temperature accumulation, long-term high-speed centrifugal, continuous alternating vibration, complex stress, which is the core part of the motor that is most likely to fail. Selection of the core of the “ultra-high heat resistance, strong mechanical vibration resistance, aging resistance, media corrosion resistance” as the first priority.
Regardless of high-voltage and low-voltage platforms, the use of ordinary polyesterimide single wire is prohibited for automotive rotor windings, and PEI+PAI double-layer composite high-temperature enameled wires are uniformly fitted as standard to ensure long-term heat resistance of over 200℃, strong paint adhesion, wear and vibration resistance, and to avoid cracking of the paint film, shedding, and short-circuiting in high-speed operation.
The rotor winding of high-end high-voltage and long-lasting models is further upgraded to corona-modified composite enameled wire, which takes into account the multiple reliability of electrical, thermal and mechanical properties, and meets the requirement of 8-10 years’ long service life of the whole vehicle.
High-quality wire performance needs to be matched with a mature mass production process, otherwise processing damage, performance degradation and consistency deviation will easily occur. Automotive enameled wire on the winding, embedded wire, shaping, curing, termination of the entire process process requirements are much higher than industrial wire.
Automotive windings require neat rows of wires, even turn spacing, no cross-pressure, no hard bending. Disordered wiring can lead to localized electric field concentration, uneven current distribution, localized overheating, as well as paint film extrusion and abrasion. Mass production requires the use of fully automatic precision winding equipment, matched with uniform tension control, to ensure uniform force on the wire to eliminate micro-damage. Flat wire windings need to be accurately molded and bent to control the bending rate and prevent the film from stretching and cracking.
The embedded wire process is a high incidence of mechanical damage to the paint film. Special embedded wire tooling and flexible protection structure should be adopted to avoid hard scraping and rubbing; high-quality slot insulating paper, slot wedge, and phase-to-phase insulating structure are provided in the groove to realize multi-layer insulation isolation, which can avoid direct contact between the windings and the iron core and cause a short-circuit to the ground, and at the same time, equalize the distribution of electric field and reduce the probability of partial discharge.
VPI vacuum pressure immersion paint curing process must be adopted for automotive grade motors, replacing ordinary atmospheric pressure immersion paint. The air inside the winding is extracted by vacuum negative pressure, and then the insulating varnish is injected at high pressure to make the insulating varnish completely penetrate the wire gaps, grooves, and end gaps. After curing at high temperature, the winding, core and insulating varnish form an integrated and rigid whole, which greatly improves the mechanical vibration resistance, thermal conductivity and insulation integrity of the winding, and eliminates the loosening of the rotor by high-speed vibration and wear and tear inefficiency.
The winding end lead and neutral welding position is the weak point where high temperature, stress and electric field are concentrated. Vehicle production needs to use precise paint stripping process to prevent excessive paint stripping damage to the conductor, paint stripping residues affect welding; welding using laser welding, resistance welding high-precision technology to ensure that the weld joints are full, no welding, no overheating burns, after welding to do the insulation reinforcement encapsulation, to eliminate the end of the insulation short board.
Automotive motor enameled wire is the core material of the new energy power system, and its performance and adaptability directly determine the reliability of the motor and the vehicle range, power and durability performance. Unlike traditional industrial wires, automotive enameled wires need to cope with multiple extreme loads of electrical, thermal, mechanical and chemical loads at the same time, and must be accurately selected, structurally matched and technically adapted according to the differences in voltage platforms, rotational speeds, and stator and rotor working conditions.
By popularizing composite heat-resistant insulation, corona-resistant modification, high slot fullness flat wire system, with standardized VPI curing, precision molding, vibration-resistant reinforcement process, it can comprehensively solve the core pain points of automotive winding such as high temperature aging, corona breakdown, vibration abrasion and dielectric corrosion.
