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I'm making a bicycle generator, and am trying to search for the right motor. I need to know at how many rpms a motor would need to be rated to generate a significant amount of electricity (like, to charge a laptop). Also, is there any type of motor from an old device that would work well for this (like an electric weed wacker motor). Edit: I'm planning on using a 2750RPM scooter motor (24V, 250W)

Thanks

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  • \$\begingroup\$ Motor or generator? In any case, it is really not (only) about RPMs... \$\endgroup\$
    – Eugene Sh.
    Nov 5, 2015 at 20:06
  • \$\begingroup\$ Motor, sorry, used a bad tag \$\endgroup\$
    – Kemosabe
    Nov 5, 2015 at 20:09
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    \$\begingroup\$ Not at all. If you want to generate, it is definitely generator. \$\endgroup\$
    – Eugene Sh.
    Nov 5, 2015 at 20:10
  • \$\begingroup\$ @EugeneSh.: I think he's looking for a device that's designed to be a motor so that he can use it as a generator. Kemosabe: That doesn't always work in every case. AC induction motors, for example, are incapable of generating electricity except for regenerative braking by a controller that excites them that way. \$\endgroup\$
    – AaronD
    Nov 5, 2015 at 20:10
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    \$\begingroup\$ Do you understand, that loading the generator with somewhat significant load will increase the physical effort required for pedalling? \$\endgroup\$
    – Eugene Sh.
    Nov 5, 2015 at 20:24

4 Answers 4

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This got a bit long for a comment:


All motors have a different base speed for the same voltage because of their geometry. The formal specification for this is Kv, or RPM/volt.

Look for something that has a permanent magnet in it, like Brush[ed/less] DC, and then look for a Kv rating that is roughly close to what you can achieve. (Note: it may be difficult to find in some cases and must be calculated from other specs)

The permanent magnet ensures the motor does not require external excitation to generate electricity. Two types of motors that can't do this are Universal and Induction, both of which are very common in AC-powered equipment. A Universal motor may also appear in DC-powered equipment because it works there too and may have some advantages over a permanent magnet in its intended application.

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  • \$\begingroup\$ "Generate electricity all by itself" is a bit of a misnomer. I understand what you are trying to say, but someone not well versed in motors might not... \$\endgroup\$ Nov 5, 2015 at 20:59
  • \$\begingroup\$ @KurtE.Clothier: I think I see what you mean, but I'm not sure how to edit without introducing other uncertainties. (I wasn't entirely confident in the original either.) Would you like to take a shot at it? \$\endgroup\$
    – AaronD
    Nov 5, 2015 at 21:06
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The problem you will face is that there are few common motors that will generate high enough voltages for your application while spinning only at 100 RPM or so. For instance, your 2750RPM scooter motor (24V, 250W) will be outputting less than 1 volt at the 100 RPM you are likely to pedal. So you will either need a gearing system or a very small RPM/volt constant.

You could also use a 3 phase brushless dc motor and run the leads through a three phase rectifier to get DC. This will open up a lot of motors that are used for hobby boats/cars/helicopters. If you are really gung ho, you could even wind your own to get a voltage constant in your wheelhouse.

Unless you are in really good shape, you can probably only get around 100 Watts average on a good bike with gears. Probably 300 Watts for around a minute before you give/throw up.

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  • \$\begingroup\$ I think you're assuming I'm just attaching a chain to a gear (correct me if I'm wrong), but I'm planning on having a belt around the whole back wheel and the motor. If I used the latter, would this change things? \$\endgroup\$
    – Kemosabe
    Nov 5, 2015 at 23:16
  • \$\begingroup\$ I wasn't sure if your bicycle had gears or what your mode of attaching the crank to the motor would be so I assumed 1:1. In any event, consider what your gear ranges are and pick a motor such that when you are pedaling 100 RPM in the "downhill gears" you almost overvoltage your motor. Cause once you start to load it you want to be able to shift down which will lower your motor RPM. \$\endgroup\$
    – Matt
    Nov 6, 2015 at 14:08
  • \$\begingroup\$ Right, but (I didn't mention this) the idea is to be spinning on a low gear for a sustained period, like half an hour of a rather easy gear. Also, I don't see how shifting down will over-voltage the motor? (correct me if I read your response wrong) \$\endgroup\$
    – Kemosabe
    Nov 7, 2015 at 1:32
  • \$\begingroup\$ In the downhill gear you get many wheel revs for 1 pedal rev. I'm just saying you will want that to be very close to the max rpm of your motor because when you start changing to more uphill gears your rpm and voltage will decrease. \$\endgroup\$
    – Matt
    Nov 7, 2015 at 16:13
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A permanent-magnet, brushed DC motor would be best. It will work as a generator without needing a battery to initiate power generation. The output voltage will be proportional to RPM. At the motor's rated speed for motor operation it will generate something close to its rated voltage. If you turn it at half of that speed you will get about half of rated voltage, but you can load it with its rated current and get about half the rated power.

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You could use a rear wheel hub motor. Pick a non-geared one, without freewheel (or jam the freewheel).

These usually reach peak efficiency at 20 km/h. Just add a bicycle and a solid stand to keep the rear tyre away from the ground.

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