swbluto wrote:Thanks for the illustration. I was recently under the impression that the caveat you brought up affected performance in the region of 5-10%, but it certainly seems to be fairly important.
Yes, well the level of importance really depends on the motor and RPM under question. But when the inductive impedance of the windings at the commutation frequency starts to reach the same order as the ohmic resistance, then the effect can be very significant, and as illustrated with the geared eZee/BMC motors it is more than a factor of 2. If you look at an oscilloscope trace of the waveform, the current never comes close to leveling out at the value predicted by (V-emf)/R. It is still ramping up when the commutation event takes place, then it takes a big hit during the commutation as the current steers away from one phase to the other. The graph here shows a Nine Continent motor spinning at 510 RPM, with both the actual drive voltage when the controller is on (pink) and the back-emf voltage when it is off (green) showing. The blue line is the phase current when the controller is on.
Notice how the current through the phase is always ramping upwards? With a sinusoidal motor driven by a 6-step controller as is the case here, we would expect the current waveform to look a bit like a 'w', but here it is a 'w' on a pretty big slant.
By the way... can you tell me how the "commutation" period is so long at low speeds?
It seems obvious that the phases are activated for longer periods of time at lower RPM, but it also seems that you're encountering PWM at the lower speeds due to current limiting. Is the "commutation period" of the PWM waveform not particularly important (Once current limited has deceased at higher speeds, the PWM disappears and the phase waveform becomes the only one existent), and it's mostly just the phase current's period?
I think on my 6th read over that I see what you're getting at! In the context above, by "commutation period" I just meant the time period between one commutation event and the next, not the actual time duration that it takes the current to switch over from the non-driven phase to the newly driven phase. The actual time required for the commutation to take place is dependent on a) the current through the windings, b) the inductance of the windings, c) the back emf voltage, and c) the supply voltage.
I'm completely ignoring PWM in all of this, it's not part of the equation at all. At PWM frequencies the winding inductance is assumed to be for all intents and purposes infinite, and that approximation seems fine.