Five things to note when driving motors at high speeds for new energy vehicles

Analysis on the trend of high-speed drive motors for new energy vehicles

High-speed motors in the industry generally refer to motors with speeds exceeding 10,000 r/min. The high-speed drive motors of new energy vehicles can increase power, thereby improving vehicle dynamics, or maintain the same power and dynamics while reducing the size of the motor and reducing costs. With the upgrading of new energy vehicles in recent years, it can be seen that the speed of drive motors at home and abroad has been developing towards high speed, as shown in the following table (my incomplete statistics): The speed of the three generations of Toyota Prius models has gradually increased, the maximum speed of Lucid Air motors abroad has reached 20,000 rpm, and the maximum speed of Changan 630/BAIC 647V (Huawei three-in-one) in China has reached 17,300 rpm.

However, the high speed of motors will also bring some problems. The following five points are the issues that I think need to be paid attention to:

01. High speed brings great centrifugal force to the rotor

When the rotor rotates at high speed, the punching sheet and the magnet will be affected by centrifugal force. The higher the speed, the greater the centrifugal force. When there are more and larger holes on the punching sheet, the gap between the holes will be smaller. At this time, under the action of the larger centrifugal force, the strength of the fragile part of the punching sheet may exceed its yield strength, and its deformation may also affect the reduction of the air gap, thereby causing performance changes or even sweeping. In addition, the tensile strength of NdFeB permanent magnet material is very small. Therefore, rotor strength analysis is one of the keys to high-speed motor design.

Total rotor deformation cloud map

Rotor total stress cloud diagram

In order to adapt to the high speed of 17900rpm, Tesla Plaid version of the motor directly eliminated the stress-fragile magnetic isolation bridge and adopted a rotor sleeve, which can also reduce magnetic leakage. A unique machine-wound carbon fiber sleeve process is adopted, because the commonly used energized coil heats the sleeve, and the higher temperature will demagnetize the permanent magnet. Although the carbon fiber rotor sleeve increases the upper limit of the speed, the cost of carbon fiber composite materials and the difficulty of machine winding are both challenges.

Carbon fiber protective cover

02. Selection of high-speed bearings

For high-speed motors, there is an important parameter of the bearing, the dn value, which needs special attention. The dn value is equal to the product of the diameter of the bearing gear and the speed. Usually, the dn value does not exceed 2*10^6. In addition, new technology bearings such as air bearings, magnetic bearings, ceramic bearings, etc. can be considered, but the load capacity, precision, reliability and cost need to be weighed. Bearing Rotor with bearing

03. Winding eddy current loss caused by high speed

Under high speed/high frequency conditions, the stator winding will produce obvious skin effect and proximity effect, collectively known as winding eddy current loss. Especially for the flat wire motors that are now very popular, the winding eddy current loss needs to be carefully analyzed and treated at high speeds.

4-layer winding loss cloud map 8-layer winding loss cloud map

04. NVH deterioration

NVH is an important technical indicator of new energy vehicle motors. Due to the high speed of high-speed motors, the excitation frequency is very high. In addition, the motors are slender, light, have low damping, and have rich vibration modes, which are prone to vibrations of various frequencies. In addition, high speeds are also prone to generate large air noise. Therefore, NVH is a problem that must be solved for high-speed motors, and it needs to be comprehensively considered in terms of electromagnetic design, structural design, motor control, etc.

05. Calibration test bench resources

The testing of high-speed motors requires a high-speed motor test bench. According to incomplete research, the current maximum speed of the test benches for drive motors is 25,000 rpm, and several are 23,000 rpm. The 23,000 rpm test bench of the National Innovation Center should be the first 23,000 rpm test bench in China, and has been running normally for external and internal tests for a year. If there is a motor with a higher speed, it may be a headache to find test bench resources to test it.