Turning a hub dynamo to a hub motor?

[SamTexas]

Hello all,

There is absolutely no practical/economical gain from this discussion. I just want to pick your brain for the sake of learning and tinkering.



How do I turn a hub dynamo into a hub motor? I have a Shimano HB-NX20 hub dynamo in my front wheel. I rarely every use the head light because I ride mostly in daylight. In use or not, it always generates some drag (probably around 2 watts). So instead of letting it rob me of my precious power, I would like to turn the situation around, applying some battery power to it to make it a motor. I know it's more complicated than that and that's why I'm here.

Thanks and happy New Year.
Sam

 

[Jeremy Harris]

I guess it's possible to do this, with some effort, but the benefit will most probably be very small, maybe not even enough to really notice. Hub dynamos are usually alternators, often single phase, with no rectification built in - they normally feed raw AC to the lights, often at a pretty low voltage around 6V nominal.

The big problem to overcome is to generate frequency synced AC single phase (I think these things are normally single phase AC) to feed to the dynamo. No off-the-shelf ebike controller will do the job, as far as I know, so it means making a custom single phase AC controller and adding a method for detecting the dynamo rotor position with enough accuracy that the drive signal can be kept in sync to give power. It might be possible to add a Hall sensor to the dynamo and feed that signal back to trigger the controller, but it seems like a lot of effort to go to to gain maybe 5 watts or so of useful power.

Doing the opposite, using a direct drive hub motor as a three phase alternator, is easy, not that it's any consolation in your case.......................

Jeremy

 

[SamTexas]

..., but the benefit will most probably be very small, maybe not even enough to really notice.......

I know. It's just for the sake of learning and tinkering.

 

[spinningmagnets]

There was a lot of discussion over at otherpower.com about the similarities between motors and generators, because a frequent hobby was to convert large surplus auction motors into generators/alternators as the heart of a wind-generator. So, you needed to identify the best candidates to make even a cheap purchase worthwhile.

I am still learning, but the biggest problem I see is that the dynamo is likely single-phase, as Jeremy has indicated. The reason they used single-phase is to make a light generator that is very simple and cheap. A filament-bulb can accept very crude pulsing dirty energy. Perhaps if you are pedaling slowly you could even see the light flicker?

Since it is probably brushless, in order to use it as a motor, you would have to find a single-phase brushless controller. I dont know if anyone makes something like that. If it has brushes, it may be possible to adapt a cheap brushed controller to work with it.

Another major consideration is brushless motor/generators can be fiddled with using a wider variety of voltages. A brushed is limited by the spacing of some of the parts. Lower voltage devices can have some parts close together, higher voltage parts must be spaced farther, therefore a low-voltage dynamo being used as a motor would reach a voltage limit where sparks were jumping across components and causing shorting.

Putting more amps through tiny brushes than they are sized to take, will lead to rapid erosion and failure. Even if you dont mind it dying in a week due to accelerated brush wear (due to over-amping), the coils and body are only sized to absorb and shed a certain amount of heat.

Over-volting a component is a hobby around here, and cheap magnets will de-magnetize if gotten too hot. The magnet glue will soften, and the enamel on the copper wire in the coils fry, leading to a short. using too many watts will fry it in less than 5 minutes.

When you are pedaling fast, is the light as bright as a 60W bulb?, perhaps a 40W bulb? If the dynamo gets warm at a high constant load, then that load is roughly the amount of watts you could expect from it as a motor (if you could get a controller to run it as a motor).

A third problem I see is the tiny diameter and that it spins at the low wheel RPMs. Giving the the magnet/coil interaction a very tiny leverage to spin a full-diameter tire will strain it, like a cordless drill spinning a large diameter bit in a deep piece of wood. Just a guess, I hope this has helped...

 

[SamTexas]

Thanks for the input, spinningmagnets.

 

[docnjoj]

There are several very small and light geared motors in the 2-3 kilo range that are literally about the same size as that generator but are actually designed as motors. Tongxin Nano and the small Cute series from Cellman would save a lot of fruitless effort. I hope this helps.
otherDoc

 

[fruktstav]

Reviving an old thread here, because I'm also experimenting with this.

I've got a Shimano 6v dynamo, two stroke, in the sense that it goes between positive and negative 6v on one revolution. So I tried to just hook up a battery to the inputs, which obviously yielded no success what so ever, the wheel is actually kept in place slightly by the magnetic field.

What I need to do in order to make the dynamo useful as a motor, is to build a circuit that analogously changes the polarity of the battery in unison with the 'natural' current of the coil in the dynamo.

So I came up with this, I might be making any number of fatal mistakes with my schematic, but please review it and if anyone needs a further explanation, please ask.

Any thoughts?

 

[major]

Consider using an H bridge. http://en.wikipedia.org/wiki/H_bridge

 

[ak74]

Just recently found this thread when pondering a possibility to arrange golf style 'handicapped' races on two wheels. More on that at the end of the post, now to the point:

What if you used an ordinary bike speedometer sensor (a magnetic switch) to cut the current.
- Normal DC battery, a bit overvoltage of course
- Other wire straight to the hub, other via a speedometer sensor.
- Magnets (magnet) in dynamo hub are probably rotating and coil is static.
- Hub magnet is probably strong enough to switch the speedometer sensor on/off from a distance of cm or two.
- And all you need to do is to find such a place for the speedometer sensor it connects the current when time is right, and fix it's position relative to the hub axle.

Understood? Need a drawing? Possible to make it work?

------------------

And to the handicapped races:
I'm involved in a small sport, speed scooting or footbiking. In smaller local events level of participants is really dispersed (right word here?)
I'm sure same applies for many small local bike events. I thought about having a handicap series where winners of each race are given extra weights until the weakest start to score first places too. The winner of the series would be the one who at the end has the highest penalty weights and differences were undoubtedly clear, but events could be much more interesting when riders actually raced head to head and not minutes apart.
But then. Riding with extra weights sucks. Would be much nicer for all if weaker riders had extra boost of energy instead of stronger getting extra slow. If hub dynamo could make enough difference it would be a whole lot cheaper than a normal hub motor (thinking if organizer needed to buy several wheels).
I'm sceptical though if a power in a hub dynamo would be enough. Any idea how much one could crank out of a dynamo that is supposed to give 4 watts to the bulb? Or other ideas for handicapping?

 

[Drunkskunk]

Most hub dynos have an output in the 3 watt range.so their output potential as a motor would be the same. That's about as much power as the little vibrator motor on your cellphone, or about as much thrust as farting while riding.

If you dramatically overpower them by 300%, you're still looking at 9 watts power. 9 watts might be able to move a toddler across a smooth kitchen floor if you put the motor in a tiny wheel. But it might very well melt trying to do so.

Turning a dyno into a hub motor is a cool idea as an intellectual exercise, but it wouldn't serve any practical purpose.