Six Key Technologies For High-speed Motors

                                               Six key technologies for high-speed motors

"Driven by the pursuit of smaller size and higher power, the speed of the motor has climbed all the way, from two or three thousand revolutions in the early days, all the way to tens of thousands to hundreds of thousands of revolutions, and the higher speed has improved the power density and raw material utilization." Therefore, high speed is a strong trend, taking new energy drive as an example, the maximum speed of the first generation of Toyota Prius is only 6000 r/min, and the speed of the fourth generation product reaches 17000 r/min. In this issue, we take a higher perspective to look at the application of speed motors and the key technologies behind them. ”

High speed and ultra-high speed have broad application prospects but at the same time bring extremely high challenges to the motor, we combine these problems into the same category and find that there are six categories: heat dissipation, selection, rotor structure, vibration noise, efficient design, bearings.

 

01. The problem of heat dissipation

 

The motor loss increases with the geometric number of speeds, and the heat generated by high loss increases the temperature rise of the motor extremely rapidly, in order to maintain high-speed operation, it is necessary to design a cooling method with good heat dissipation. We can see that the common high-speed motor cooling methods are:

 

"Internal forced air cooling" as shown in the figure below, strong cold air can directly blow into the motor to take away the heat on the winding and core, this way generally appears in air compressors, blowers, aircraft motors and other occasions where strong wind can be used.

2 "internal oil cooling" In the application environment where the motor must be closed and protected, or the strong wind, the most used is the internal oil cooling method, such as the combination of oil cooling in the stator groove used in the high-speed motor designed by AVL. Some motors also adopt a combination of winding oil injection cooling + stator oil cooling + rotor oil cooling and so on.

In order to achieve high power density, heat generation and cooling are important issues that high-speed motors must face.

 

02. Motor selection problem

 

Permanent magnet motor or induction motor? Whether it is other types of motors such as switched reluctance, the choice of high-speed motor type has always been a question with no standard answer. Generally, from the perspective of power density and efficiency, it is advantageous to choose permanent magnet motors, while induction motors and switched reluctance motors are selected from reliability. However, because of the large vibration noise, the application of switched reluctance is less.

 

 

The figure below is the type distribution law of high-speed motors under different speeds and powers, and the "power * speed value" of the motor is drawn as a contour curve, and we can find some general context: "In ultra-high applications, induction motors are most, and induction motors and permanent magnet motors coexist in high-speed applications". As long as this principle is followed, we can choose the type of motor according to the needs within the range.

03. Problems with rotor structure

 

The centrifugal stress that the rotor structure of the high-speed motor must overcome is generally used in the "high-speed" range, the metal sheath, the rotor itself structure (such as the fish skeleton of lpm, the rotor structure of IM), etc., and the carbon fiber winding is used in the "ultra-high-speed" range, or simply make the rotor into a solid integrated structure, such as the motor of the energy storage flywheel.

Most permanent magnet high-speed motors use the structure of the rotor sheath, and this design is also very particular, that is, to protect the permanent magnet and prevent the sheath from failing. Therefore, try to avoid stress concentration, as shown in the figure below, if the magnet does not fill the entire circumference, there will be stress concentration on the sheath and magnet, which is why the high-speed permanent magnet motor uses a complete ring magnet, if not a complete ring is also used to fill the circumference.

04. The problem of vibration noise

 

The problem of vibration noise is a major obstacle for high-speed motors. Compared with ordinary motors, there are vibration problems caused by rotor dynamics, such as the critical speed problem of the rotor and the deflection vibration problem of the shaft. There is also the problem of howling caused by high-frequency electromagnetic force, and the electromagnetic force frequency of high-speed motor is higher, the distribution range is wider, and it is easy to stimulate the resonance of the stator system.

In order to avoid critical speed vibration, the rotor design of high-speed motors is very critical and requires rigorous modal analysis and testing. The length-to-diameter ratio needs to be used as an optimization variable in the design: the rotor design is too thick and short, which can increase the upper limit of the critical speed and is not prone to resonance, but the difficulty of the rotor to overcome centrifugal stress increases. In turn, the rotor design is slender, the centrifugal strength problem is improved, but the critical speed is shifted, the probability of resonance increases, and the electromagnetic power will also decrease. Therefore, the design of the rotor needs to be repeatedly balanced, which is the top priority of high-speed motor design.

 

 

05. Efficient questions

 

Motor loss increases with the geometric number of speeds, high losses make the motor efficiency decay rapidly, in order to achieve high efficiency, all kinds of losses must be managed. Taking iron consumption as an example, in order to reduce eddy current loss, ultra-thin silicon steel sheets of 0.10mm and 0.08mm are generally used. Ultra-thin wafers can reduce eddy current losses but cannot improve hysteresis losses, so the iron hysteresis loss of ultra-thin sheets accounts for the majority, while the eddy current loss in ordinary sheets accounts for the majority. To improve hysteresis loss, you can start from the following three ways:

 

1. Optimize the magnetic circuit design to improve the sinusoidity of the magnetic field and reduce the harmonic iron consumption;

2. Reduce magnetic load, increase heat load, and reduce fundamental iron consumption;

3. Starting from material selection, select silicon steel sheets with small hysteresis loss.

In addition to iron consumption, high-speed motors also pay extra attention to AC losses, which are caused by the penetration of dry high-frequency alternating magnetic fields, often appearing outside the magnet, metal sheath, and stator winding surfaces. Taking the AC loss of magnet as an example, the commonly used method is to divide the magnet into multiple segments, which can be radial segments or axial segments. Segmentation can reduce the eddy current circulation area and reduce AC loss, the figure below is the simulation of the eddy current field after segmentation, it can be seen that the more segmented particles, the smaller the AC loss. There are more solutions than segmentation, which are limited to space and do not expand.

The magnetic field component with the highest frequency in the high-speed motor is introduced by the PWM carrier of the inverter, because the working principle of pulse modulation inevitably produces high-frequency current harmonics, which in turn further produce a high-frequency magnetic field, and the high-frequency magnetic field penetrates into the surface of the magnet and the stator and rotor to produce high-frequency loss. Some high-speed motors use a multilevel drive structure to improve PWM side band harmonics.

06. Bearing problems

The bearing selection of high-speed motors is a key issue, and generally there are four categories of magnetic levitation, air bearings, sliding mechanical bearings, and ball mechanical bearings. Magnetic bearings are used in applications with higher power, and air bearings are used in applications with smaller power and size. Mechanical bearings often require oil lubrication and are limited in many oil-free applications.

There are still many key problems and technologies of high-speed motors, and these problems need to be managed at the same time, which is relatively high and difficult compared to ordinary motors. The need to adopt force-magnetic-thermal-NVH multiphysics coupling design is a new challenge and a new opportunity.

This article introduces eight categories of applications and six key technologies of high-speed motors. In general, high-speed motors are a promising and technically challenging application. Some technologies seem far away from us, but from the perspective of development, we can see that the vein of "shallow high-speed - medium-high speed - ultra-ultra-high speed" has been evolving. Compared with ten years ago, 10,000 or 20,000 rotary motors are now commonplace. Therefore, high-speed is "long-termism" and will slowly change the pattern of the industry. Therefore, whether it is looking for opportunities in new fields or improving the competitiveness of existing products, high-speed technology is an area worth long-term investment.