High-Efficiency and High Power Density Motors

Table of Contents

How to Improve Motor Power Density?

To achieve higher power output within a fixed volume, the following methods are commonly adopted:

Increase motor speed (design high-speed motors)

Based on the formula (P/V)=Kn, the motor’s output power is proportional to its speed at a constant volume. The design data of Toyota Prius from Generation I to IV shows a continuous increase in rotational speed and a corresponding rise in power density.
Enhance torque density

Improving torque density also boosts power density, with two main approaches:
- Increase the proportion of reluctance torque, as applied in products like Toyota Prius and BMW models.
- Inject electrical harmonics (3rd, 5th, etc.) from the control side to utilize harmonic torque generated by these harmonics, while improving NVH (Noise, Vibration, and Harshness) performance.
Develop hairpin motors (flat wire motors)

Compared with traditional round wire motors, hairpin motors feature a higher slot fill factor. A higher fill factor allows more copper wire to be placed in the same space, generating a stronger magnetic field and increasing motor power density.
Develop axial flux motors

With a unique axial magnetic flux direction, axial flux motors differ structurally from conventional radial flux motors and offer advantages such as compact size, high speed, high power density, and excellent heat dissipation. A typical application is the YASA motor adopted by Mercedes-AMG.

Key Challenges of High Power Density Motors

High-efficiency maintenance issues

A core characteristic of high power density motors is high rotor speed, which typically ranges from 18,000 rpm to 20,000 rpm (compared with a conventional stable threshold of around 15,000 rpm). Such high-speed rotation leads to a sharp increase in supply frequency—rising from a normal 200 Hz to over 1000 Hz at maximum.

Motor speed is proportional to core magnetic flux frequency. At high speeds, the alternating frequency of the core magnetic flux increases, keeping iron loss at a high level. Additionally, high-speed rotation causes bearing friction loss and windage loss on the rotor surface. Thus, elevated supply frequencies result in increased high-frequency additional losses and iron loss.
Heat dissipation challenges

High power density motors also feature high electromagnetic load, which is much higher than that of traditional motors and leads to increased total losses. Higher losses cause temperature rises in all motor components, imposing stricter requirements on cooling methods—improper cooling will directly affect motor performance.

Internal oil cooling is the most widely used method (e.g., the stator slot oil cooling adopted in high-speed motors designed by AVL). Some motors use a combination of cooling methods such as winding oil spray cooling, stator oil cooling, and rotor oil cooling, which increases the design complexity of the cooling system.
Rotor strength issues

High-speed rotation generates enormous centrifugal force, subjecting the rotor to high shear stress.

Practical experience shows that traditional laminated rotors cannot withstand extreme centrifugal force when the linear speed exceeds 250 m/s. Therefore, protective measures must be taken to enhance rotor strength in the design of high-speed rotors for high power density motors—for example, the carbon fiber-wrapped rotor core used in Tesla Model S Plaid.
Vibration and noise issues

Compared with conventional motors, high-speed motors face dual vibration and noise challenges:
- Rotor dynamics-related vibrations: critical speed of the rotor, shaft runout vibration, etc.
- High-frequency electromagnetic force-induced issues: electromagnetic whistling, amplified vibration from the superposition of motor electromagnetic harmonics and IGBT harmonics.
  
  The electromagnetic force of high-speed motors has a higher frequency and wider distribution, which easily excites resonance in the stator system. Rotor design is crucial for avoiding critical speed vibration and requires rigorous modal analysis and testing.
  
  The aspect ratio is a key optimization variable in design:
- A short and thick rotor raises the upper limit of critical speed (reducing resonance risk) but increases the difficulty of withstanding centrifugal stress.
- A slender rotor improves centrifugal strength but lowers the critical speed (increasing resonance probability) and reduces electromagnetic power.
  
  Thus, rotor design requires repeated trade-offs and is the top priority in high-speed motor development.
Bearing challenges

High-speed bearings face significant limitations: ball bearings cannot withstand ultra-high speeds, air bearings have limited load capacity, and magnetic suspension bearings feature complex control and high costs.
Simulation challenges

The technical indicators of high-speed motors must be analyzed through a multi-physics field coupling method that integrates electromagnetic field, stress field, rotor dynamics, fluid field, and temperature field.
Hairpin motor-specific challenges

High technical thresholds, strict flat wire forming requirements, and easy damage to insulating layers result in a lower yield rate for flat wires compared with round wires. Additionally, hairpin motors involve more processing procedures, higher equipment requirements, large upfront investment, difficult serialized design, and high R&D costs.

Amorphous Alloy Soft Magnetic Materials

Applying amorphous alloy materials to motor cores instead of conventional silicon steel sheets can significantly reduce motor iron loss, improve efficiency, and achieve remarkable energy-saving effects—especially for high-frequency motor applications where iron loss accounts for the main loss (e.g., electric vehicle drive motors, high-speed electric spindles, aerospace generators, ship generators, and other military fields). Amorphous alloys have broad application prospects, and their gradual popularization will inevitably challenge the market position of traditional silicon steel sheet motors in the long run.

Amorphous alloys are characterized by high hardness and thin thickness, requiring breakthroughs in stator stamping technology to address cost and efficiency issues in mass production. After annealing, amorphous materials become highly brittle, prone to chipping during stator processing, assembly and operation—thus, surface protection technology for stators is a key consideration. Under the same structure, amorphous alloy motors reduce iron loss by more than 50% compared with silicon steel motors but suffer from lower peak torque and a significant increase in copper loss, making them more suitable for high-speed and high-frequency design.

Compared with traditional processes, amorphous alloy materials feature a cooling rate of up to 1,000,000 °C/s (1,000 times faster than iron-based silicon steel). Their atomic arrangement is long-range disordered (amorphous state) , different from the ordered structure of conventional crystalline materials, and their magnetic permeability is 20 to 100 times that of ordinary silicon steel sheets.

The loss density of amorphous alloy soft magnetic materials is only 1/5 to 1/10 of that of traditional silicon steel sheets, with the following key characteristics:

Physical properties: Thin, hard, limited bandwidth, high brittleness after annealing.
Mechanical properties: Low stiffness, low stacking factor, high strength.
Electromagnetic properties: Low saturation density, large magnetostriction coefficient.

Amorphous alloys have obvious advantages and broad application prospects in the field of high-speed/high-frequency motors and axial flux motors.

Iron Loss of Amorphous Alloy Cores

Amorphous alloys are highly sensitive to processing techniques, and their magnetization and loss properties change after processing:

At 1.0T and 800Hz, the relative magnetic permeability of processed cores is only 48.3% of that of raw strips.
The iron loss of processed amorphous alloy cores is 5 to 8 times that of raw strips, but still significantly lower than that of conventional silicon steel sheets.

Performance Comparison: Stacked Cores vs. Wound Cores

Wound cores have better magnetization performance (the magnetic permeability of stacked cores is only 40% of wound cores at 1.0T).
Wound cores have a lower loss density than stacked cores.

Impact of Different Processing Techniques on Amorphous Alloy Core Loss

Annealing reduces core loss by 22.6%.
Varnish impregnation and curing increases core loss by 44.7%.

Impact of Interference Fit on Amorphous Alloy Core Loss

Excessive interference fit leads to more severe deterioration of core magnetization performance.
Loss density increases with interference fit, but the growth rate slows down.
At a magnetic density of 1T, an interference fit of 0.1mm increases core loss density by 183.8%.

Performance Comparison: Amorphous Alloys vs. Silicon Steel Sheets

As representative soft magnetic materials, silicon steel sheets have an obvious advantage in saturation magnetic induction intensity, but their magnetic permeability is far lower than that of amorphous alloys. Amorphous alloys are referred to as “double green” soft magnetic functional materials by scholars at home and abroad due to their ultra-low loss density (1/5 to 1/10 of traditional silicon steel sheets).

Amorphous alloy motors offer superior performance and obvious advantages in high-speed/high-frequency and axial flux motor fields, with promising application prospects. China has industrial advantages in both amorphous alloy materials and rare earth permanent magnet materials, making the development of amorphous alloy permanent magnet motors highly in line with national conditions.

In terms of loss, the ultra-low iron loss of amorphous alloys is unquestionable: in low-frequency and low-load applications of main drive motors, iron loss can be reduced by about 70%. A 180kW, 15,000RPM amorphous alloy drive motor achieves high efficiency, with a comprehensive efficiency advantage of more than 3% in the optimal range compared with a silicon steel motor of the same specification.

Summary

Material Performance Requirements for High Power Density Motors

Silicon steel sheets
- High magnetic induction intensity
- Low iron loss
- Higher mechanical strength
Stator conductors
- Low resistivity
- Excellent insulation performance
- Corona resistance
Permanent magnets
- Higher residual magnetism (Br) and maximum energy product ((BH)max)
- High intrinsic coercivity (Hcj) for demagnetization resistance
- High resistivity to reduce eddy current loss

The design of high power density motors for electric vehicles is a complex task. High-speed operation of such motors leads to various losses (e.g., heat dissipation loss, windage loss) and imposes higher requirements on components such as bearings and oil seals. Therefore, designers must prioritize high-quality, high-performance material selection to ensure that material properties meet the operational requirements of the motor.