Differences, Similarities, and Practical Considerations
Brushless DC (BLDC) motors and Permanent Magnet Synchronous Motors (PMSMs) are two of the most widely used motor types in modern electric drive systems. At first glance, they appear to be distinct technologies, often presented as alternatives with different strengths and weaknesses. In practice, however, the distinction between BLDC and PMSM motors is far less clear-cut than many descriptions suggest.
Both motor types use permanent magnets on the rotor and electronic commutation rather than mechanical brushes. Much of the perceived difference lies not in the physical motor itself, but in how it is controlled, described, and applied. Understanding where they truly differ — and where they are effectively the same — is key to making informed motor and controller choices.
Fundamental Similarities
From a construction standpoint, BLDC motors and PMSMs share the same core architecture:
- A permanent magnet rotor
- A three-phase stator winding
- No brushes or commutator
- Electronic commutation via a motor controller
In both cases, torque is produced by interaction between the magnetic field generated by the stator currents and the magnetic field of the rotor magnets. Because the rotor magnetic field is fixed relative to the rotor, these motors operate synchronously, meaning the rotor speed is directly linked to the rotating magnetic field produced by the controller.
Both BLDC and PMSM motors therefore:
- Offer high efficiency compared to brushed motors
- Have long service life due to the absence of mechanical wear components
- Are well suited to variable-speed operation
- Require an electronic controller to function
From a purely electromagnetic perspective, both are permanent magnet synchronous machines.
Where the Terminology Comes From
The distinction between BLDC and PMSM originated historically from control strategy and back-EMF shape, rather than from motor physics.
- BLDC motor is a term that emerged primarily from the electronics and appliance industries.
- PMSM is a term more commonly used in industrial drives, automation, and automotive sectors.
Over time, these terms became associated with different control methods, even though motor construction often overlaps.
Back-EMF Shape: Trapezoidal vs Sinusoidal
One commonly cited difference is the shape of the motor’s back electromotive force (back-EMF):
BLDC (Traditional Definition)
- Back-EMF is approximately trapezoidal
- Optimised for six-step (trapezoidal) commutation
- Two motor phases are energised at a time
PMSM (Traditional Definition)
- Back-EMF is approximately sinusoidal
- Optimised for sinusoidal commutation or field-oriented control (FOC)
- All three phases are driven continuously with sinusoidal currents
In theory, motors designed for trapezoidal back-EMF perform best with six-step commutation, while motors with sinusoidal back-EMF perform best with sinusoidal current waveforms.
In practice, many modern motors fall somewhere between these idealised shapes, and advances in control algorithms allow a wide range of motors to be driven effectively using FOC regardless of back-EMF profile.
Control Strategy: The Real Differentiator
The most meaningful difference between BLDC and PMSM systems lies in how they are controlled, not in the motor hardware itself.
BLDC Control (Six-Step / Trapezoidal)
- Simple commutation logic
- Lower computational requirements
- Often uses Hall sensors for rotor position
- Produces torque ripple due to phase switching
- Generates more acoustic noise at low speeds
This approach is robust and cost-effective, making it suitable for fans, pumps, and appliances where smoothness and precision are less critical.
PMSM Control (Sinusoidal / FOC)
- Uses mathematical transformations to control torque and flux independently
- Requires higher processing power
- Typically uses encoders or sensorless observers
- Produces smooth torque with minimal ripple
- Higher efficiency across a wider operating range
Field-oriented control is now widely used for both PMSMs and motors marketed as BLDCs, further blurring the distinction.
Sensors vs Sensorless Operation
Both BLDC and PMSM systems can be implemented with or without physical position sensors.
- Sensored systems use Hall sensors or encoders to provide absolute rotor position.
- Sensorless systems estimate position using back-EMF or model-based observers.
Historically, BLDC motors were more commonly associated with Hall sensors, while PMSMs were associated with encoders. Today, both motor types are frequently operated sensorless, particularly in cost-sensitive or harsh environments.
Sensor choice affects:
- Startup behaviour
- Low-speed torque control
- Cost and reliability
- System complexity
Again, this is a system-level decision rather than a motor-type limitation.
Efficiency and Performance
When properly controlled, both BLDC and PMSM motors can achieve very high efficiency. Differences in real-world performance are usually driven by:
- Quality of current control
- Accuracy of rotor position estimation
- Motor design (magnet type, winding layout, thermal design)
- Operating speed and load profile
PMSM systems using FOC generally offer:
- Better low-speed smoothness
- Lower acoustic noise
- Higher efficiency under partial load
However, a BLDC motor driven with FOC can achieve very similar results, reinforcing the idea that control method matters more than naming convention.
Industry Usage and Naming Conventions
Different industries tend to favour different terminology:
- Consumer appliances / HVAC: BLDC, EC motor
- Industrial automation: PMSM, PMAC
- Automotive / EV: PMSM
- Electronics / robotics: BLDC or PMSM depending on context
This often reflects market expectations rather than fundamental technical differences.
Practical Rule of Thumb
A useful and accurate way to think about this is:
BLDC and PMSM describe the same class of permanent magnet motor.
The name usually reflects the intended control strategy rather than the motor itself.
- Six-step commutation → commonly called BLDC
- Sinusoidal or FOC control → commonly called PMSM
Modern controllers increasingly support both approaches, allowing the same motor to be driven in different ways depending on application requirements.
Making the Right Choice
When specifying a motor system, it is more important to focus on:
- Required torque and speed range
- Efficiency targets
- Smoothness and acoustic requirements
- Sensor vs sensorless operation
- Controller capability and tuning flexibility
Rather than asking “BLDC or PMSM?”, a better question is:
“What level of control performance and efficiency does my application require?”
Conclusion
Brushless DC motors and permanent magnet synchronous motors are far more alike than their names suggest. They share the same fundamental construction and operating principles, with most meaningful differences arising from control strategy, application context, and historical naming conventions.
As motor controllers become more powerful and flexible, the boundary between BLDC and PMSM continues to fade. Understanding this allows engineers and system designers to focus on what truly matters: selecting the right motor-controller combination to deliver reliable, efficient, and optimised performance in real-world applications.
Need to know more? Contact our motor control experts today.