Permanent Magnet Synchronous Motors are used in EVs, robotics, automation, HVAC, compressors, and high-speed systems for their high efficiency, rapid response, precise control, and compact design.

However, despite their advantages, PMSMs commonly face bearing noise and vibration problems, which directly affect motor performance, lifetime, and user experience.

Why Bearing Noise Matters in PMSM

Bearings are responsible for supporting the rotor, reducing friction, enabling smooth rotation, and maintaining correct alignment. In PMSMs, which often operate at high speeds and require precise rotor positioning for synchronous operation, bearings play a critical role in:

• Rotor stability
• Torque smoothness
• Minimizing friction losses
• Preventing demagnetization from mechanical collisions
• Extending motor lifetime

Any abnormality in bearing behavior — such as noise, vibration, or overheating — leads to:

• Increased energy consumption
• Loss of efficiency
• Reduced accuracy in servo systems
• Higher acoustic noise (unacceptable for EVs and home appliances)
• Premature motor failure

Therefore, diagnosing bearing noise early and implementing corrective solutions is essential for PMSM reliability and performance.

Types of Bearing Noise in PMSM

Bearing noise in PMSM is generally classified into the following categories:

Mechanical Noise

Caused by physical defects or damage inside the bearing:

• Surface wear
• Cracks or pitting
• Cage looseness
• Ball deformation

Mechanical noise usually sounds like:

➡ Grinding, rattling, or knocking

Electromagnetic-Induced Noise

Although bearings are mechanical parts, electromagnetic forces in PMSM can indirectly contribute:

• Magnetic radial forces
• Unbalanced magnetic pull (UMP)
• Cogging torque vibration

This often creates:

➡ Humming, whining, or resonance

Lubrication-Related Noise

Occurs when lubrication is insufficient, contaminated, or broken down:

• Dry rubbing
• Oil starvation
• Grease hardening

Audible symptoms:

➡ Squealing or chirping

Structural Noise

Poor assembly or imbalance in surrounding components:

• Misalignment
• Loose housings
• Incorrect shaft fit

Produces:

➡ Intermittent metal contact sounds

Common Causes of Bearing Noise and Vibration in PMSM

This section provides an engineering-level analysis of factors contributing to PMSM bearing noise.

Overload and Excessive Radial/Axial Force

Bearings experience higher stress when:

• The motor drives heavy loads
• Misalignment increases shaft deflection
• Rotor imbalance produces uneven radial force
• Belt transmissions apply excessive axial load

High radial loads cause premature wear.

High axial loads destroy thrust bearings.

Rotor Imbalance and Unbalanced Magnetic Pull (UMP)

PMSM rotors experience UMP due to:

• Uneven air gap
• Assembly errors
• Magnet tolerance variations
• Rotor eccentricity

UMP pulls the rotor toward one side, increasing bearing stress and causing:

• Vibration
• Audible humming
• Premature bearing fatigue

This is especially common in surface-mounted permanent magnet (SPM) rotors.

Contamination Inside the Bearing

Dust, metallic particles, and moisture create surface abrasion and rust.

Typical contamination sources include:

• Poor sealing
• High-humidity environments
• Manufacturing machining debris
• Aging lubricant breakdown

Contaminated bearings produce an unmistakable rough grinding noise.

Lubrication Failure

Lubrication problems occur due to:

• Grease aging or oxidation
• Excessive temperature
• Over-greasing or under-greasing
• Chemical contamination

High-speed operation beyond grease capability

When lubrication fails, friction increases, leading to:

• Squealing noise
• Sudden temperature rise
• Rapid wear

Misalignment Between Rotor and Stator

Misalignment may result from:

• Incorrect mounting
• Bent shafts
• Poor machining tolerances
• Bearing seat deformation
• Housing warpage under thermal expansion

Misalignment produces:

• Vibration
• Uneven loading on bearings
• Increased acoustic noise

Electrical Current Passing Through Bearings (EDM Damage)

Stray electrical currents may flow through bearings due to:

• Improper grounding
• High-frequency PWM inverters
• Shaft voltage induced by switching radiation
• Poor insulation design

This leads to Electrical Discharge Machining (EDM) pitting on bearing surfaces.

Symptoms:

• Buzzing noise
• Vibration
• Fluting marks on bearings

High-Speed PMSM Rotor Dynamics

High-speed PMSMs (30,000–120,000 rpm) amplify:

• Centrifugal force
• Rotor bending
• Resonance
• Thermal expansion

These factors make bearings sensitive to:

• Imbalance
• Lubricant breakdown
• Incorrect preload
• Noise amplification

Diagnostic Techniques for Bearing Noise and Vibration

Engineers use several quantitative and qualitative diagnostic methods.

Audible Noise Inspection

A simple but effective method.

Operators listen for noises:

• Grinding → mechanical damage
• Whining → electromagnetic excitation
• Chirping → lubrication failure
• Knocking → cage looseness

Often used during routine maintenance.

Vibration Spectrum Analysis (FFT)

Vibration signals are decomposed using Fast Fourier Transform (FFT).

Helps identify:

• Ball pass frequency defects
• Inner/outer race wear
• Cage defects
• Resonance
• Rotor imbalance

FFT is essential for high-speed PMSMs used in EV and robotics.

Temperature Monitoring

Abnormal temperature rise indicates:

• Friction increase
• Lubrication failure
• Overloading
• EDM damage

Thermal imaging cameras or embedded sensors are commonly used.

Shaft Runout Measurement

Measures rotor shaft deviation using:

• Dial indicators
• Laser alignment tools

High runout → bearing preload problems or misalignment.

Acoustic Vibration Sensors (AE Sensors)

Acoustic emission sensors detect micro-fractures inside bearings before failure.

Beneficial for:

• PMSM servo motors
• Robotics
• Medical equipment

Oil/Grease Condition Analysis

Checks:

• Particle contamination
• Moisture content
• Viscosity

Used mainly in industrial motor maintenance.

Symptoms vs Causes of Bearing Noise in PMSM

Symptom	Likely Cause	Diagnosis Method
Grinding noise	Surface wear, contamination	Vibration analysis, disassembly
Whining/high-pitch noise	UMP, rotor imbalance, electromagnetic forces	FFT, air gap measurement
Squealing	Lubrication failure	Grease test, thermal monitoring
Knocking	Cage looseness, misalignment	Shaft runout, visual inspection
Buzzing	Electrical discharge (EDM) damage	Shaft voltage test
Irregular vibration	Shaft misalignment	Laser alignment
Temperature rise	Overload, lubrication failure	Temperature sensors

Engineering Solutions to Reduce Bearing Noise and Vibration

Solutions fall into several categories: design improvements, operational adjustments, and maintenance practices.

Improve Rotor and Stator Machining Accuracy

Manufacturing tolerances significantly affect PMSM bearing performance.

Actions:

• Reduce rotor eccentricity (<10–20 microns)
• Maintain uniform air gap
• Use precision grinding and CNC machining
• Adopt high-accuracy stamping and stacking for laminations
• Better precision reduces UMP, lowering bearing loads and noise.

Optimize Rotor Balancing

Dynamic balancing is essential for high-speed PMSMs.

Methods:

• ISO G2.5 or G1 balancing grade
• Multi-plane balancing
• Compensation slots
• Magnet weight adjustment

Balance correction significantly reduces vibration amplitude.

Use High-Quality Bearings

Key selection criteria:

• Precision grade: P5, P4, P2
• Material: Chrome steel, stainless steel, hybrid ceramic
• Sealing type: Contact/semi-contact seal
• Cage type: Polyamide for low noise
• Internal clearance: C3, C4 for high-speed PMSM

Hybrid ceramic bearings are preferred for:

• EV motors
• High-speed compressors
• Medical centrifuges

They reduce EDM damage and improve noise performance.

Ensure Proper Lubrication

Solutions:

• High-speed synthetic grease
• Automatic lubrication systems
• Low-temperature grease for HVAC PMSM
• Anti-oxidation additives

In high-speed PMSMs:

• Oil mist lubrication
• Oil-air lubrication system
• are commonly used.

Prevent Electrical Current Through Bearings

To avoid EDM damage:

• Use insulated bearings
• Apply shaft grounding rings
• Improve inverter filtering design
• Increase stator insulation

This prevents bearing pitting and reduces buzzing noises.

Improve Motor Assembly Process

Assembly quality directly affects noise.

Key requirements:

• Correct preload control
• Accurate bearing seat tolerance
• Avoid excessive press-fit force
• Ensure parallelism of bearing housings
• Eliminate shaft burrs

Assembly defects are a major cause of early bearing failure.

Reduce Electromagnetic Vibration Forces

Electromagnetic noise can be reduced by:

• Skewing rotor or stator slots
• Increasing slot number
• Optimize magnet geometry to reduce electromagnetic vibration.
• Minimize harmonic currents for smoother motor operation.
• This solution addresses whining or humming noise.

Structural and Housing Improvements

To avoid resonance or structural amplification:

• Strengthen housing stiffness
• Add damping layers
• Avoid thin-wall housings

Apply finite element analysis (FEA) to predict resonance frequency

Solutions to Common PMSM Bearing Issues

Bearing Issue	Root Cause	Effective Solution
Wear & pitting	Contamination	Improve sealing, clean assembly
Squealing	Lubrication failure	Use proper grease, schedule relubrication
Buzzing noise	EDM discharge	Use insulated bearings, grounding ring
Excessive vibration	Rotor imbalance	Dynamic balancing
Overheating	Overload, friction	Reduce load, improve cooling
Resonance noise	Weak housing	Structural reinforcement
Whining	Electromagnetic forces	Reduce harmonics, optimize rotor/stator design

Preventive Maintenance Strategies

Proper maintenance ensures long motor life.

Routine Noise & Vibration Monitoring

• Install vibration sensors
• Perform FFT analysis quarterly
• Maintain noise trend records

Scheduled Lubrication

Re-lubrication intervals based on:

• Speed
• Load
• Ambient temperature

Regular Bearing Replacement

Typical PMSM industrial motors replace bearings every:

• 8,000 – 20,000 hours (general)
• 5,000 – 10,000 hours (high-speed)
• 2,000 – 5,000 hours (extreme environment)

Seal Inspection

Replace seals if:

• Cracked
• Hardened
• Oil leakage occurs

Application-Specific Recommendations

EV PMSM Motors

Requirements:

• Low noise
• High speed (up to 18,000 rpm)
• Low friction

Solutions:

• Hybrid ceramic bearings
• Precision balancing
• Noise-optimized rotor skew

Industrial PMSM Motors

Focus on:

• Load capacity
• Easy maintenance

Solutions:

• C3 clearance bearings
• Heavy-duty sealing
• Reinforced housing

Robotics PMSM Motors

Key needs:

• Ultra-low vibration
• Precision positioning

Solutions:

• High-precision P4 or P2 bearings
• Low harmonic windings
• Perfect alignment

Improving PMSM Reliability Requires Engineering + Maintenance

Bearing noise and vibration in Permanent Magnet Synchronous Motors (PMSMs) come from a combination of mechanical, electromagnetic, lubrication, and assembly-related factors. Effective diagnosis requires a combination of:

• Vibration analysis
• Temperature monitoring
• Shaft alignment checks
• Acoustic inspection
• Lubricant condition testing

Meanwhile, long-term solutions include:

• Better rotor balancing
• High-quality bearings
• Improved lubrication systems
• Preventing electrical discharge through bearings
• High-precision manufacturing
• Better assembly processes

By addressing these areas comprehensively, manufacturers and maintenance teams can significantly improve PMSM performance, reduce acoustic noise, extend motor life, and enhance user satisfaction—especially in noise-sensitive industries such as electric vehicles, robotics, and household appliances.

Permanent Magnet Synchronous Motors (PMSMs) deliver exceptional efficiency, compact size, and high torque density, making them ideal for electric vehicles, robotics, and industrial automation—performance depends on precise control strategy.

Both techniques aim to optimize torque production and efficiency while minimizing ripple and response time. Yet, their underlying principles, implementation complexity, and performance characteristics differ significantly.

Overview of PMSM Control

Basics of Permanent Magnet Synchronous Motors

PMSMs feature permanent magnets on the rotor that create the magnetic field, while the stator’s three-phase windings produce a rotating field that synchronously drives rotation.

Key equations governing PMSM dynamics include:

Te​=2/3​p(ψd​iq​−ψq​id​)

where:

• Te = Electromagnetic torque
• P= Number of pole pairs
• ψd = Flux linkages in the d- and q-axis
• id,iq= Current components along d- and q-axis

The control system’s main goal is to manage id and id​ precisely to achieve desired torque and flux levels.

Field-Oriented Control (FOC)

Principle of Operation

Field-Oriented Control, also known as Vector Control, transforms the three-phase stator currents into a rotating reference frame (d–q frame). This transformation decouples torque and flux, enabling independent, DC-motor-like control of PMSM currents.

The steps involved are:

• Measure stator currents ia,ib,ib, ic​.
• Convert them into id and iq​ using Clarke and Park transformations.
• Control idi_did​ (flux) and iqi_qiq​ (torque) independently using PI regulators.
• Inverse transform back to three-phase voltages for PWM modulation.

This decoupling enables precise torque and speed control under dynamic load conditions.

FOC Control Structure

Stage	Description	Function
Current Measurement	Captures phase currents ia,ibi_a, i_bia​,ib​	Inputs for transformations
Clarke Transformation	Converts 3-phase to 2-phase (α–β)	Simplifies calculations
Park Transformation	Converts α–β to d–q rotating frame	Separates torque and flux
PI Controllers	Controls idi_did​ and iqi_qiq​	Maintains desired torque and flux
Inverse Park Transformation	Converts control outputs back to 3-phase signals	Feeds PWM inverter
PWM Generation	Modulates inverter switching	Applies voltage to PMSM

​Advantages of FOC

• Smooth Torque Output – Torque ripple is minimal due to sinusoidal current control.
• High Efficiency – Magnetic field alignment minimizes copper and iron losses.
• Wide Speed Range – Effective field weakening for high-speed operation.
• Stable Control – Proportional-integral (PI) regulators provide steady performance under variable load.

Limitations of FOC

• Complex Implementation – Requires multiple coordinate transformations and rotor position sensors.
• Parameter Sensitivity – Dependent on accurate motor parameters (resistance, inductance, flux linkage).
• Moderate Dynamic Response – Slightly slower torque response compared to DTC due to current regulation loops.

Direct Torque Control (DTC)

Principle of Operation

Direct Torque Control directly regulates the torque and stator flux of the PMSM without relying on current control loops or PWM modulation. Instead, it selects inverter voltage vectors based on real-time torque and flux feedback.

Core concept:

• Calculate instantaneous stator flux and torque.
• Compare with reference values using hysteresis controllers.
• Select the optimal voltage vector from a predefined table to correct deviations instantly.

DTC Control Structure

Stage	Description	Function
Voltage and Current Sensing	Measures stator voltages/currents	Inputs for flux and torque estimation
Flux Estimation	Calculates stator flux vector	Determines magnetic field level
Torque Estimation	Computes electromagnetic torque	Monitors motor output
Hysteresis Controllers	Compare actual vs. reference torque/flux	Generate switching signals
Switching Table	Selects appropriate inverter vector	Controls torque and flux directly
Inverter	Applies selected voltage vector	Adjusts motor electromagnetic state

Advantages of DTC

• Fast Torque Response – Excellent dynamic performance due to direct control.
• No Coordinate Transformations – Simplifies computation compared to FOC.
• No Need for PI Regulators or PWM – Reduces processing delay.
• Robustness – Less sensitive to motor parameter variations.

Limitations of DTC

• Higher Torque Ripple – Hysteresis-based control produces torque and flux oscillations.
• Variable Switching Frequency – Makes inverter design and filtering more complex.
• Lower Efficiency at Steady-State – Ripple losses may reduce system efficiency.
• Difficult Flux Control at Low Speed – Accuracy of flux estimation declines at low voltage.

Comparative Analysis: FOC vs DTC

Aspect	Field-Oriented Control (FOC)	Direct Torque Control (DTC)
Basic Principle	Vector control with decoupled current control	Direct torque and flux control via hysteresis
Control Variables	id,iqi_d, i_qid​,iq​ (current components)	Torque and stator flux
Dynamic Response	Moderate	Very fast
Torque Ripple	Low	High
Switching Frequency	Constant (via PWM)	Variable
Implementation Complexity	High (transformations + PI control)	Moderate (lookup tables + estimation)
Parameter Sensitivity	High	Low
Efficiency (steady-state)	High	Moderate
Low-Speed Performance	Excellent	Poor (flux estimation issue)
Hardware Requirement	Rotor position sensor, current sensors	Voltage and current sensors
Computational Load	High	Lower
Use Case Examples	Precision motion control, servo drives, robotics	Traction, EVs, applications needing fast torque response

Dynamic Performance Comparison

To illustrate differences, the following example compares FOC and DTC control in a PMSM rated at 5 kW, 3000 rpm, under a step torque command:

Performance Metric	FOC	DTC
Torque Rise Time	2.8 ms	1.1 ms
Torque Ripple	2%	8%
Speed Overshoot	3%	6%
Efficiency at Rated Load	95%	91%
Switching Frequency	Fixed (10 kHz)	Variable (5–20 kHz)

These results highlight that DTC offers superior transient response, while FOC provides smoother and more efficient steady-state operation.

Practical Considerations in Implementation

Sensor Requirements

• FOC typically uses a rotor position sensor (resolver, encoder, or Hall sensors) for coordinate transformations. Sensorless FOC methods exist but require complex observers.
• DTC, in contrast, can function sensorless using voltage and current measurements for flux estimation, but this becomes less accurate at low speeds.

Computational Demand

FOC requires real-time transformations (Clarke, Park, inverse Park) and PI controllers for both d and q axes. DTC avoids these computations, but frequent torque and flux estimations still demand high sampling rates.

Inverter and Switching Design

Since DTC employs variable switching frequency, inverter design must accommodate a wider operating range, often resulting in increased thermal stress on power devices. FOC, using constant-frequency PWM, simplifies inverter thermal management.

Application Areas

Application	Preferred Control Strategy	Reason
Electric Vehicles (EVs)	DTC	Rapid torque response, better acceleration control
Robotics and Automation	FOC	Smooth motion and precise torque regulation
Machine Tools	FOC	Low torque ripple essential for precision machining
Aerospace Actuators	FOC	High reliability and low noise operation
Elevators & Cranes	DTC	High dynamic response to sudden load changes
HVAC and Compressors	FOC	Energy-efficient constant-speed operation

Hybrid and Modern Improvements

Recent research aims to combine FOC’s smoothness and DTC’s speed through hybrid FOC-DTC methods or model predictive control (MPC) frameworks.
Some trends include:

• Model Predictive Torque Control (MPTC) – Enhances DTC with predictive algorithms for fixed-frequency switching.
• Sensorless FOC-DTC Hybrids – Integrate flux estimation for sensorless operation while maintaining FOC smoothness.
• AI-Based Controllers – Machine learning and adaptive neural controllers are emerging to automatically tune gains and hysteresis thresholds.
• Space Vector Modulation (SVM) in DTC – Reduces torque ripple and stabilizes switching frequency, bridging the gap between the two strategies.

Choosing Between FOC and DTC

The choice between FOC and DTC depends on specific application requirements:

Design Priority	Recommended Strategy
High Torque Response	DTC
Minimal Torque Ripple	FOC
Simplified Control Implementation	DTC
High Efficiency & Precision	FOC
Low-Speed Accuracy	FOC
Sensorless Operation	DTC
Cost-Effective Hardware	DTC
Stable Thermal Load	FOC

Future Outlook

With advancements in digital signal processors (DSPs) and field-programmable gate arrays (FPGAs), implementing both FOC and DTC has become more practical and cost-effective. Engineers can now achieve hybrid schemes that exploit DTC’s rapid dynamics and FOC’s smooth performance. Moreover, AI-driven parameter identification and adaptive control are paving the way for self-optimizing PMSM systems, reducing dependency on manual tuning.

The ongoing focus is on achieving:

• Higher power efficiency
• Reduced torque ripple
• Simplified hardware
• Unified hybrid control models

As electrification expands into mobility, manufacturing, and renewable systems, selecting the optimal control strategy remains a key determinant of system performance and reliability.

• Both Field-Oriented Control (FOC) and Direct Torque Control (DTC) are proven strategies for PMSM operation, each offering distinct advantages.
• FOC excels in smooth torque generation, precise control, and energy efficiency, making it suitable for robotics, automation, and servo applications.
• DTC provides faster torque response and simpler implementation, ideal for traction drives and systems requiring rapid dynamic performance.

In modern motor control design, the line between FOC and DTC continues to blur as hybrid systems and predictive algorithms evolve — combining the best of both worlds to deliver smarter, faster, and more efficient PMSM drives.

Permanent Magnet Synchronous Motors (PMSMs) have become a cornerstone of modern motion control systems, offering high efficiency, compact size, and superior dynamic performance compared to induction and brushed DC motors. They’re commonly used in EVs, robotics, automation, and renewable energy systems.

However, PMSMs are not all the same — their rotor design fundamentally influences performance characteristics. Two main PMSM types—Surface-Mounted and Interior—differ in structure and function, crucial for choosing the right motor.

Understanding PMSM Fundamentals

A PMSM works by synchronizing stator and rotor magnetic fields. The stator carries a three-phase winding powered by an AC supply, producing a rotating magnetic field (RMF). The rotor’s magnets synchronize with the stator field, rotating at the same speed seamlessly.

Unlike induction motors that rely on rotor current to generate torque, PMSMs use permanent magnets to establish the magnetic field, leading to higher efficiency and lower losses. Removing rotor windings and slip rings boosts reliability and lowers heat.

What is a Surface-Mounted PMSM (SPMSM)?

In a Surface-Mounted Permanent Magnet Synchronous Motor, the permanent magnets are affixed directly onto the rotor surface, typically in a circular array. The magnetic field generated by these surface magnets interacts directly with the stator field to produce torque.

This design offers simplicity — both in construction and magnetic behavior — since the rotor’s magnetic field distribution is nearly sinusoidal. The air gap between rotor and stator is uniform, resulting in smooth torque production and low cogging torque.

Advantages include:

• Simple mechanical design and manufacturing
• High torque accuracy and smooth operation
• Ideal for servo applications requiring precise speed and position control

Common applications: CNC machines, industrial robots, actuators, and small electric vehicles where high precision and compactness are critical.

What is an Interior PMSM (IPMSM)?

An Interior Permanent Magnet Synchronous Motor differs significantly in rotor design. The permanent magnets are embedded within the rotor’s iron core, often arranged in V-shaped or U-shaped cavities. This configuration introduces magnetic saliency — a difference between the rotor’s direct-axis (d-axis) and quadrature-axis (q-axis) inductances.

The magnetic saliency allows IPMSMs to generate not only magnetic torque (as in SPMSM) but also reluctance torque, resulting in higher overall torque density. Embedded magnets resist mechanical stress and demagnetization during high-speed operation.

Advantages include:

• Higher torque density and efficiency
• Wide speed range due to field weakening capability
• Enhanced mechanical strength and thermal stability

Typical applications: Electric vehicles, industrial drives, compressors, and wind power generators.

Key Structural Differences

The structural difference between the two types forms the foundation for their contrasting characteristics.

Feature	Surface-Mounted PMSM (SPMSM)	Interior PMSM (IPMSM)
Magnet Placement	On rotor surface	Embedded inside rotor iron core
Torque Type	Magnetic torque only	Magnetic + reluctance torque
Saliency Ratio (Lq/Ld)	≈1 (no saliency)	>1 (high saliency)
Field Weakening Capability	Limited	Excellent
Mechanical Strength	Moderate	High (magnets well protected)
Cooling Efficiency	Poorer (exposed magnets)	Better (iron acts as thermal path)
Manufacturing Complexity	Simple	Complex (requires precision slotting)

This structural difference means IPMSMs can handle higher speeds and loads, whereas SPMSMs excel in precision and simplicity.

Electromagnetic Performance Comparison

Electromagnetic performance dictates how a motor behaves under different operating conditions. SPMSMs have a relatively linear torque-speed relationship, offering excellent control at low to medium speeds. However, their inability to perform field weakening restricts high-speed operation.

In contrast, IPMSMs exhibit nonlinear behavior due to their saliency. The additional reluctance torque improves efficiency and torque density, particularly in field-weakening regions, making them ideal for traction drives.

Example performance data (simulation results):

Parameter	SPMSM	IPMSM
Rated Power (kW)	5	5
Rated Torque (Nm)	15	18
Peak Torque (Nm)	28	35
Base Speed (rpm)	1500	1500
Max Speed (rpm)	2500	4500
Efficiency at Base Load	91%	95%

The embedded magnet design enables IPMSMs to deliver higher torque and extended speed range with less demagnetization risk.

Control and Drive Considerations

Control strategies differ due to rotor saliency and torque composition. Both SPMSMs and IPMSMs commonly use Field-Oriented Control (FOC), but with varying emphasis:

SPMSM Control:

• Simpler, as Ld = Lq, resulting in a purely magnetic torque.
• Control involves maintaining rotor flux alignment.
• Ideal for applications needing smooth, predictable torque.

IPMSM Control:

• Exploits Maximum Torque per Ampere (MTPA) control to balance magnetic and reluctance torque.
• Requires dynamic adjustment of current vector for optimal efficiency.
• Enables efficient high-speed field-weakening operation for EVs.

Thus, IPMSMs require more complex algorithms and real-time feedback systems but deliver superior torque utilization.

Efficiency and Power Density

Power density and efficiency determine how much torque or power can be extracted per unit mass. SPMSMs, with their simpler magnetic circuit, achieve high efficiency at low and steady speeds, while IPMSMs maintain higher efficiency across a broader speed range.

Example comparison:

Speed Range (rpm)	SPMSM Efficiency	IPMSM Efficiency
1000	94%	95%
2000	91%	94%
3000	85%	92%
4000	75%	90%

The difference becomes more evident at high speeds where the IPMSM benefits from field-weakening, avoiding back-EMF saturation.

Cost, Manufacturing, and Maintenance Aspects

The choice between SPMSM and IPMSM also depends on manufacturing cost, maintenance complexity, and material utilization.

SPMSM Manufacturing:

The rotor construction involves surface gluing or bonding of magnets, often requiring protective sleeves (e.g., carbon fiber or stainless steel). This design is straightforward but limits maximum rotational speed due to centrifugal stress on magnets.

IPMSM Manufacturing:

The rotor needs precise machining to create magnet slots and alignment angles. The complexity increases cost but ensures robust performance and longer lifespan.

Maintenance Considerations:

• IPMSMs are less prone to magnet chipping or delamination.
• SPMSMs are easier to disassemble and remagnetize if necessary.

Material cost also varies. IPMSMs typically use less magnetic material for the same torque output due to additional reluctance torque, leading to better utilization of expensive rare-earth magnets like neodymium.

Application Suitability

Each motor type has distinct advantages based on performance priorities.

Application	Recommended Motor Type	Reason
Servo Systems	SPMSM	Simple control, low torque ripple, high precision
Electric Vehicles	IPMSM	High torque density, wide speed range, field weakening
Robotics	SPMSM	Compact design, fast dynamic response
Industrial Drives	IPMSM	Efficient under variable loads
Household Appliances	SPMSM	Cost-effective and quiet operation
Wind Turbines/Generators	IPMSM	Robust structure, better cooling, efficiency at variable speeds

These distinctions make SPMSMs the preferred option for low-inertia precision systems, while IPMSMs dominate high-power applications like EVs and industrial drives.

Case Study: EV Traction Motor Comparison

To illustrate, consider a 100 kW electric traction system tested with both SPMSM and IPMSM configurations under similar voltage and current limits.

Performance Metric	SPMSM	IPMSM
Continuous Torque (Nm)	220	270
Peak Torque (Nm)	380	440
Field Weakening Ratio	1.3	2.8
Max Speed (rpm)	6000	12000
Efficiency at 75% Load	92%	96%
Magnet Cost	100%	85% (due to less volume)

The IPMSM clearly outperforms in torque, speed, and energy efficiency, explaining why major EV manufacturers, such as Tesla and Toyota, employ IPMSMs in their traction systems. However, SPMSMs remain relevant for auxiliary systems (e.g., pumps and fans) requiring smooth, low-torque operation.

Future Trends and Innovations

Recent advancements in PMSM technology are narrowing the gap between the two designs. Engineers are experimenting with hybrid PMSMs, combining surface and interior magnet arrangements to harness the best of both worlds — high torque at low speed and efficient field weakening at high speed.

Other innovations include:

• Segmented magnets to minimize eddy current losses
• High-temperature magnets (SmCo or NdFeB alloys with dysprosium) for stability
• AI-driven motor control optimizing current vector orientation for real-time torque management
• Additive manufacturing techniques that reduce rotor assembly complexity

As material costs and energy efficiency regulations evolve, hybrid PMSMs may become the standard for next-generation EVs and high-performance servo drives.

Both Surface-Mounted PMSMs (SPMSMs) and Interior PMSMs (IPMSMs) share the same operational principle but differ significantly in performance and application scope due to their rotor configurations.

SPMSMs excel in simplicity, precision, and smooth torque output — making them ideal for low-speed, high-accuracy applications such as robotics and automation. In contrast, IPMSMs deliver superior torque density, mechanical robustness, and wide speed range — perfectly suited for electric vehicles and heavy industrial drives.

When choosing between them, engineers must weigh priorities such as torque requirement, efficiency range, control complexity, and cost. As design tools, magnet materials, and control algorithms advance, both motor types will continue to evolve — driving innovation across electrification and automation sectors.