How to make high-speed motor manufacturing solutions more efficient?

Generally, any motor with a speed exceeding 10,000 rpm can be called a high-speed motor. It is also defined by the linear speed of the rotor. The linear speed of a high-speed motor is generally greater than 50 m/s. The centrifugal stress of the rotor is proportional to the square of the linear speed. Therefore, the classification by linear speed reflects the difficulty of the rotor structure design. At present, the speed of the mainstream high-speed motors in mass production on the market is 20,000 to 22,000 rpm, and the speed of the next generation of motors will develop towards 25,000 to 30,000 rpm.

1. What are the benefits of high-speed motors?

The high-speed drive motors of new energy vehicles can increase power and thus improve vehicle power, or maintain the same power and power while reducing the size of the motor and reducing costs.

High-speed motors have higher output power and energy conversion efficiency, which can bring faster and more powerful acceleration and extreme speed performance to drivers at a limited power consumption level.

High-speed motors can carry higher currents and voltages, and the upper limit of motor power is also higher.

2. The overall idea of high-speed motor

First, it needs the support of high-current power devices such as SiC, because the main strategy to achieve high-power and high-torque output is to increase the maximum current.

Secondly, ultra-thin silicon steel sheets are used. The thinner the silicon steel sheet, the lower the eddy current loss, which is conducive to improving efficiency. In order to reduce eddy current loss, ultra-thin silicon steel sheets of 0.10 mm and 0.08 mm are generally used.

Ultra-thin sheets can reduce eddy current loss but cannot improve hysteresis loss. Improving hysteresis loss can start from the following three ways:

① Optimize the magnetic circuit design to improve the sinusoidality of the magnetic field and reduce harmonic iron loss;

② Reduce the magnetic load, increase the heat load, and reduce the fundamental iron loss;

③ From the perspective of material selection, choose silicon steel sheets with smaller hysteresis loss.

Third, using a 6-pole design to reduce iron loss is also the key to improving the efficiency of the high-speed range.

Fourth, eddy current loss can also be reduced by segmenting the permanent magnet. Taking the reduction of AC loss as an example, the common method is to divide the magnetic steel into multiple segments, which can be segmented radially or axially. Segmentation can reduce the eddy current circulation area and reduce AC loss.

Fifth, in terms of technology, the burrs of the punching sheets can be minimized through the improvement of equipment and punching and shearing dies; the consistency of the punching sheets can be ensured by adjusting the punching method, ensuring that the iron core after lamination is smooth enough, reducing and eliminating unnecessary setbacks; and so on.

3. High-speed motor solution: Replace the magnetic isolation bridge with a rotor sleeve

The rotor sleeve of a high-speed motor increases the air gap between the rotor magnet and the stator. The larger the air gap, the lower the electromagnetic power and efficiency of the motor. In other words, the ideal sleeve should be as thin as possible while increasing strength to improve motor efficiency.

Compared with steel, carbon fiber reinforced composites have much lower conductivity, and low conductivity can reduce interference with the magnetic field. The combination of thin wall thickness and low conductivity makes it possible for carbon fiber sleeves to maximize the power density of permanent magnet motors. And the lower thermal expansion coefficient of carbon fiber can also achieve a strong mechanical bond between the sleeve and the rotor.

4. Motor high-speed solution: low-noise gear technology

Vibration is the source of noise. If the vibration is large, the structural noise will inevitably be large. When the motor runs at high speed, it will cause rotor vibration problems, rotor critical speed problems, and shaft yaw vibration problems.

New energy vehicles have lost the "masking effect" of traditional fuel vehicle engine and intake and exhaust noise. The meshing noise of the electric drive gear transmission system has become one of the important factors affecting the comfort of the car. Therefore, the NVH of the gear transmission system of new energy vehicles has become particularly important.

Compared with the reduction gear set of traditional spoke technology, the high-strength special-shaped spoke technology can reduce noise for the gear set when rotating at high speed, improve lubrication efficiency, withstand greater impact force, and improve reliability.

5. Motor high-speed solution: suppress torque pulsation

The torque pulsation of the motor refers to the fluctuation of torque generated by the motor during operation due to problems such as the motor rotor structure, electromagnetic excitation method, and casing stiffness. Generally speaking, the size of the torque pulsation is related to the rotor crosstalk vibration and the unevenness of the magnetic field distribution. The size of the torque pulsation of the motor directly affects the performance and service life of the motor, and will also cause problems such as noise and vibration during operation.

The torque pulsation of the motor will show periodic fluctuations, and its frequency is related to factors such as the power and pole pair number of the motor. In actual operation, the torque pulsation generated by the motor will cause vibration and noise of the whole machine, and will also affect the operating accuracy and life of the equipment.

The electromagnetic force of high-speed motors has a high frequency and a wide distribution range. High-frequency electromagnetic force will cause howling problems and easily arouse the resonance of the stator system.

In order to reduce the torque pulsation of the motor, it is necessary to improve and optimize the design, manufacturing and use of the motor. Specifically, the following measures can be adopted:

a. Optimize the rotor structure and magnetic field design to reduce rotor crosstalk vibration and magnetic field inhomogeneity;

b. Optimize the circuit structure and control method of the motor to reduce the fluctuation of current and magnetic field;

c. Improve the rigidity of the casing and reduce coupled vibration;

d. Select suitable bearings and vibration reduction devices to reduce the mechanical vibration of the motor;

e. Perform dynamic balancing and vibration testing during the operation of the motor to promptly discover and eliminate torque pulsation problems.

6. Motor high-speed solution: bearing selection

For bearings, there are generally four categories: magnetic bearings, air bearings, sliding mechanical bearings, and rolling mechanical bearings.

Magnetic bearings do not have mechanical contact, the rotor can reach a very high speed, with low mechanical wear, low energy consumption, low noise, long life, and no lubrication required. They are used in high-power applications.

Air bearings form an air film by injecting high-pressure gas between the bearing and the shaft, which plays a role in isolation and lubrication, reduces friction and resistance, and is used in applications with small power and size.

Mechanical bearings often require oil lubrication and are limited in many oil-free applications.

7. Heat dissipation and lubrication problems of high-speed motors

When the motor works under extreme conditions such as high temperature, high speed, and high power density, its heat and temperature rise are serious. Excessive temperature rise of the motor causes irreversible demagnetization of the permanent magnet, damage to the insulation layer of the enameled wire, and even winding burnout accidents.

Accurate calculation of loss and temperature rise is one of the key technologies for the design and analysis of high-speed permanent magnet motors, and the heat and temperature rise of the motor is also the most important factor affecting the reliability and life of the motor.

At present, the most effective cooling method for automotive high-speed motors is internal oil cooling, which is a direct cooling technology. It can be divided into two categories according to stator cooling and rotor cooling, which generally adopt a combination of winding spray cooling + stator oil cooling + rotor oil cooling and other methods.