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I am looking for a cheap (preferably under 2000 USD) electrical motor that can spin freely in either direction when the motor is unpowered. The motor will be used as an alternative source of power to a hand-cranked ferry.

There are three cranks: one on the ferry + one at each docking station. All three cranks are connected - when one turns, all turn the same direction.

At this point, we're just looking for a quick proof-of-concept hack that interferes minimally with the current operation of the ferry. What we have in mind is a small electrical motor mounted near each of the two cranks on shore, directly connected to the crank axles using belts or bike chains. To keep it as simple and cheap as possible for this quick proof-of-concept, we were hoping to find a motor that spins with minimal resistance in either direction when unpowered.

Our first idea was to use a direct drive brushless motor designed for bikes, since they presumably do not need a clutch to spin freely, but from what I have read, such motors don't like high load at low RPMs, so they don't seem ideal for our use-case. Our cranks will spin at about 30 RPM. The load varies depending on the weather, but rarely more than an adult person can crank with one arm.

Does anyone know of an off-the-shelf product that meets our requirements?

I have googled for a geared motor with an internal magnetic clutch that automatically releases when the motor is unpowered, but I've found no good candidates so far.

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Any motor that does not have permanent magnets should work fine, given the low rpm.

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  • $\begingroup$ Thanks for the feedback. I assume such motors are called induction motors? In that case, would something like this work for us? The rated speed is 30RPM but I'm a bit concerned about the 60:1 gear ratio. Does that mean that when someone hand cranks the unpowered motor at 30 RPM, it has to spin internally at 30×60 = 1800RPM? I would think that would generate a lot of resistance. I have no experience in this area so I hope my intuition is wrong because it would make things simpler. $\endgroup$ – Bob Builder Jun 17 at 10:56
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I sounds like what you might want is an electrostatic motor. if there is current running through the motor then there will be a magnetic field which will help the motor lock in place. if there is no current, then the motor should spin freely.

Note: this is just for the motor, if there is a gearbox on the output shaft of the motor, it will still take some force to spin the output of the gearbox. If you need a gear box, but you still want the shaft to free spin when it's not in use, then you might need to have a clutch type system which engages and disengages the output of your gearbox.

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  • $\begingroup$ Thanks for the tip, the characteristics of these motors seem to be ideal for our use-case. Unfortunately, it also looks like high torque electrostatic motors is a new technology, because the only thing close to a product that I have found is developed by C-Motive, and it doesn't exactly look like an off-the-shelf thing that we can try out. :( $\endgroup$ – Bob Builder Jun 18 at 20:26
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There are plenty of motors that spin freely when unpowered. BUT, given the low RPMs you're going to need a gear reduction, and that is not going to spin freely when unpowered.

I would recommend using a freewheel gear or sprocket, that way the gearbox only turns when the motor is running. One designed for a bicycle might work.

If you can't make this work with only one direction of rotation, look into centripetal clutches (or electronic ones).

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