I would like to ask a question about zero crossing event in a trapezoidal commutation on a brush-less DC motor. Here is the waveform that shows that the zero crossing event occurs every 180 electrical degrees in a sinusoidal commutation:

But what about trapezoidal commutation. Here is the waveform that I found about the trapezoidal commutation:

So as you see, the zero crossing occurs 30 electrical degrees after the previous commutation and 30 electrical degrees before the next commutation. In a motor with one pole pair, we would have 30 electrical degrees = 30 mechanical degrees, so we would have this waveform: zero crossing You see that the zero crossing in phase A occurs when the magnet faces the phase C, or in other words, after 30 electrical degrees from the last commutation. My question in why does the zero crossing happen at that moment, why not after 60 electrical degrees, or 15 electrical degrees? Is it related to some law's of induction? What are those law and how do this law's appear in this motor? Can someone explain to me this with some pics?


Browse other questions tagged or ask your own question.