In the rapidly evolving electric vehicle (EV) industry, the demand for high-performance traction motors is paramount. Permanent Magnet Synchronous Motors (PMSMs) are widely adopted in China EV applications due to their high power density and efficiency. However, the reliance on rare-earth materials like neodymium in PMSM designs poses economic and supply chain challenges. To address this, we propose a novel rotor topology for a less-rare-earth PMSM, which reduces rare-earth usage while maintaining torque performance. This article analyzes the electromagnetic torque characteristics and introduces an asymmetric pole design to mitigate torque ripple, supported by finite element simulations and experimental validation.
The proposed less-rare-earth PMSM integrates ferrite magnets with neodymium-based rare-earth magnets, strategically positioned to enhance reluctance torque. The electromagnetic torque of a PMSM consists of permanent magnet torque and reluctance torque, expressed as:
$$T_e = T_{pm} + T_{re} = \frac{3}{2}p\left[\psi_f i_q + (L_d – L_q)i_d i_q\right]$$
where $T_e$ is the electromagnetic torque, $T_{pm}$ is the permanent magnet torque, $T_{re}$ is the reluctance torque, $p$ is the number of pole pairs, $\psi_f$ is the permanent magnet flux linkage, $i_d$ and $i_q$ are the d-axis and q-axis currents, and $L_d$ and $L_q$ are the d-axis and q-axis inductances. By optimizing the inductance difference $(L_d – L_q)$, the reluctance torque can be amplified, compensating for reduced rare-earth content.
We developed finite element models for three PMSM configurations: the proposed less-rare-earth PMSM, a V-type rare-earth PMSM, and a rare-earth PMSM with a flux barrier. The electromagnetic properties were analyzed under identical conditions to ensure fair comparison. The air-gap flux density and no-load back-EMF were evaluated, with key results summarized in Table 1.
| Parameter | Less-Rare-Earth PMSM | V-Type Rare-Earth PMSM | Rare-Earth PMSM with Flux Barrier |
|---|---|---|---|
| Air-Gap Flux Density Fundamental (T) | 0.909 | 0.917 | 0.892 |
| No-Load Back-EMF Fundamental (V) | 53.01 | 54.06 | 52.50 |
| THD of Air-Gap Flux Density (%) | 22.35 | 22.28 | 23.03 |
| THD of No-Load Back-EMF (%) | 8.67 | 8.13 | 9.75 |
The less-rare-earth PMSM exhibited a slightly lower fundamental air-gap flux density but a comparable total harmonic distortion (THD) to the V-type configuration. The d-axis and q-axis inductances were calculated to assess the impact on torque. The inductance values are presented in Table 2.
| Inductance | Less-Rare-Earth PMSM (mH) | V-Type Rare-Earth PMSM (mH) | Rare-Earth PMSM with Flux Barrier (mH) |
|---|---|---|---|
| $L_d$ | 1.9686 | 1.9919 | 1.9249 |
| $L_q$ | 4.3193 | 4.3152 | 4.3157 |
| $L_d – L_q$ | -2.3507 | -2.3233 | -2.3908 |
The larger negative inductance difference in the less-rare-earth PMSM indicates a higher potential for reluctance torque. By adjusting the stator current advance angle $\gamma$, the torque output was optimized. The relationship between average output torque and $\gamma$ is shown in Figure 1, where the less-rare-earth PMSM achieved a peak torque of 13.54 N·m at $\gamma = 23^\circ$, a 7.24% improvement over the symmetric pole design.
Torque ripple, a critical issue in EV applications, was addressed through an asymmetric pole design. The equivalent magnetic circuit model of the less-rare-earth PMSM was derived to analyze the flux distribution. The magnetic circuit equations are:
$$\begin{cases}
F_y = F_{g1} (R_{g1} + R_{\delta1}) \\
F_{nd} = F_{g2} (R_{g2} + R_{\delta2} + R_{\delta3}) \\
F_y – F_{st} = F_{g1} R_{g1} + F_{y} \\
F_{nd1} – F_{st} = F_{g2} R_{g2} + F_{nd}
\end{cases}$$
where $F_y$ and $F_{nd}$ are the magnetomotive forces of ferrite and neodymium magnets, $F_{g1}$ and $F_{g2}$ are the air-gap fluxes, and $R$ denotes reluctances. By varying the pole angles $\alpha_1$, $\alpha_2$, $\alpha_3$, and $\alpha_4$, the air-gap flux density distribution was modified to reduce torque ripple. The torque ripple coefficient $K_{mb}$ is defined as:
$$K_{mb} = \frac{T_{\text{max}} – T_{\text{min}}}{(T_{\text{max}} + T_{\text{min}})/2} \times 100\%$$
A multi-objective optimization using the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) was employed to determine the optimal asymmetric pole angles. The sensitivity analysis revealed that N-S pole asymmetry had a higher impact on torque ripple reduction. The Pareto front for single-pole and N-S pole asymmetry was generated, and TOPSIS scores were calculated as:
$$C_i = \frac{D_i^-}{D_i^+ + D_i^-}$$
where $D_i^+$ and $D_i^-$ are the positive and negative ideal distances. The optimal parameters for single-pole asymmetry were $\alpha_1 = \alpha_3 = 12^\circ$ and $\alpha_2 = \alpha_4 = 5^\circ$, yielding an average torque of 14.52 N·m and a torque ripple of 7.54%, a 35.72% reduction compared to the symmetric case.
To validate the simulations, a prototype less-rare-earth PMSM was fabricated and tested. The experimental setup included a dynamometer and back-EMF measurement under no-load conditions at 3000 rpm. The results showed a close match with simulations, with a back-EMF fundamental of 54.06 V (simulation: 53.01 V) and an output torque of 14.18 N·m (simulation: 14.52 N·m). The torque ripple measured 7.88%, confirming the effectiveness of the asymmetric pole design. These findings demonstrate the potential of less-rare-earth PMSMs for enhancing the performance and sustainability of electric vehicles in the China EV market.
In conclusion, the proposed less-rare-earth PMSM topology significantly reduces rare-earth material usage while improving torque output through optimized reluctance torque. The asymmetric pole design effectively suppresses torque ripple, making it suitable for high-performance electric vehicle applications. Future work will focus on thermal management and cost-benefit analysis for mass production in the evolving China EV industry.