Are BLDC motors and BLAC (PMSM) motors same?

Brushless AC (BLAC) or Permanent Magnet Synchronous Motors (PMSM) seems to be similar in construction and in working as well. Then aren't both one and the same thing? What is the difference between them?



  • edited May 2020

    Although a BLDC motor and PMSM motor look very similar, the way their stator wound is different and also their control mechanisms are different. I'm not saying BLAC motor here, because technically an induction motor is also a Brushless AC motor. Isn't it?

    What is the difference between BLDC motor and PMSM motor?

    • BLDC motors have Concentrated stator winding; while PMSM motors have Distributed stator winding.
    • As torque is a function of current and back-emf (Eb), and Eb depend on how the stator is wound -- BLDC motors with concentrated winding will have Trapezoidal back-emf; and PMSM motors with distributed winding will have Sinusoidal back-emf.
    • Driving current for BLDC motors is in Trapezoidal waveform; whereas, driving current in PMSM motors is in Sinusoidal waveform.
    • The supplied currents must be synchronized with rotor position feedback which can be achieved from sensors (such as hall effect sensor) or by evaluating motor's back-emf signal. In BLDC motors, six-step trapezoidal commutation is achieved with the help of three hall sensors feedback signal or back emf signal. Basically, rotor position is monitored in 6 steps in a BLDC motor. Hence, this six-step communication causes small torque ripples in the torque output of the motor.
    • On the other hand, a PMSM motor requires only one hall effect sensor, as the rotor position can be monitored continuously by evaluating sinusoidal back emf. Therefore, torque ripples are absent in PMSM motors which results in smooth torque output and increased efficiency.
    • Trapezoidal commutation is a bit easier to control than controlling PMSM motors which require complex three-phase sinusoidal PWM circuitry.

    Above image credit (concentrated and distributed winding):

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