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Can you run a BLDC motor backwards without damage?

Is it OK to drive a model airplane BLDC engine backwards while landing, so it gets a little "reverse thrust" and come to a stop a little quicker on the runway?

Is it OK to drive a model helicopter BLDC motors backwards so it can hover upside-down?

Or do I need to design the hardware so that it never drives the motor backwards, under any circumstances, no matter what the pilot on the ground does at the transmitter?

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  • \$\begingroup\$ The motor should be fine, but if you are talking about a helicopter, not a small quadcopter or anything like that you'll have to do this mechanically by changing pitch anyway. The blades themselves can not run backwards for aerodynamic reasons, and starting/stopping them takes some time where the helicopter would crash if not high enough. Some helicopters also have freewheeling mechanics that can not be driven backwards. \$\endgroup\$ – Andreas Wallner Mar 16 '14 at 20:15
  • \$\begingroup\$ Some 3D quadcopters actually do reverse their direction of spin and it works surprisingly well, e.g. Heli-Max Voltage 500 3D and Blade 200 QX. Though these probably came out after this question was written. Also there are many model helicopters which hover inverted due to changing blade pitch instead of motor direction, and not just large ones; take the Blade Nano QX for example, a micro indoor heli with a working swashplate and collective pitch for flying inverted. \$\endgroup\$ – Ricket Jun 8 '16 at 5:28
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BLDC motors usually just use permanent magnets on the rotor (be it in-runner or out-runner) and use a set of windings on the stator connected in a three-phase delta or wye configuration. The speed controller just generates a variable-frequency, three phase waveform to power the motor. Since the windings are symmetric, electrically there's no reason you can't turn the motor in either direction.

As for whether it's a good idea to run a prop backwards on landing, that's more of an aeronautics problem than anything inherently electronic. Having flown some r/c planes, it seems to make sense to me that if you reverse the prop on landing, you're basically just applying a braking force along the line of the axis of rotation. If that line passes above the center of gravity (not below it), that should torque things so that the tail will stay down, so you should be stable if that's the case. If the prop axis is below the CG, though, you're looking at forward torques that would drive the nose down, which would result in damage.

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  • \$\begingroup\$ Excellent. My understanding is that both pusher props and tractor props are designed so they apply force directly through the center of gravity, precisely for the reasons you've mentioned here. I'm assuming that the line of force doesn't change when you reverse direction -- does that sound reasonable? If the airplane is stable when you go from cruising down to the runway and you suddenly kill the power to the motor on touchdown, then I'm guessing that a little bit of reverse thrust is going to be just as stable. \$\endgroup\$ – davidcary May 29 '10 at 6:16
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Yes, you can drive a brushless DC motor in both directions.

See, for example, the On Semiconductor MC33035 brushless DC motor control chip, which has a pin to control direction.

Here's a little explanation from p. 9 of the datasheet:

The Forward/Reverse input (Pin 3) is used to change the direction of motor rotation by reversing the voltage across the stator winding. When the input changes state, from high to low with a given sensor input code (for example 100), the enabled top and bottom drive outputs with the same alpha designation are exchanged (AT to AB, BT to BB, CT to CB). In effect, the commutation sequence is reversed and the motor changes directional rotation.

I believe you do have to be careful about "shoot-through"-- if you're trying to switch the direction of current flow in a winding, you have to be sure to turn one set of FETs off completely before you turn on the other set, or you may inadvertently short your power supply.

You might google "adaptive gate drive" or "dead time" for more details.

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  • \$\begingroup\$ I have to handle dead time properly in order to avoid shoot-through even in the normal forward-running case. I'm glad you reminded me to check this, though -- it's quite possible that covering the full range from fast-forward to fast-reverse may require tweaking the dead time. \$\endgroup\$ – davidcary May 29 '10 at 6:36
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Thrust on helycopters is controlled by varying the propeller pitch, not the motor speed/direction. In an helycopter, the main rotor motor turns almost always at the same speed.

Inverted hovering needs a special designed swash plate that allows positive and negative blade pitch.

For the original question, yes, you can drive a brushless dc motor in both direction. Stopping it quickly (with a propeller connected to it), change direcion to the thrust, and keeping your plane on the track, is another story :-)

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  • \$\begingroup\$ Thrust on full-size manned aircraft is controlled by varying the propeller pitch or the articulated rotor pitch, keeping the petrol-powered engine running at a constant speed. Thrust on model quadcopters and other model multi-rotor helicopters and many model rigid-wing aircraft is controlled by varying the electric motor speed. Each drive motor is connected to a rigid propeller. Such helicopters don't have a swash plate. Inverted hovering requires changing the direction of the motor. spectrum.ieee.org/automaton/robotics/robotics-software/… \$\endgroup\$ – davidcary May 29 '10 at 6:11
  • \$\begingroup\$ True for every single point you've cited. But the original question was about model RC planes (so thrust -> motor speed) and model RC heli and inverted hovering (so thrust -> pitch). Coaxial multi-rotor heli AFAIK can't do inverted hovering. Quadcopters are another matter, and I've yet to see a quad that can REALLY go upside-down during flight... \$\endgroup\$ – Axeman May 29 '10 at 11:43
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What you have to worry about most when you reverse direction of a motor, is that you do not put too much current into either the motor or the electronics/switches that control it.

When you connect a voltage source across a motor that is at rest and either has a large inertia or a locked rotor, you get a large current flowing through it = V / R where R is the stator winding resistance of the motor. This is called the stall current.

If you are running at full speed with a voltage source across a motor, and you immediately reverse the polarity of the voltage source, you can get up to 2x the stall current, because the voltage source is then at the opposite polarity of the motor's back-emf. This can be too much current, and if that's the case then you have to control the rate at which you reverse voltage across the motor, by using PWM or some other way besides a hard voltage reversal.

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