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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?

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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.

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  • \$\begingroup\$ Interesting idea, to just add an alternate set of connections to my existing ESC. That wouldn't have even occurred to me. It's easy enough to set up, so I'll try it and see what happens. Good point about the number of poles, I will experimentally investigate that at the same time. \$\endgroup\$
    – Doug McClean
    Commented May 12, 2011 at 22:45
  • \$\begingroup\$ @Doug McClean - just make sure you don't try to drive the motor with both the ESC and whatever else you devise, at the same time. There will almost certainly be smoke. \$\endgroup\$
    – JustJeff
    Commented May 13, 2011 at 2:15
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    \$\begingroup\$ I think the proper way to do this would be to reprogram the ESC to activate the necessary FETs for this mode. However, current draw is going to be involved; this may result in deeper discharge of your drawn down battey than is "healthy" for the typical Lipo cells, and if you are planning to operate the radio off the same battery, could result in drawing it down to the point where you loose the radio. Plus without at least one sensor, it's as likely to cog in and undesired position as a desired one. \$\endgroup\$
    – Chris Stratton
    Commented May 19, 2011 at 7:14

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