Why is BLDC used more than PMSM for model airplane?

In the final analysis, it is the difference between PMSM and BLDC. The difference between the two. Let's simply say.

Permanent magnet synchronous motor (PMSM) can rotate at a constant speed synchronized with the power frequency, and is not affected by the load and line voltage. Therefore, PMSM is an ideal choice for high precision constant speed drive. The speed / torque characteristics of PMSM are very suitable for directly driving high horsepower, low speed (RPM) loads.

The magnetization of BLDC motor permanent magnet and its distribution on the rotor are selected, which can make the waveform of back EMF (voltage induced by rotor movement in stator winding) trapezoid. In this way, the DC voltage with rectangular waveform can be used to generate the rotating magnetic field with low torque ripple. Because the applied rectangular voltage is easy to produce, such as any MCU, or C2000 series DSP or even 51 single chip microcomputer can produce this PWM wave, so the control and drive of the motor become simple. However, the position of the rotor must be known at an angle so that the applied voltage is aligned with the back EMF. The alignment of back EMF and commutation is very important. Only in this way, the motor can operate with the highest efficiency as a DC motor. Therefore, BLDC motor becomes the best choice for low cost and high efficiency application.

Is PMSM a low level BLDC in terms of control? No, you can also use foc (vector control) and DTC (direct torque control) to achieve the control effect and performance similar to DC motor, but the cost is really large, and the algorithm is really not simple.

Summary: both are synchronous motor, BLDC DC power supply, back EMF trapezoidal wave; PMSM sinusoidal current power supply, back EMF sinusoidal wave.

So the main question is: if I have a motor now, how can I judge which is PMSM and which is BLDC?

Is it observable in terms of performance? Performance is not very accurate, you can first look at the motor driver, power is DC, or three-phase AC. One is to see the waveform of the back EMF.