trapezoid bemf winding motor
We have two main types (may be more, pls correct me if I'm wrong) of permanent magnet (PM) brushless motor (BLM), i.e. PM-DC-BLM and PM-AC-BLM. We always refer PM-DC-BLM as trapezoidal type and PM-AC-BLM as sinuisoidal type. Apart from the motor structure and winding topology, I agree with you that the shape of the motor's back-EMF waveform will tell whether the motor is sinusoidal type or trapezoidal type. The back-EMF waveform across stator winding (for BLM) actually tells how the magnetic field (generated by rotor, e.g. PM) changes when it's spinned (by drill or latch machine). For BLM, the variation of rotor's magnetic field can be determined either with sensor or sensorless scheme. To drive the motor efficiently, the stator winding should be powered (drived) in such a way that the magnetic fields generated by stator winding and rotor are synchronized. Therefore, in my opinion, if we power a trapezoidal type of PM-BLM with sinusoidal control scheme, the performance of the motor will not be optimum.