How Air Gap Affects PMSM Motor Performance?

Quick Answer

The air gap is one of the most important design parameters in a Permanent Magnet Synchronous Motor (PMSM) because it directly affects magnetic flux, torque, back EMF, efficiency, torque ripple, vibration, and mechanical reliability.

In general, reducing the air gap improves magnetic coupling between the rotor permanent magnets and stator, which can increase torque density and torque per ampere. However, an excessively small air gap makes the motor more sensitive to rotor eccentricity, bearing tolerance, shaft runout, thermal expansion, and manufacturing variation.

A larger air gap provides more mechanical clearance and can improve manufacturing tolerance, but it generally reduces magnetic coupling and may require stronger magnets, a larger magnetic circuit, or higher current to achieve the same torque.

The key engineering principle is simple: the best PMSM air gap is not necessarily the smallest possible air gap; it is the gap that provides the required electromagnetic performance while maintaining sufficient mechanical reliability.

What Is the Air Gap in a PMSM?

The air gap is the small radial space between the rotor and stator of a radial-flux PMSM.

Inside the motor, the permanent magnets mounted on the rotor generate magnetic flux that passes across this gap and enters the stator. Because air has much higher magnetic reluctance than the steel components used in the magnetic circuit, even a relatively small change in air-gap length can significantly influence the motor’s magnetic performance.

For this reason, air-gap design cannot be considered only a mechanical clearance issue. It is also an important electromagnetic design parameter.

When engineers develop a PMSM, the air gap is normally evaluated together with:

Rotor diameter
Stator inner diameter
Permanent magnet thickness
Magnet arc
Pole count
Slot count
Stator tooth geometry
Winding configuration
Motor speed
Required torque

Source: PMSM motor engineering design principle

How Does Air Gap Affect PMSM Motor Performance?

Air Gap and Motor Torque

One of the most important effects of air gap is its influence on electromagnetic torque.

When the air gap becomes smaller within a practical design range, the rotor magnets can couple more effectively with the stator magnetic circuit. This generally increases the useful magnetic flux available for producing torque.

As a result, a properly optimized smaller air gap can provide:

Higher torque density
Higher torque per ampere
Better utilization of permanent magnets
More compact motor dimensions

This is especially important when designing motors for applications where installation space is limited but substantial output torque is required, including robotics, AGVs, industrial automation, medical equipment, stairlift systems, pumps, and electric lawn equipment.

However, reducing the air gap does not provide unlimited torque improvement. Once parts of the magnetic circuit approach saturation, additional magnetic flux produces progressively smaller performance benefits.

Air Gap and Back EMF

The air gap also affects the motor’s back electromotive force (back EMF).

Back EMF is the voltage generated by the motor when the permanent-magnet rotor rotates. It is an important parameter when selecting the motor voltage, operating speed, and motor driver.

A smaller practical air gap generally produces stronger magnetic coupling and therefore higher back EMF at the same rotational speed.

A larger air gap generally reduces effective magnetic flux and therefore tends to reduce back EMF.

This relationship becomes particularly important in high-speed BLDC and PMSM motors, because excessive back EMF at high speed can reduce the available voltage margin of the motor driver.

For OEM motor development, engineers therefore need to evaluate air gap together with:

DC bus voltage
Maximum speed
Motor winding
Permanent magnet strength
Driver voltage capability
Control strategy

Small Air Gap vs. Large Air Gap

Performance Factor	Smaller Air Gap	Larger Air Gap
Magnetic coupling	Generally higher	Generally lower
Air-gap flux	Generally higher	Generally lower
Torque density	Higher potential	Lower potential
Torque per ampere	Higher potential	Lower potential
Back EMF	Higher potential	Lower potential
Mechanical clearance	Lower	Higher
Manufacturing tolerance	More demanding	More forgiving
Rotor eccentricity sensitivity	Higher	Lower
Rotor-stator contact risk	Higher	Lower
Magnet utilization	Generally better	Generally lower
Design complexity	Higher	Lower

These are general engineering trends. Actual results depend on the PMSM topology, rotor structure, magnet material, stator design, winding configuration, operating speed, and motor controller.

Does a Smaller Air Gap Always Improve Efficiency?

No.

A smaller air gap can improve magnetic coupling and allow the motor to produce the required torque with lower current. Lower current can potentially reduce copper losses and improve overall efficiency.

However, an excessively small air gap can introduce other problems.

These may include:

Magnetic saturation
Increased iron loss
Higher torque ripple
Greater cogging torque
Increased electromagnetic noise
Higher sensitivity to manufacturing tolerances
Greater risk of rotor-stator contact

Therefore, engineers should not optimize the air gap solely for maximum magnetic flux.

The correct approach is to find the best balance between torque, efficiency, thermal performance, acoustic behavior, manufacturing tolerance, and mechanical reliability.

Air Gap and Torque Ripple

Air-gap geometry also affects the distribution of magnetic flux between the rotor and stator.

If the magnetic field is not sufficiently uniform, the motor can experience:

Cogging torque
Torque ripple
Electromagnetic vibration
Acoustic noise
Low-speed speed fluctuation

These effects can become particularly important in precision applications.

For example, a motor used in a servo system may need very smooth low-speed operation, while a lawn mower drive motor may place greater emphasis on torque, efficiency, noise, and overload capability.

Therefore, the optimum air gap depends heavily on the application requirements, rather than simply the motor’s rated power.

How Engineers Determine the Correct PMSM Air Gap?

Step 1: Define the Application Requirements

Engineers first establish the complete operating requirements, including:

Rated voltage
Rated speed
Maximum speed
Continuous torque
Peak torque
Duty cycle
Ambient temperature
Installation space
Noise requirements
Expected service life

A motor operating continuously at 3,000 rpm, for example, requires a different design balance from a motor that operates intermittently at 500 rpm.

Step 2: Design the Magnetic Circuit

The air gap must be considered together with the complete electromagnetic structure.

Engineers evaluate:

Rotor diameter
Stator diameter
Magnet thickness
Magnet arc
Pole count
Slot count
Stator tooth width
Stack length
Winding configuration

Changing the air gap may require changes to other motor dimensions.

Step 3: Use Electromagnetic Simulation

Finite Element Analysis (FEA) can be used to compare different air-gap configurations before building prototypes.

Typical evaluation parameters include:

Air-gap magnetic flux
Electromagnetic torque
Back EMF
Torque ripple
Cogging torque
Stator saturation
Magnet demagnetization margin
Core loss

Instead of selecting one theoretical air gap, engineers can compare several candidate designs and identify the best overall performance.

Source: Motor electromagnetic simulation and engineering design practice

Manufacturing Tolerance Is Critical

One common mistake is designing a PMSM around the nominal air gap only.

The actual production air gap can be affected by:

Rotor eccentricity
Bearing clearance
Shaft runout
Rotor balancing
Lamination tolerance
Magnet positioning
Housing tolerance
Thermal expansion

For example, if a motor is designed with an extremely small nominal air gap, even a small amount of rotor eccentricity can significantly reduce the clearance at one location.

This is why engineers need to consider both the nominal air gap and minimum operating clearance.

A motor that performs well in simulation may still experience rotor-stator interference if manufacturing tolerances are not properly controlled.

Common PMSM Air-Gap Problems

Problem	Possible Cause	Engineering Check	Potential Solution
Low torque	Air gap too large	Measure actual air gap	Re-optimize magnetic circuit
High motor current	Insufficient magnetic coupling	Check magnetic flux	Optimize magnet and air-gap design
Excessive back EMF	Excessive magnetic flux	Perform no-load voltage test	Rebalance magnet and air-gap design
High torque ripple	Uneven magnetic field	Perform FEA analysis	Optimize rotor/stator geometry
Rotor rubbing	Insufficient mechanical clearance	Check rotor eccentricity	Increase minimum clearance
High vibration	Rotor eccentricity	Measure shaft runout	Improve rotor/bearing tolerance
High temperature	Excessive current or saturation	Check current and thermal performance	Optimize magnetic loading
Poor efficiency	Excessive copper/core losses	Test efficiency at multiple loads	Rebalance electromagnetic design

Common Engineering Mistakes

Mistake 1: Making the Air Gap as Small as Possible

A smaller air gap can improve magnetic performance, but it also reduces mechanical clearance and increases sensitivity to manufacturing tolerances.

Better approach: optimize the air gap based on electromagnetic performance and the complete mechanical tolerance stack.

Mistake 2: Looking Only at Rated Torque

OEM applications rarely operate at exactly one operating point.

Engineers should also consider:

Starting torque
Acceleration torque
Peak torque
Continuous torque
Low-speed performance
Maximum speed
Thermal conditions

Mistake 3: Ignoring the Motor Driver

The PMSM or BLDC motor should be evaluated together with its controller, particularly when using FOC (Field-Oriented Control).

The motor’s back EMF, inductance, speed range, current requirements, and driver voltage should all be compatible.

Mistake 4: Ignoring Temperature

Thermal expansion can change the effective mechanical clearance between rotor and stator.

This is particularly important for high-speed motors and applications with significant temperature variation.

Air Gap in BLDC and PMSM Motor Applications

PMSM and BLDC motors share many fundamental permanent-magnet motor principles, although their back-EMF waveforms, commutation methods, and control strategies can differ.

For OEM applications, air-gap optimization should therefore be considered as part of the complete motor design rather than as an isolated parameter.

UNITED MOTION INC. provides BLDC motor solutions covering approximately Φ28–Φ110 mm, with customization options including:

Gearboxes
Encoders
Integrated drivers
Customized voltage
Customized torque
Customized speed
Application-specific motor configurations

These options allow a BLDC motor manufacturer to adapt the motor system to different industrial requirements rather than relying on a one-size-fits-all motor.

For example, adding a gearbox can provide higher output torque and lower output speed, while an encoder can provide speed and position feedback for closed-loop motion control. An integrated driver can further simplify system integration where space and wiring requirements are important.

This makes air-gap optimization one part of a broader custom electric motor and motion control solution.

How to Optimize Air Gap for an OEM Motor?

A practical OEM development process can follow this sequence:

Application requirements → Motor geometry → Air-gap design → Electromagnetic simulation → Prototype → Performance testing → Tolerance validation → Mass production

Prototype testing should evaluate:

No-load back EMF
Torque-speed performance
Efficiency
Phase current
Temperature rise
Vibration
Acoustic noise
Torque ripple
Mechanical clearance

This approach is more reliable than selecting an air gap from a generic specification because the optimum value depends on the complete electromagnetic and mechanical architecture.

For OEM customers, working directly with a BLDC motor supplier or custom motor manufacturer can also simplify the process of matching motor geometry, gearbox ratio, encoder configuration, driver requirements, and application performance.

FAQ

What happens if the PMSM air gap is too large?

A larger air gap generally reduces magnetic coupling between the rotor and stator, which can reduce torque density and back EMF and may require higher current or additional magnet material.

Does a smaller air gap always improve PMSM efficiency?

No. A smaller air gap can improve magnetic coupling, but an excessively small gap can increase saturation, torque ripple, electromagnetic losses, manufacturing sensitivity, and mechanical risk.

What is a typical PMSM air gap?

There is no single standard PMSM air-gap value. The appropriate value depends on motor diameter, rotor structure, magnet configuration, operating speed, manufacturing capability, and required mechanical clearance.

How does air gap affect PMSM back EMF?

A smaller practical air gap generally increases magnetic coupling and can increase back EMF, while a larger air gap generally reduces effective magnetic flux and back EMF.

How does air gap affect PMSM torque?

Reducing the air gap within a practical design range generally improves magnetic coupling and can increase torque per ampere, although the benefit becomes limited when magnetic saturation and other design constraints become significant.

Can the air gap be customized for an OEM motor?

Yes. Air-gap design can be optimized as part of a custom electric motor project together with the permanent magnets, stator, winding, rotor, gearbox, encoder, and integrated driver.