I understand that back emf of PMSM is sinusoidal and BLDC is trapezoidal. So I am wondering is there any difference between stator windings of PMSM & BLDC or only driving technique makes difference?
PMSM motors are quite similar to Brushless DC (BLDC) Motors, in that they have the same structure and the same components. Both motors have permanent magnets in the rotor that interact with the magnetic field produced by the stator coils. PMSMs have a different Back-Electromotive Force EMF, since the flux linkage between the stator and the rotor is not trapezoidal. In the PMSM, the EMF is sinusoidal. This is an intentional effect produced by the way the coils in the stator are wound (in a sinusoidal fashion), while in the BLDC motor the stator coils are evenly wound, figure below shows EMF waveforms for both motor types.
Hope that help you
Dear Sudhir Suryavanshi,
Related to your question "is there any difference between stator windings of PMSM & BLDC ", yes there is. BLDC's have concentrated stator windings (coils wound around each tooth), while PMSM's have distributed stator windings (coils spanning more than one tooth).
Theoretically the difference lies in the back-emf as stated by the previous readers. However in a practical machine the back-emf of a BLDC will be filtered out to some extend by the parasitic components in the windings.
Most BLDCs are driven by a six-step commutation process as this is quite easy to implement when a hall-effect sensors is available or a sensorless observer is integrated. However the power quality of the stator is quite poor in this case. A BLDC even if it is a low-cost inrunner/outrunner can be driven using vector control which keeps the stator electromagnetic field at 90 electrical degrees with the permanent rotor magnetic field for maximum torque. The power quality of this approach is significantly improved as the three phase currents are of a sinusoidal nature. This is not typically used on low-power high speed motors as an absolute measurement of the rotor angle is required. The measurement can be obtained through a magnetic/optical encoder however this may be cumbersome to fit mechanically.
The sensorless control of a BLDC with vector control has been reviewed in the following works:
Conference Paper Modelling of a Sensorless Rotor Flux Oriented BLDC machine
Thesis Design and Implementation of an Electronic Speed Controller ...
The stator windings has its impact on the difference between BLDC and PMSM Motors. The BLDC Motor generates trapezoidal back emf based on the windings of the motor with respect to the slot pole combination chosen for it. Similarly the PMSM Motor generates sinusoidal back emf based on the windings of the motor with respect to the slot pole combination chosen for it too.
The concentrated windings can produce trapezoidal and even sinusoidal back emf based on the slot-pole ratio. And the distributed windings generate the sinusoidal back emf. But all motors cannot be wound with distributed windings to generate the sinusoidal back emf so the slot pole ratio plays a major role.
In a BLDC motor, the stator winding is concentrated whereas, a PMSM has a distributed winding. Hence, the BLDC motor has a trapezoidal back-emf and the PMSM has a sinusoidal back-emf. The BLDC motor is driven by a six-step electronic commutation process, which is relatively simpler compared to the control of a PMSM, but suffers from poor power quality. This can be improved by opting for vector control of the drive, and is suitable only for high-power drives.
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