If you were asked what the key technologies for new energy vehicles in my country are, what would your answer be?
800V platform? SiC power devices? Or a hub motor?
As one of the hottest technology trends today, flat-wire motors are at the forefront. Since Tesla also adopted this technology, discussions surrounding flat-wire motors have intensified. We have previously published several technical articles about flat-wire motors:
To the general public, the difference between round wire motors and flat wire motors is merely a change in appearance and a few key parameters. However, for manufacturing companies, it is precisely this change that necessitates adjustments to the entire process and production equipment.
In actual production, flat wire motors require higher stability and yield rates than current round wire motor production. Today, let's take a look at the six core processes currently in place for flat wire motor stator production!
Let's first take a look at the overall process flow of the stator production line:
The basic production process is as follows: slot paper → hair clip manufacturing → hair clip insertion → end ring shaping → end ring welding → star point connection → insulation treatment at the welding point. Among them, the wire forming and paper forming and paper insertion are carried out simultaneously.
1. Paper insertion process
Insulating paper is installed between the stator slots and conductors of the flat wire motor to ensure insulation between dissimilar conductors and between the conductors and the stator core.
Generally speaking, the paper insertion process of flat wire motors mainly includes paper forming, paper cutting, and paper insertion. Common slotted paper forming processes include cold forming and hot forming.
The main shapes of paper are O-type, C-type, B-type, and S-type, with O-type being the most common. B-type and S-type can significantly improve insulation performance, but their manufacturing processes are more complex, and the pure copper slot fill rate is low, resulting in poor stability.
For some motors, due to manufacturing processes, two sheets of insulating paper must be used to separate two adjacent conductors of different phases in the stator slot. This results in the insulating paper occupying a large space and reducing the power density of the motor.
Furthermore, since the two insulating sheets are not connected to each other, it is currently inconvenient to use equipment to automatically insert them into the stator slots, which has become a major pain point in the automated manufacturing process of flat wire motors.
2. PIN coil forming
The manufacturing of flat wires for the stator core mainly includes several process categories such as I-PIN, Hairpin, and Wave Winding.
Pin coils require a series of processes including straightening, paint removal, cutting, and shaping. Laser paint removal and traditional methods are commonly used. While the traditional method is cheaper, it also has drawbacks such as incomplete paint removal and damage to the copper wire.
The main forming processes include stamping and spring forming. The latter is more expensive, but causes less damage to the copper wire.
3. Coil insertion
Insert the hairpin coils into the conforming fixture, then grip all the hairpin coils together and insert them into the iron core, pressing them into the corresponding design dimensions.
In this part of the process, the automatic insertion across layers has been upgraded from the previous 2-layer and 4-layer processes to the latest 6-layer and 8-layer processes. Some domestic companies are already able to manufacture this product.
4. Flaring, twisting, and welding
Flaring process
The fixture positioning mechanism with the stator is moved to the flaring station. The flaring mechanism covers the upper end of all the flat wires except the innermost two layers and pulls the flat wires outward to complete the flaring of all the flat wires in succession.
Twisting process
Move the twisting mechanism and stator to the working position. Extend the flaring mechanism to abut the ends of the two innermost layers of flat wire, aligning the ends of the two innermost layers of flat wire with the twisting mechanism. Then, move the flaring mechanism away from the top of the flat wire and retract it. Insert the ends of the two innermost layers of flat wire into the twisting mechanism. The inner and outer molds of the twisting mechanism rotate in opposite directions to complete the twisting process of the two innermost layers of flat wire. Repeat this process to complete the twisting of all flat wires.
Welding process
Existing welding methods mainly include laser welding and argon arc welding. Both methods melt copper at a high temperature to form weld points, thereby achieving electrical connection of the windings. Other companies also use CMT cold welding or other welding methods.
Existing laser welding or argon arc welding technologies have two main drawbacks:
Firstly, laser welding and argon arc welding require instantaneous high temperatures to melt the copper, which can easily damage the enamel coating around the weld point, reducing insulation reliability.
Secondly, the number of hairpin coils or single-sided coils that make up the stator windings of flat wire motors is relatively large, requiring a large number of solder joints. Laser welding or argon arc welding generally involves welding each solder joint individually, which seriously affects the production efficiency of flat wire stators.
5. Coating and impregnation
The detailed process for this step is as follows:
Coating materials are currently mainly available in two forms: powder and liquid. Impregnation processes mainly include traditional impregnation, vacuum impregnation, vacuum pressure impregnation, drip impregnation, and EUV impregnation.
6. Stator potting
For a long time, motor thermal management has been a major challenge for many car manufacturers. The increasing driving range and power density have placed higher demands on motor heat dissipation. Currently, motors have always relied on cooling systems to achieve thermal management.
Currently, motor manufacturing mainly uses the impregnation process, but this has problems such as poor heat dissipation, susceptibility to damage, and insensitivity to organic oils. In this context, the vacuum potting process has emerged.
For vacuum potting, the potting resin needs to have the following characteristics:
Before curing, it should have good fluidity. It can penetrate into the uneven gaps on the winding surface. After potting, the outer surface of the workpiece should be smooth and flat, so that the rotating parts of the motor have basically the same moment of inertia when rotating. This reduces the vibration caused by sudden changes in mechanical stress and heat when the motor speed and direction change suddenly, and reduces the resistance of the cooling medium to the rotating parts of the motor.
The motor windings should have strong adhesion, strong resistance to thermal shock, and sufficiently high mechanical strength.
It should have a high thermal conductivity to reduce the temperature difference between the inner and outer surfaces of the potting compound. On the one hand, this can quickly conduct the heat generated by the motor windings during operation to the outer surface of the workpiece. On the other hand, it can reduce the internal stress caused by the temperature difference.
It has good electrical insulation and oil resistance. After potting, the motor stator forms a whole, which improves heat dissipation, stator modal stiffness and damping performance, reduces temperature rise and vibration noise, and improves moisture resistance, shock resistance and corona resistance.
