Home built alternator

Yeah, but it'll be very inefficient. There's no magnetic path between poles of the stator when PM's are merely mounted around the periphery.

Look up motors on Google and see if you can find some elementary treatise on motors, such as How Electric Motors Work.

Thank you for the comment

"Yeah, but it'll be very inefficient."

What is "very"?

How can a competent engineer overcome the deficiency you suggest?

http://www.kreuzotter.de/english/espeed.htm

Let's say you use button magnets stuck to a good magnetic steel attached to the sidewall of a bicycle wheel alternating north pole and south pole. Now you have a magnetic path for the rotor. The stator pole (or poles, in the case of three phase, which needs to be spaced so that you get 120^o phase shift between phases) need to be in close proximity to the magnets, wound with enough wire to produce at least enough voltage to make it useful (such as 12 volts), and aligned with the button magnets. It will be an alternator, but the waveform will not be a good, clean sine wave. Your alternator voltage may be stepped up or down with a transformer, but you will incur some losses by doing this.

If the magnets are placed side by side, there won't be much cogging.

The amount of power you can expect to get from a human-powered machine is probably going to be about 1/4 HP for any extended length of time.

I'll take back the "very," if you plan to use the generator connected to a heater as an exercise machine.

Thanks Bill! This is what I need.

Many of the folks answering assume that I want to peddle this thing, which may or my not be true.

What I am really trying to determine is just what makes a dynamo, generator or alrternator cut in sooner - start producing usefull electricity - enough to at least trickle charge a battery bank or light a small bulb - or more?

I have seen examples of the wind turbine crowd that use disc brakes assemblies to make alternators. I have even seen a motor made of wood that cuts in very quickly used for a wind turbine,- but why is it that these cut in sooner than say a stepper motor when used as an alternator?

Is quick cut in allowed by more poles? more phases? more magnets? More powerful magnets? Bigger stator/rotor? Different shaped magnets? . . . or what?

I don't know what you mean by "cut in." Would that be cut-in frequency???

Actually, there's been a fair amount of engineering done for automobile alternators, and they produce usable 3-phase power at around 800 RPM or less (of course, the output is immediately rectified and used as DC). If you want to 'make' an alternator, study the construction, and redesign it for a larger diameter machine with permanent magnets. Neodymium-Iron-Boron magnets have the highest (?) energy product of any magnetic material. The stator windings of an automobile alternator and their associated laminated steel (reduces eddy currents and much improves efficiency) magnetic paths can be replaced with some hefty NeFeB magnets. Of course, the number of poles and the speed will determine the frequency of the output power, so anything less than 50-60 Hz can't be easily transformed to the voltage you want.

Anyway, you're on the right track.

Thanks Bill. The wind turbine literature use the "cut in" phrase to indicate when "useable" (evn if small) electricity begins generating.

If you do a Google search for homemade wind turbines you will find many who are constructing their own alternators, with good descriptions and photos. Some can be found also at "Forcefield" web site.

Theoretically, yes.

Physically, yes.

Practical, that depends.

I've seen bicycle-powered alternators or generators but these are only used, at the most, for a few minutes. I heard that they were also used back in World War II here in the Philippines. The guerrillas powered their radio transceivers with these contraptions with a man pedalling away as the radio operator made his call. The bicycle also doubled as the radio's transportation.

Your project is different, I know. You're proposing to convert your bicycle into a generator, not connect a generator to it. I lack the expertise to advise you on how to do it but experimentation should be interesting. My only problem with your proposal is that you want to use human power to turn it.

Such a contraption might be a little heavy to pedal. You're not thinking of using this for long periods, are you? Like an hour or two?

It is not about money, even though the ROI seems excellent - it is about clean energy. The rim of a bicycle made into a generator of such a diameter should begin to generate power at a very low rpm.

The science of using bikes for generators is a well explored line of science. But a bicycle bottom bracket can also serve as crankshaft and power delivery system for many devices including vertical wind machines (HAWTS), mini hydro, waterwheels and pumping devices.

Bicycle parts are ubiquitous and cheap. A bicycle bottom bracket is self lubricating, the guts can be replaced and it is cheap to incorporate. A chain drive transfers power with little loss of efficiency. A quality ½" mild steel bicycle crank has a high torsion strength designed to endure a man's weight combined with the considerable power his body can generate.

A bicycle crank is usually hardened steel, 1/2" mild will not support a persons full weight.

Accepted generalizations in the bio-mechanics world (from a bio-mechanical engineer friend working on a similar project for a University).

Average adult male can deliver 0.1HP (75W) of mechanical power continuously for about 2 hours before tiring to the point of having to rest. (No data on the rest time, sorry).

A "well trained cyclist" (undefined) can produce 0.25 to 0.40 HP (190 to 300W) for 4 hours.

A world champion cyclist can produce up to 0.6HP (450W) for periods of 1 hour or more, but averages out over time to about 400W.

That is the mechanical energy, to which you would need to apply any losses in the electrical conversion, which I can't imagine are much less than 20% (just an educated guess).

So if we assume that after pedaling for months on-end trying to power your laptop to read these posts, you become a "well trained cyclist", you can expect to harvest about .96kWh in the morning, another .96kWh after lunch (remember to wait an hour before getting back on), totaling a whopping 1.92kWh/day. So at an exorbitant rate of let's say, $0.20 US per kWh for electricity, you would produce/save about $0.38 US per day. Assuming the bike, magnets and wire would cost you maybe $200 used / salvage and you ride 5 days a week for 50 weeks per year (2 weeks vacation), you could pay for it in 2.08 years, not too bad of an ROI really.

Oh wait, then there's the cost of food...

A back-of-the-envelope calculation on a cyclist aged 25 climbing a hill, given the gearing, cadence rate, gradient, and weight of cyclist and machine produced a shaft brake horsepower figure of about 1/6^th for 4 minutes. At the top of the hill, power output dropped and the down-grade took over.

This sort of performance is unlikely to be sufficient to win the Tour de France.

Average adult male can deliver 0.1HP (75W) of mechanical power continuously for about 2 hours before tiring to the point of having to rest. (No data on the rest time, sorry).

This means peddle in the dark for 2 hours, and if you find a means for 100% conversion you will have 2 hours of light if using an incandescent 75w bulb, but almost 11 hours if you switch to poisonous mercury 13w efficiency bulbs.

Main benefit would be the positive use of time making use of getting in shape AND producing some energy, figure out how to make 5w bulbs that produce 1000lumens of light to pass the time while peddling and then you have really made an energy savings!

Good luck.

Sounds like you're building a slow 3 phase version of a Harley Davidson alternator. They have a magnet filled basket rotating around the induction coils. At 1000 RPM quite a bit faster than you'll pedal they can generate up to 300W. It'll take a lot of pedal power to generate good power though!

I should have considered in my cost/benefit analysis that you would save money on a gym membership!

In addition to all of the good comments made by others -- you could charge a bank of batteries during the productive hours of pedaling then use the batteries for power during your rest periods. Maybe????

http://www.scoraigwind.com/pmgbooklet/itpmg.pdf