I have a BLDC motor on an autopiloted model airplane, running with an ordinary PWM-controlled ESC.
I have a feature I'd like to add which the ESC I have doesn't support, so I either want to find one that does or to learn enough to build one that is integrated with the rest of my (mostly home cooked) autopilot.
The feature I'd like to add is a mode that stops the prop and holds it in a horizontal position (it is a 2-blade prop) both to reduce drag during glides and for landing so that the pusher prop on my flying wing won't be as easily damaged by the ground.
From what I understand of how these motors work, it should be possible for the usual 6-MOSFET driver circuit to hold the output shaft in any of 6 (maybe even 12, if you leave one high side and the other two low sides on etc?) positions. Of course, without sensors, I'm not sure if it is possible to get it to that position in the first place? (I mean, if the motor were entirely off and I just connected certain leads and left them on — with appropriate current — would the shaft spin to that position? Or might it not have enough torque to move at all from some starting positions?)
Has anyone tried this? Thoughts or advice?
Best Answer
I think if the speed controller was completely off, and you connect it to the battery, it would snap to some position. However, depending on the number of poles in the motor, you might find that it's not just one of six positions, but one of a multiple of six positions. In other words, you could get it to snap into line with one of the phases, but there may be multiple points that represent that phase, and the motor will likely just seek the nearest one.
Of course, if the ESC were to become activated while you had this alternate connection in place, you'd probably smoke at least a couple of the MOSFETs, so, hazard there to worry about. As long as it's off though, I don't think the speed controller would be harmed, b/c if you think about it, those MOSFETS routinely switch the battery voltage to make approximately 3-phase AC out of it. There should be protection diodes in the 6-way bridge that will protect it from the inductive kick-back when you break your alternate connection, too.