rpm to power question?

[drummer boy]

Can someone give me a yay or nay please.

is it possible to produce 24v 250watts from a constant 200rpm input?

thanks in advance,

stu.

[wpowokal]

Ya!

Stu, Power produced is relevent to the generator and driving force, rpm is relevent to generator type/construction/frequency.

allan

[spinningmagnets]

Watts is a measure of power, one horse-power is 745 Watts.

You can get one HP from 12 volts (electrical pressure?) at 62 amps of current (volume of flow?), OR...

From 48 Volts at 16 amps.

If Watts are Amps X Volts (4 X 2 and 2 X 4 both equal , then,

250 Watts @ 12 volts is 21 Amps. To get 21 Amps (of 12V) from 200 RPM's will probably require a large diameter wind-gen and strong winds.

The answer is yes, but I don't know the size you will need, or how hot it will run.

[Flux]

The answer is yay.

As the others have said the only requirement is that your power source providing the 200rpm is capable of a power greater than the 250W that you want out otherwise your 200rpm will not be maintained and it won't happen.

It is possible to get any power at any voltage at any speed as long as the power is available. ( within reasonable limits of practical devices).

You can produce 1Mw at 2v and 10rpm if the source can provide it. It may be a rather big costly and not very efficient set up but is possible.

Certainly 250W at 200 rpm is within the range of many of the designs here, the lower the speed and the higher the power the more it will cost you and the bigger the thing becomes but there is no technical limitation.

Flux

[drummer boy]

thanks guys, now i know its possible, can you help me out on the next question.

what i am producing will not be wind powered so no issues there. basically i have a wheel which can spin at a constant 200rpm which i need it to run a 24v 250w motor. if i can produce more power at this speed, then even better. can someone point me in the right directions as where to go next.

i shouldnt go into detail really, but my friend is wheelchair bound and has a hand cycle with a motor running on batteries which give him a range of about 13 miles.

my design is to incorporate the solid state motor into the rear wheel to power the motor on the small front wheel.

by my calculation, he can hand cycle upto 10mph, which at this point, the rpm of the 'motor' is 200rpm, producing the power he needs to keep the motor running. as long as he keeps the speed above 10mph, he should, in theory, be self powering???

i know this sounds like perpetual motion, but i believe my design will work.

any designs or tips to help me will be really appreciated.

thank you for your time once again.

stu.

 

[zeusmorg]

 In a basic response to your idea of self powering this , no(qualified).. Yes it would work but the additional strain would cause your friend to no longer be able to maintain 10 MPH. "as long as he keeps the speed above 10mph, he should, in theory, be self powering???"

 250W = .335HP actually that is at 100% efficiency. A healthy human can produce about 1.2 hp briefly and sustain about 0.1 hp indefinitely, and trained athletes can manage up to about 0.3 horsepower for a period of several hours.

 I imagine the current design of this chair is hand cranked assist with a battery driven motor. a hand cranked generator would increase the range of a battery only system.

 What you're attempting to do will DECREASE the overall range. You'll just tire your friend out needlessly. Using human power directly to the wheels would be the most efficient manner. Adding in the losses of a generator, and then the losses of the battery storage system and the motor losses, plus the losses of any additional weight would just make this setup inefficient.

 If you're looking to increase the range of the electric system,add battery storage.

It is possible to build a device that you could charge those batteries by hand, before any trips.

[kurt]

i have a friend who has a very fancy watt meter on his recumbent exercise bicycle he can maintain about 100 watts comfortably for 15-20 minutes and that is with his legs. i think your gonna make your friends chair so hard to move that it won't go anywhere. it is a basic rule of physics that you cannot get back more than you put in. otherwise you are attempting to build something called an overunity devise and that is doomed to failure and off topic on this forum.    

[spinningmagnets]

drummer boy, I appreciate your enthusiasm, but...

Motors and generators easily spin freely without a load, as soon as you apply a load to either of them, they will bog down. Series hybrid car research showed that the generator had to be twice the Kw size as the motor to maintain speed. (real world conditions and performance)

If you carefully select the perfectly sized motor and generator, you will probably still lose 50% of your power in the conversion.

Flux, Kurt, and zeusmorg are right. 100 years or so of electro-mechanical experiments by many clever people. Many others have had the same idea, results not even close.

Read about electric bicycles over at endless-sphere (Google it), and consider boosting the wheelchair with DeWalt/Milwaukee lithium battery packs.

[drummer boy]

thanks for the input guys, really helpful!

at the moment, the bike has been a handcrank to which a heinsmann 24v motor has been fitted (i am a cycle dealer, so i built it into the wheel hub!)...

he has the option of either peddling or running on battery power, not pedal assist.

with a fully charged battery, the chair will cruise along at 13mph, while carrying my friend (about 16 stone in wieght, his bags and batteries (about another 20kg) although within about 5 miles the speed will start to drop to 10mph where it stays till the batteries go flat and he has to pedal home!....

my thoughts are based on the mechanics of a car engine. by using a starter motor and fuel (hand biking to 10mph?) the engine will turn the alternator to produce the power needed to give the car electricity. in theory, if the engine has enough fuel supply it could run forever??, so by keeping momentum and rpm above 200 (to supply the fuel), the power needed will be produced?. if the speed is dropped below the threshold, the engine will stop and would need restarting using the starter motor.?

..... or is all that utter tosh?

thanks all, is there a way of charging a second set of batteries in the time the first set are being used?... thats the next plan!

[spinningmagnets]

The starter motor draws a lot of amps, but only for about 3 seconds to start the engine, the alternator can be small because it has at least a few minutes to charge up the battery after starting.

The amount of electricity needed to make the sparks in the cylinder is actually very small.

There have been a lot of electric vehicles made, and since the battery pack is expensive, they are usually short range. adding more lead/acid batteries adds a lot of weight, and converting to lithium batteries (to get more range in a smaller-lighter package) is very expensive.

Since most trips for most people are usually short range, this isn't a big problem. For somepeople who insist on occasionally being able to travel longer distances, it has been useful to make a series-hybrid trailer.

Most trips the trailer stays at home, hook it up and an engine turns a generator, so you can ride until you run out of fuel.

http://visforvoltage.org/forum-topic/hybrid-systems/731-series-hybrid-suitable-generator

[Airstream]

It is not clear whether the "peddling" is a purely mechanical direct drive or a electro-mechanical drive system...

The one sure thing that can be done is have a dynamic or regenerative braking system.

In a completely automated system when slowing is requested a friction brake is 1st applied, then with a valid feedback of the motor successfully configured as generator AND is either charging the batteries (2nd set?) or dumping into a resistance load the friction brakes release. Then when the vehicles' momentum is reduced to the point the generator current no longer yields the deceleration required friction brakes feather back in until they are sole source of holding effort.

This could also be a manual push button that only enables when machine is coasting - or, if your cyclist is willing, be enabled when peddling with either no driving force pushing vehicle so just charge batteries, or the double-whammy of moving vehicle and charging batteries.... if the system is configured that way.

[Airstream]

I know it's not very eco-friendly to suggest this, but perhaps it would keep the worry and gray hairs down among the people who care for your client..

I have seen golf carts with the batteries removed powered by twin Honda EU2000 inverter generators feeding an industrial battery charger to yield DC motor voltage, and since they lack the weight of all the batteries they truly fly!

The Inverter style generators come in less than 1000 watt models, have nearly zero noise and vibration, the hot surfaces are well shielded, very miserly with fuel from a smart-throttle, weigh 30~ pounds and are built to last a long time. Using one to produce 120VAC and down converting to system voltage would be the ultimate "emergency power pack" or "get home safely without assistance" accessory for someone pushing the 'green' envelope

[drummer boy]

hi guys, thanks again to all the input and helpful pointers. we have uprated the batteries to 2x 36ah 12v and we have been able to produce a constant 13mph for over 38 miles on the open road, before the speed starts to drop. as we have ditched the second set of batteries, we have been able to reduce the weight of the chair and improve the range of the machine, so all is going well.

we are now looking into other options and are leaning toward a small honda silent generator to hook up trailerlike for longer journeys. this will produce plenty of power to let him pootle around for a while. .... now, next dilemma....

is it possible to charge the batteries while the generator is running the motor? the batteries will charge fully within 4 hours off his home charger, so im thinking.. while the petrol motor is running the wheel, can i simultaniously charge the batteries. IE... run the wheel on batteries for 3 hours, run the gen for 4 hours before switching back onto battery power. as the generator produces 900w cont and the engine needs 250w can i use the remainder to charge the batteries?

thanks again for your input, you have been very helpful so far.

ah, also, can someone give me a quick answer. was the first idea flawed due to the drag effect of the magnets? its bugging me now!

cheers, stu.

[ghurd]

You asked.

It looks to me as if the first idea was flawed due to overunity, or almost overunity, thinking.

The drag effect on the magnets is due to the power they are making in the generator.

Power comes from somewhere, and it takes power to make power.

The magnets only have 'drag' if they are making power.

Making power means power is moving.

Moving power means losses.

Losses mean it won't work.

If it takes 250W to move, and 250W is supplied to the generator, then the 'moving thing' will stop.  Watts are used up in the generator, bearings, etc.  It will not keep moving by itself.  Losses to run the thing are greater than just running the thing by hand.

Otherwise, we could use a weedwhacker engine to power an alternator, and the alternator to make a lot more power than the weedwacker engine,

Then a DC motor to run some alternators, which make more power, fed to larger motors, repeat, repeat, repeat.

Then there would be plenty of powr to have a DC motor can replace the weedwacker engine, then we can all sell power back to the electic company.

We'll tell everyone who would do the same thing we did.

They will send everyone $100,000 a month.

We'll all be rich from what the electric company pays us.

And nobody would need the electric company, except to mail us those checks everybody is getting, and the money they send would be coming from the poeople who didn't hear about this wonderful device we all have.

It is possible to make a terrawatt at 250RPM.  It will weigh a lot.

G-