PMSM or BLDC Motor: What is the difference between these two types of permanent magnet motors?

The technical specifications sometimes refer to "BLDC" motors, while others use "PMSM," even for motors that appear identical. This is not a coincidence: these two terms refer to very closely related types of motors. This guide explains what truly distinguishes them, and why this distinction is particularly important for the controller.

What they have in common

The two motors have:

• permanent magnets are mounted on the rotating part, the rotor;
• certain windings on the stationary part, the stator, distributed across three phases;
• `brushless`: This refers to the controller, which switches the current electronically, hence the name "brushless," meaning "without brushes."

The hub motor in an electric scooter is of this type, with a particular characteristic: the rotor is located on the outside. The magnets are glued to the inside of the rim, which rotates around the stator that is fixed to the axle. This is known as an outrunner motor.

The real difference lies in the counter-electromotive force

When you manually rotate a permanent magnet motor, it generates a voltage at its terminals: this is a dynamo effect. This voltage is called the back electromotive force (back EMF). Its shape depends on how the windings are arranged and how the magnets are configured.

With square wave control, the current is delivered in steps across two phases simultaneously. If the motor voltage has a similar stepped waveform, the torque remains almost constant: this is the principle behind BLDC motors. With sinusoidal control, the current varies smoothly across the three phases; if the motor voltage is also sinusoidal, the torque is perfectly smooth: this is the principle behind PMSM motors.

In reality, the boundary is blurred

Most real-world motors fall somewhere between these two extremes: their voltage isn't a perfect trapezoid nor a pure sine wave. That's why the same scooter motor can be sold under different names.

In practical workshop experience, most scooter hub motors perform very well using vector control (FOC), which makes them quieter and smoother than when using square wave control. The exact shape of their voltage only has a minor impact on the overall result.

And what about synchronous motors and asynchronous motors?

"Synchronous" means that the rotor rotates exactly at the same speed as the magnetic field created by the windings: this is the case for motors with magnets, such as BLDC (Brushless DC) and PMSM (Permanent Magnet Synchronous Motor). The asynchronous motor, on the other hand, does not have magnets: the magnetic field of the rotor is induced by the stator, and the rotor rotates slightly slower than the magnetic field. Robust and without magnets, it is less compact for the same power output, and it is rarely found on small vehicles.

Surface-mounted or internal magnets

Among PMSM motors, we can further distinguish between surface-mounted permanent magnet (SPM) motors and those with magnets embedded within the rotor (IPM). This difference significantly affects high-speed performance, which is the subject of our guide IPM and SPM motors.

What changes when you choose a motor

• The terms BLDC or PMSM are less important than the voltage, current capacity, cooling system, and speed constant of the motor. Refer to our guide on Kv, Kt, and motor poles.
• The controller must be able to read the sensors that the motor is equipped with.
• Our electric scooter motors are brushless DC motors, available in High Speed and High Torque versions.