I. Core Principles and Classification
The essence of a motor is a device that converts electrical energy into mechanical energy (and vice versa) in accordance with Faraday’s Law of Electromagnetic Induction. It operates in two modes:
- Motor mode (Electromagnetism-driven rotation): Three-phase current is supplied to the stator to generate a rotating magnetic field, which drives the rotor to rotate.
- Generator mode (Magnetic variation-induced electricity): The rotating rotor magnetic field induces an electromotive force in the stator windings.
Among various motor types, permanent magnet synchronous motors (PMSMs) stand out with outstanding advantages demonstrated by comparative performance data. They are rated “excellent” in efficiency, power density, load capacity and control performance, achieving the highest comprehensive score – a key reason why they have become the mainstream choice for high-performance applications such as electric vehicles.
II. Structure and Operating Mechanism
Surface-mounted PMSM
It features a simple structure, easy manufacturing and low moment of inertia, but has lower power density and limited flux-weakening speed expansion capability.
Inserted PMSM
It combines good power density and flux-weakening performance with a significant contribution of reluctance torque, yet suffers from high magnetic leakage and relatively high manufacturing costs.
Interior PMSM
It can achieve high torque and high power density with excellent operational safety, but has the most complex structural design.
III. Key Performance Indicators and Design Requirements
A series of core indicators are defined to quantitatively evaluate motor performance:
- Back EMF (Electromotive Force): It directly affects the motor’s performance in generating mode or fault conditions.
- Torque ripple: Calculated by the formula: (Maximum torque – Minimum torque) / Average torque. This indicator exerts a decisive influence on the vehicle’s NVH (Noise, Vibration, and Harshness) performance, a key concern for customers – the smaller the value, the better.
- Operating characteristics: The design pursues a wide constant power speed regulation range, as well as high-speed and high-torque output capabilities.
- Thermal management: Motor heating is a critical design constraint, as it directly impacts the reliability and service life of core components such as permanent magnets and windings.
- NVH performance: Electromagnetic, mechanical and aerodynamic noise of the motor are important sources of vehicle NVH issues.
IV. In-depth Analysis: AC Losses
AC losses are the superposition of DC losses, skin effect losses and proximity effect losses, mainly caused by two types of harmonics: high-frequency harmonics from PWM inverters (external harmonics) and harmonics generated by the motor’s own slotting and saturation effects (internal harmonics).
- Harmonic impact: Harmonic current is the primary cause of eddy current losses; increasing the inverter carrier ratio helps reduce such losses.
- Structural parameters: Slot opening dimensions (losses increase with height and decrease with width) and the radial position of conductors in the slot (losses peak when conductors are placed near the slot opening) both have a significant impact on losses.
- Conductor selection: As the conductor diameter increases, AC losses surge to the fourth power, while DC losses decrease in inverse proportion to the square. Therefore, there exists an optimal diameter that minimizes the total loss.
- Parallel winding strategy: With the total cross-sectional area unchanged, increasing the number of parallel windings can reduce losses within a specific frequency range. However, at excessively high frequencies, losses may increase instead due to the intensified proximity effect.
Conclusion
In summary, this paper constructs a complete knowledge framework for understanding permanent magnet synchronous motors, ranging from basic theories to advanced topics. It not only explains the operating principles of PMSMs, but also conducts an in-depth analysis of the key design variables and loss mechanisms that affect their performance, efficiency and reliability. This provides practical guidance for the design, selection and optimization of electric drive systems.
