What changes will the third-generation flat wire motor bring to new energy vehicles on the eve of its outbreak?

Flat wire motor is actually a kind of square wire motor, but the shape of the wire is changed from round to square, the circles are in line contact, and the squares are in surface contact, which effectively improves the space utilization and power density, improves the power density, and does not need to use notches for wire laying, which improves NVH from axial wire insertion. At the same time, it also brings a disadvantage, that is, the flat wire motor changes from multi-strand wire to single-strand thick wire, which cannot be made by hand, so it leads to a greater investment risk. The high investment risk is reflected in two aspects. The first is the high price of the equipment, and the second is the difficulty of serialization. Design changes often require an increase of 1/3 of the total equipment cost.

Cao Hongfei, Chief Engineer, Shanghai E-Weike Motor Technology Co., Ltd.

In fact, flat wire motors were not originally used in the automotive industry. High-power industrial motors all use flat wires. The difference between flat wire motors and automotive flat wire motors is that conventional industrial flat wire motors do not use enameled wires, but silk covered wires, and they originally entered through the slots. For products that do not have a very large demand, the cost-effectiveness is not high.

Overview of Flat Wire Motor Development

The advancement of motor technology is driven by three factors: demand, materials, and processes.

The first driving force is new demands. Only with new demands can there be motivation for research and development, including changes in controller levels, changes in peripheral power requirements, etc. This is the source of innovation and change in motors.

The second driving force is new materials. The materials of motors are mainly silicon steel sheets, copper wires, magnetic steels, etc., as well as insulating materials. In the past 10 to 20 years, the performance of basic motor materials has not changed much, except that the silicon steel sheets have become thinner and the wires have changed from round to square.

Since the first two methods are more difficult, we can only rely on the third method - process breakthrough. The most important thing to change from round wire to square wire is the process change. The process change brings about an increase in the power density of high power.

The second generation of flat wire motors has the following characteristics: 1. Number of layers: more than 6 layers, effectively reducing AC losses; 2. Multi-branch + busbar-commutation to ensure balanced motor branches; 3. Winding form: wave winding or stacked winding; 4. Coil element shape: I PIN or U-PIN, continuous UPIN; 5. Wire direction: mostly twisted end (production process can be improved). But even so, there are still many problems. The new generation of flat wire motors must meet the requirements of higher power density, higher low voltage, etc. At present, flat wire motors are still evolving, but Cao Hongfei believes that the third generation of flat wire motors will soon appear, which will be a revolutionary progress.

At the same time, whether the asynchronous motor is suitable for flat wire: the asynchronous motor is very sensitive to harmonics, and the flat wire has a smaller inductance than the round wire, so the flat wire is slightly worse. But because of its small inductance, its characteristics are hard, and the asynchronous motor has both electronic and rotor losses. With the flat wire motor, you can create more space for the rotor, which is good for solving the heating of the asynchronous motor. In general, as long as the PWM is well controlled, whether from the motor level or the control level, I believe that the asynchronous motor is also suitable, or more suitable for the flat wire technology, but no one is doing it at present, which is a pity.

In fact, many electric vehicles at low speeds use asynchronous motors, including those of Lao Toule, many of which are asynchronous motors with flat wires, but they are rarely used in regular car manufacturers.

The latest powertrain demand for flat wire motors

The powertrain needs to be lower cost, higher power density, and obviously higher speed. In this case, the flat wire motor needs to further reduce the AC loss, which is a challenge.

To achieve higher efficiency, should we increase the number of layers to solve the problem of loss? Tesla has now made 10 layers, but it is said that it is very difficult and may have to change to 8 layers. From the perspective of technology and slot fill rate, it does not mean that the number of layers can be increased indefinitely. It is not the case.

The slot voltage of flat wire motors is a big problem. Unlike round wire, which can be filled with insulating paper, flat wire motors have a high power density, and the insulating paper pad has a great impact on slot utilization. Referring to the thermal management of the whole vehicle, this is also a demand for oil-cooled motors. Because Tesla was the first to recover the heat of the motor to heat the battery, other domestic car manufacturers making electric vehicles will not want to use such a function, which is actually challenging for the motor.

The controller adds a positive harmonic current to the motor, but the voltage is not electrical harmonics, but a lot of chopped waves. After these chopped waves are added to the motor, due to the electrical modification of the motor, it will cause obvious pressure between the input wires, which can easily burn the motor. Increasing the working voltage is more special for flat wires. The high working voltage may cause a relatively high voltage between the first wire and the second wire in the slot for a long time. It may be 400 volts and 200 volts between layers before, but now it is 800 volts and 400 volts between layers. After actual measurement, if the paint film of the enameled wire is damaged, it cannot withstand 800 volts. But it is very difficult to make the enameled wire without any holes, which is a challenge to the technology of flat wire motors.

In view of high working voltage, Yiweike must ensure that the paint film is not damaged during the coil forming manufacturing and welding process; especially during welding, after welding, the burned paint film is no longer an insulator but a semiconductor, which is not acceptable for high voltage.

Oil cooling is a requirement for flat wire motors, because there is an oil pump, which is very sensitive to the impurities inside. But why do flat wire motors produce excess impurities? It is because of welding. If welding cannot be solved, where should the slag of copper splashing be collected? It is impossible to control when it falls off. In this case, how to do oil cooling?

The oil cooling technology is based on energy recovery, especially in winter, to collect heat as quickly as possible. This does not require the motor to be the most efficient, but to generate the most heat. How to coordinate the thermal design of the vehicle, how to arrange thermistors, and how to implement thermal management strategies require motor companies, powertrain companies, and even thermal management companies to work together to do these things. I am sorry that I have not seen any good cases in China so far.

Finally, we need higher efficiency. What will the next generation of flat wire motors look like? We need higher speeds, of course. How can we reduce AC losses? To save energy at high speeds, we just need to make the wires in the slots thinner. I just said that we can’t make all the wires thinner, but in theory, we can make some parts of the wires thinner, which is also effective. However, there are actually many patents for this, and the armature manufacturing process is very complicated.

Possible approaches to third generation flat wire motor technology

First of all, the raw materials. Are there better materials to help the third generation? The punching sheets can only become thinner and thinner. Some people in the industry use 0.2, which is expensive, but it seems that the supply and demand density has not increased much; the second is the enameled wire. Some people wanted to use Litz wire at the time, but they couldn’t solve the process problem after a long time, and welding was really impossible. What should I do? Is there a better qualified wire? In fact, some people recently proposed to use single crystal copper, higher purity copper, which increases the cost several times and improves the conductivity by only 10%. It is not suitable for the development of the automotive industry. Although there are many, the military industry can have it, but the automotive industry can’t afford it. Regarding whether there will be better insulation materials in the future, there are infinite possibilities, because insulation materials are not only expected to be insulating, but also to be thermally conductive and high-strength.

The third generation of flat wire motors must have a higher power density, be suitable for higher speeds, and also meet higher voltages. Cao Hongfei, the founder of Shanghai Eweike Motor, is also the earliest pioneer in the field of flat wire motors. He believes that my country's flat wire motor technology still needs to be improved, and there is still a certain gap from the real flat wire, and there is a lot of room for improvement. At present, the power density of each generation of flat wire motors increases by about 20%, so the goal of Eweike Motor is to produce third-generation products with a 20% increase in power density, and to solve the problems of high voltage, better flexibility and higher speed.