Bicycle Power Generation Using Hub Motor???

[mauimart]

I, with some students, would like to build a stationary bicycle power generation station to be used as part of an energy course in a high school. I have built one of these before using a bike on a trainer that spun a car alternator through a belt drive. The alternator regulated to about 14V which was fed into an inverter (cheap modified sine wave variety) to drive AC loads. This setup was very inefficient in converting human power into AC power. My guess is 50% overall at best considering the alternator by itself is less than 60%.

I thought a more efficient setup would be to use a hub motor e-bike in regen mode. I have no experience using hub motors or regen as all of my ebike builds have been non-hub drive without regen. Has any one done this before? What kind of efficiency could I expect? I would still need to have some amount of battery power to energize the controller and get it to regen. Maybe a low kV wound hub motor with a 3-phase rectifier feeding a DIY step down converter? Ideally the key is to get regulated 10V-15V power to feed the common <1000W inverters, preferable pure sine wave inverter. It would be nice to be able to put as many off-the-shelf components together for a rapid (< 2 weeks) solution. Subsequently I will add an arduino based power monitoring/logging system to allow students to do some experiments. Please share any ideas you may have regarding this project.

Thanks.

Martin

 

[teklektik]

The guys at Rock the Bike have been powering concerts with a bank of bikes-with-hubmotors-as-generators for quite awhile.


Leif is their engineer and might at least be able to provide some general background for educational purposes.
(I ran into him because he designs for Yuba as well...)

They also sell an inverter box for end-to-end pedal-to-AC conversion but it's kind of pricey. Leif might be able to fill in some blanks there as well.

My knee-jerk reaction would be to skip the controller and go directly to a 24v alternator diode bridge + regulator with 24v battery and a 24v inverter - just borrowing from common 24v truck parts.

 

[teklektik]

Hmmm -- obviously can't use a common truck regulator since hub motors are PM and have no field coils for a standard alternator regulator to drive for regulation (d'oh!).

Well - I did say it was a 'knee-jerk' approach....

 

[doctorGONZO]

Keep in mind that any and every electric motor that has permanent magnets to supply the field is a generator using an alias. The old Faraday principle that any time a conductor moves in a magnetic field a current is induced in the conductor is always working, whether the motor is acting as a motor or as a generator.

That means that when you jack up the ebike like in the aforementioned concert power conversion, and pedal the bike to turn the rear wheel, a current is produced that flows OUT in the wire(s) that normally bring battery current IN to the motor when it is rolling on the road under power.

So, if you get the controller out of the way and tap the connection(s) directly from the hub motor, you will have a DC current. It is up to you to then devise whatever voltage regulators or inverters or converters to take that raw DC current and use or demonstrate it as you need to.

Regen Braking is a little bit of an enduring mystery to me. I strongly suspect that it use battery current in a reversed way so that it is trying to make the motor spin BACKWARD and thereby causing an aggressive slowing of the spinning wheel and therefore the bike as a whole. For simplicity, perhaps you should disconnect and forget about any controller or other device touted as a Regen Brake.

The simple physics basics of it is that the bare spinning hub motor is acting as a generator without any complications being needed. And the voltage and current tapped directly from the coils is all you are going to get. Unless you have invented some kind of an Over-Unity Power Amplifier, no controller or inverter or whatever is going to give you any more power than a tap directly off the coils.

BUT....some motors have a controller built in, within the hub. In such a case, tapping the raw current directly from the coils would almost certainly mean removing the controller. That may or may not be more trouble than it is worth your while.

 

[spisska]

>Regen Braking is a little bit of an enduring mystery to me. I strongly suspect that it use battery current in a reversed way so that it is trying to make the motor spin BACKWARD and thereby causing an aggressive slowing of the spinning wheel and therefore the bike as a whole.

Nah, it's a lot simpler than that, and a much older technology to boot. The more accurate term is dynamic braking -- that is, you are using the motor as a dynamo, allowing electrical resistance to create a breaking force within the motor. Dynamic brakes have been on diesel locomotives for just about ever, for example.

A locomotive, of course, works a bit differently. A diesel generator provides electricity to independent motors at the engine's drive wheels. When the dynamic brake is engaged, the system sends current from the motors to a set of resistors, converting power into heat and making the locomotive resist motion. This is in addition to the air brakes, which is a fascinating subject itself.

Regenerative braking is the same as dynamic braking, except you're recapturing the electricity rather than cooking it in a big heat-sink. Pretty much all electric cars use this. For it to work, you need the whole system from motor to battery to work both ways, which adds weight, cost, and complexity. A car is incredibly massive, and converting its kinetic energy into electric energy can capture a significant amount of power. Likewise the additional weight, cost, and complexity is not very significant compared to that of the overall car,

Bikes, on the other hand, are a different beast. The weight, cost, and complexity burden is definitely significant compared to a similar non-regen build, and the efficiency boost is only around 10 percent, or so I've read here. Plus regenerative braking also means added mechanical stresses on the dropouts, more points of failure, limited motor choice, and so on.

Of course this has little to do with OP's question, which is totally doable. Just get a non-geared hub motor, stick it on a bicycle, hook up a multimeter, spin it around and see what you get. It sounds like a really fun class project, actually.

 

[doctorGONZO]

@ mauimart......See if this link helps you:

http://www.uumotor.com/blog/2012/10...b-motor-to-a-gernerator-may-be-not-difficult/



@ spisska.......LOL. u speed reed dont u?

 

[mauimart]

Here's what we ended up with. Each bike is fitted with a hub motor acting as a generator. Two players compete in trying to generate 2Wh first. Increasing loads are automatically switched in as the game progresses. The whole setup is controlled with an Arduino. The intent is to help make people realize how much work it takes, through physical exertion, to power some of their everyday loads.

 

[John in CR]

That's great Mauimart. It really hammers home just how great electricity is, and how much energy is involved. Have you done any efficiency analysis of the charging? At low speeds regen braking isn't very efficient, so much of the braking energy still turns into heat.

 

[mauimart]

That's great Mauimart. It really hammers home just how great electricity is, and how much energy is involved. Have you done any efficiency analysis of the charging? At low speeds regen braking isn't very efficient, so much of the braking energy still turns into heat.

No formal efficiency calculations have been done at this point. In terms of losses the hub motor output is connected directly to a 3-phase rectifier. Power is then fed into a DC/DC converter to regulate the output at 12v. If we assume the hub acting as a generator is at 85% efficiency and the DC/DC is no worse than 80%, we are looking at 68% for the whole system. One of the goals of this project was to do better than your standard car alternator solution which I believe it has done.

[youtube]GGe9soy27CQ[/youtube]

 

[John in CR]

There's also an efficiency % for the motor's conversion of mechanical to electrical. It may not be too bad since the effective voltage is low. I'd try to estimate the mechanical effort input.