I am trying to understand why would the back emf in bldc be trapezoidal. Lets consider a simple motor with two stator coils placed opposite to each other. Lets say a bar magnet is kept as a rotor. If i manually crank the bar magnet and rotate it, then the back emf observed seems to be trapezoidal as per my reading. However, i have an alternate thought. If voltage induced in a coil is supposed to be proportional to rate of change of magnetic flux linkage, then when the bar magnet pole sweeps a coil area during its rotation, this should result in a constant voltage in the coil since field in coil due to magnet increases linearly as magnet pole approaches the coil. And when the pole moves away from the coil, as magnetic flux linkage decreases linearly, there would be negative voltage. Once bar magnet pole passes the coil area completely , then there will be no change in magnetic field in the coil hence zero bemf. This results in a positive and negative pulse followed by zero voltage bemf. Why does the bldc emf not behave the way i mentioned?
For best performance, the drive current should match the back EMF waveform, whether it is trapezoid or sinusoid.
As the leading edge of a rotor pole or moving magnet leaves the stator fixed pole surface the amplitude drops at a linear rate according to the surface velocity of the rotor. While the rotor pole surface is smaller than the stator magnetic field, if/when they overlap for some distance, then the BEMF is constant during that arc path.