Small 12V low RPM generator

[Brian Wood]

Knowledgable ones:
I posted this request to another (non-usenet) forum and was referred to you
by a friend in that forum:

It is difficult to present my current 'daydream' in a reasonably short post,
but allow me to try: I plan to purchase a recumbent tadpole-design tricycle
(see www.catrike.com/road.htm for photos). "Chain management" is often an
issue for recumbent trikes, and idler pulleys are often used to control the
chain as it passes under the seat. I enjoy riding before sunrise and after
sundown, and will need reasonably powerful lights on my trike. I plan to
install a 14.8 volt Li-Ion battery pack, with a (nominal) 12 V lighting
system (HID or multi-LED headlamp and tail lamp, and a 360° amber strobe
light mounted on a pole behind the seat).
I wish to consider a small 12 V generator driven by a toothed idler pulley,
mounted low behind the seat. It will serve for chain management and power
generation. My front chainring will be 32-tooth and the rear gearing will be
an internal hub, so the chain velocity will be constant (no rear
derailleur). My normal pedaling cadence is 80 RPM, so the 16-tooth idler
pulley will be driven at ~160 RPM. I need an output of only 3-4 amperes, and
might get by with only 2-3 amps. I don't need to power all of my lighting
exclusively from the generator; I need to keep the battery charged and then
extend its run-down time after dark when I turn on all the lights.
Is it feasible to try to construct a generator, using rare-earth magnets and
copper wire, that will generate 15-18 volts, and have a package something
close to the size of a can of soda pop (2-2½" diameter, 4-5" long)?
My research to date finds many designs for homemade generators, but all of
them are large diameter (6-10 inches). That is great for wind turbine
projects but not suitable for my dream. I've heard of using brushless DC
motors as generators, but all that I can find operate at very high RPM.
There are dynamo hubs available for bicycles, but they all generate 6 volts
and I am committed to a 12 V system. There are "bottle type" generators for
bicycles that drive against the lateral surface of the rubber tire, but I am
(at least for now) set on designing something driven from an idler pulley.
What do y'all think? Totally feasible? Impossible? Somewhere in between?
Just to keep the discussion on track, please understand that I am also
investigating the options of solar power to charge the battery, and of using
a larger battery and "home-charging" it. My purpose for posting here is only
to inquire about the technical feasibility of constructing a (very) small
12V generator being spun at only 160 RPM.
Thanks in advance,
Dennis

 

[Brian Wood]

When I posted, it did not occur to me that I was using an old usenet account
established by my nephew when he lived here, several years ago. Please do
not reply to the email address given (I am sure it is ficticious). My name
is Dennis McKim; I live in western Colorado, USA.
Sorry for any confusion.
Thanks,
Dennis

 

[SQLit]

When I posted, it did not occur to me that I was using an old usenet account
established by my nephew when he lived here, several years ago. Please do
not reply to the email address given (I am sure it is ficticious). My name
is Dennis McKim; I live in western Colorado, USA.
Sorry for any confusion.
Thanks,
Dennis

Is there any reason you do not just buy one that is in production now?
These have been around for some time. If you were to convert the lamps to
LED's you could further reduce your needs.

 

[Brian Wood]

Is there any reason you do not just buy one that is in production now?
These have been around for some time.

I have not found a generator in production that will produce ~15 volts (to
charge a 12V battery), at 3-5 amps, when driven at 160 RPM.
I have run numerous internet searches, with various combinations of terms,
but have come up dry. If you know of such a device by name or manufacturer,
I am more than willing to explore that option.
Thanks,
Dennis

 

[stu]

wow
thats a lot of light for a bike
4amps at 12v is 48watts(not sure what the losses would be, say 10%) so thats
53 watts
while you might think 53watts isnt a huge amount of light (for a bike it is,
i use 20w bulbs on my bike), but it is a huge % of your total power output,
depending on how fit you are 53 watts will be between 15%(if you are very
fit) and 40%(if you are not so fit) of the power output you can mantain. at
these levels of power there is a good chance you would need to change the
gearing on the bike
i think he needs to work out what lights you want to use first. the biggest
LEDs that i know of are 5w, the HID for bikes are all 20W i think. tail
lights and strodes use very little power normally. if he really needs this
sort of power i think it would be best to just stay with a battery.

 

[clare at snyder.on.ca]

Knowledgable ones:
I posted this request to another (non-usenet) forum and was referred to you
by a friend in that forum:

It is difficult to present my current 'daydream' in a reasonably short post,
but allow me to try: I plan to purchase a recumbent tadpole-design tricycle
(see www.catrike.com/road.htm for photos). "Chain management" is often an
issue for recumbent trikes, and idler pulleys are often used to control the
chain as it passes under the seat. I enjoy riding before sunrise and after
sundown, and will need reasonably powerful lights on my trike. I plan to
install a 14.8 volt Li-Ion battery pack, with a (nominal) 12 V lighting
system (HID or multi-LED headlamp and tail lamp, and a 360° amber strobe
light mounted on a pole behind the seat).
I wish to consider a small 12 V generator driven by a toothed idler pulley,
mounted low behind the seat. It will serve for chain management and power
generation. My front chainring will be 32-tooth and the rear gearing will be
an internal hub, so the chain velocity will be constant (no rear
derailleur). My normal pedaling cadence is 80 RPM, so the 16-tooth idler
pulley will be driven at ~160 RPM. I need an output of only 3-4 amperes, and
might get by with only 2-3 amps. I don't need to power all of my lighting
exclusively from the generator; I need to keep the battery charged and then
extend its run-down time after dark when I turn on all the lights.
Is it feasible to try to construct a generator, using rare-earth magnets and
copper wire, that will generate 15-18 volts, and have a package something
close to the size of a can of soda pop (2-2½" diameter, 4-5" long)?
My research to date finds many designs for homemade generators, but all of
them are large diameter (6-10 inches). That is great for wind turbine
projects but not suitable for my dream. I've heard of using brushless DC
motors as generators, but all that I can find operate at very high RPM.
There are dynamo hubs available for bicycles, but they all generate 6 volts
and I am committed to a 12 V system. There are "bottle type" generators for
bicycles that drive against the lateral surface of the rubber tire, but I am
(at least for now) set on designing something driven from an idler pulley.
What do y'all think? Totally feasible? Impossible? Somewhere in between?
Just to keep the discussion on track, please understand that I am also
investigating the options of solar power to charge the battery, and of using
a larger battery and "home-charging" it. My purpose for posting here is only
to inquire about the technical feasibility of constructing a (very) small
12V generator being spun at only 160 RPM.
Thanks in advance,
Dennis

Try a 110 or 220 volt universal or DC motor. At reduced RPM you MIGHT
manage 12 volts out of it. Remember to put a diode in like so it does
not try to drive the bike.
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[Brian Wood]

Try a 110 or 220 volt universal or DC motor. At reduced RPM you MIGHT
manage 12 volts out of it. Remember to put a diode in like so it does
not try to drive the bike.

Thanks very much for that idea. I truly appreciate it.
Regards,
Dennis

 

[Newby]

Knowledgable ones:
I posted this request to another (non-usenet) forum and was referred to you
by a friend in that forum:

[snipped]

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[Brian Wood]

I would go straight to a stepper motor, as used in 5.25" disc drives
and printers. ....
Note there are 2 versions of these steppers: ones that turn easily, and
ones that are stiff. Both take some force to turn at all, due to the
magnetics, what you dont want is the ones common in printers that are
high friction.

Thanks very much, I appreciate that idea.
Dennis

 

[FrankG]

Brian,

The issue at hand is the relative size that you've described, the reason
that the wind Gen's are 8" plus on the rotor is to a) mount a sufficient
mass of Magnets, b) a corresponding quianty of copper in the form of coils
to be acted upon by the Flux and c) to have adequate velocity of flux
cutting across the coils...

The small the diameter of the rotor the fewer the number of poles or Magnets
that can be shoe-horned into place, as well the lower the velocity of the
poles moving over the coils... as well the number of coils or total mass of
copper that can be acted upon is reduced... The only way to offset this is
to increase the velocity of the rotating flux, but your criteria is 160RPM
(which is slow)...

So now you're looking at some form of mechanical gearing to take that 160
RPM and multiply it to XXXXRPM (note that there are 4 X's implying that my
guess is in the thousands of RPM)...

12V X 3 Amps = 36 Watts assuming a total efficiency of 50% after both
electrical and mechinical losses 72watts of energy would have to go into
this unit from your efforts at the pedals...

Certainly the above is quite rough in it's approximation but I think that
you may find that the effort on your part to run this unit may be a lost
cause at the end of the day...

I'd be the last person to ever say it can't be done, it's just a matter of
what price are you willing to pay in terms of performance...

But in the course of wording this reply, it does occur to me that the
concept of a Hybrid Human/Electric drive on the same lines as the Toyota
Prius would be a neat project to consider...

A bank of Batteries, Pedals interfaced to gen & Charge Controller, Drive
motor and speed controller... The idea is that the pedals NEVER drive the
rear wheel so your optimal cadence is maintained regardless of terrain or
having to stop...

FrankG
www.theworkshop.ca
[email protected]

 

[phatty mo]

Brian,

The issue at hand is the relative size that you've described, the reason
that the wind Gen's are 8" plus on the rotor is to a) mount a sufficient
mass of Magnets, b) a corresponding quianty of copper in the form of coils
to be acted upon by the Flux and c) to have adequate velocity of flux
cutting across the coils...

The small the diameter of the rotor the fewer the number of poles or Magnets
that can be shoe-horned into place, as well the lower the velocity of the
poles moving over the coils... as well the number of coils or total mass of
copper that can be acted upon is reduced... The only way to offset this is
to increase the velocity of the rotating flux, but your criteria is 160RPM
(which is slow)...

So now you're looking at some form of mechanical gearing to take that 160
RPM and multiply it to XXXXRPM (note that there are 4 X's implying that my
guess is in the thousands of RPM)...

12V X 3 Amps = 36 Watts assuming a total efficiency of 50% after both
electrical and mechinical losses 72watts of energy would have to go into
this unit from your efforts at the pedals...

Certainly the above is quite rough in it's approximation but I think that
you may find that the effort on your part to run this unit may be a lost
cause at the end of the day...

I'd be the last person to ever say it can't be done, it's just a matter of
what price are you willing to pay in terms of performance...

But in the course of wording this reply, it does occur to me that the
concept of a Hybrid Human/Electric drive on the same lines as the Toyota
Prius would be a neat project to consider...

A bank of Batteries, Pedals interfaced to gen & Charge Controller, Drive
motor and speed controller... The idea is that the pedals NEVER drive the
rear wheel so your optimal cadence is maintained regardless of terrain or
having to stop...

FrankG
www.theworkshop.ca
[email protected]

A decent sized 12-24VDC permanent-magnet motor might work for you,if you
can gear it up a bit/somehow spin it a bit faster.

Perhaps regenerative braking? Basically using the drag from the
generator/motor to slow you down,and recharge the batteries a bit.
Great if you go down alot of hills.

I was discussing adding regen braking to a friends homebrew electric
bike. I think he had a 36V setup ~15AH,and a 350? watt motor.

 

[phatty mo]

Thanks very much, I appreciate that idea.
Dennis

A friend and I were playing with steppers one day to use as generators
for small lights on bicycles..
We got a decent amount of power from a small stepper rated at 25W!
Enough to light an automotive tail light bulb.
Stepper generators are good for low RPM stuff..they seem to start
"running out of steam" at higher RPM's.

 

[D.S.]

On the regen braking. . .Complexity for minimal result is the kicker here.

Think about this. You are driving in the hilly country and go down a
hill. So you apply the regerative braking to slow down. You get a
fraction of the power back. Now, *because* you slowed down going down
aht hill, now you have to *expend* a lot of power to get up the next
one. The loss is greater than the recovery.

It would have been better to let the vehicle 'coast' freely down the one
hill and have a bit of a 'slingshot' up the next and get further up it
before you had to apply power. This will of course bear on the speed
limits of the particular highway. . .

Now, if everywhere you drove was downhill, you could do well with regen
braking. But you eventually have to go back up. . .

DS

 

[SQLit]

I have not found a generator in production that will produce ~15 volts (to
charge a 12V battery), at 3-5 amps, when driven at 160 RPM.
I have run numerous internet searches, with various combinations of terms,
but have come up dry. If you know of such a device by name or manufacturer,
I am more than willing to explore that option.
Thanks,
Dennis


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[JoeSP]

On the regen braking. . .Complexity for minimal result is the kicker here.

Think about this. You are driving in the hilly country and go down a
hill. So you apply the regerative braking to slow down. You get a
fraction of the power back. Now, *because* you slowed down going down aht
hill, now you have to *expend* a lot of power to get up the next one. The
loss is greater than the recovery.

It would have been better to let the vehicle 'coast' freely down the one
hill and have a bit of a 'slingshot' up the next and get further up it
before you had to apply power. This will of course bear on the speed
limits of the particular highway. . .

Ah yes, been there, done that, it wasn't pretty. Picture this; a heavy load
on a trailer, being pulled by an underpowered pickup truck. Coming upon the
valley, I quickly realized how difficult it was going to be getting up the
hill on the opposite side, so I endeavored to maximixe my speed at the
bottom of the hill. It was almost a given anyway, due to the fact that the
brakes on a light pickup are no match for a trailer with six tonnes of
payload. I was certainly going a good clip when I hit the narrow, bumpy
bridge at the bottom. I made it across, but my biggest concern rapidly
became how to make it up the steep and long hill ahead. My considerable
momentum carried me about 3/4 of the way up, but then I had to rely on the
full power of the truck's engine to counteract the decelleration as much as
possible. With full throttle, I had slowed to a slow walking pace just when
the grade began to level out to the point where I could hold my own and
make it to level ground. I felt a mixture of of enourmous relief and a bit
of pride for having avoided an ugly mishap. It lasted roughly until I
arrived at the lip of another canyon of similar proportions.