I am building a thing that would be much easier to run with a DC, rather than an AC electric motor, due to variable speed being desirable. It is a thing that could certainly appeal to tree hugger types, so I figure, use a DC motor, a battery, and a generator or alternator mounted on a bike to recharge. A guy could just use a battery charger, but where is the fun in that?
My first impression is that the generator/generator needs a certain amount of rotor speed to work, so it would have to be overdriven by a pretty significant amount to work, but too much overdrive, or too high an output and it becomes too hard to pedal... What is the math, oh GRM braintrust?
www.econvergence.net/product-p/pawa-1.htm
A professional rider can generate 300 watts continuously. That works out to 25 amps @ 12V or about 0.4 HP. Figure a normal person could probably generate 1/4 of that power.
So, five or eight amps, likely. That would probably be adequate for the task.
Hmm.
Also think about how long you would need to charge the battery, or how long the thing should run for at a time. A reasonably fit person could probably ride for an hour, less than that as fitness level drops. In addition, will your thing tolerate being fed less current as the rider gets tired?
Ransom
PowerDork
12/24/17 11:57 a.m.
I'm not sure how to describe what mathematical function describes perceived output vs watts, but I know the last time I was doing structured indoor cycling, my FTP (Functional Threshold Power, or the average I'm theoretically able to maintain for an hour) was 239 watts (as a slowest-end-of-the-spectrum masters-category racer at 190 lbs), but half that wasn't half as hard, meaning that 120 watts is more or less like sitting on the couch with my legs moving (this is all relative; 120 watts can be real power for a smaller person)
bearmtnmartin's link looks interesting, though I wish they made one like a lot of the newer trainers that replaces the rear wheel and is driven directly by the chain rather than being driven off the tire. It's always annoying, and you're definitely going to lose some of your power there. I'm *so* looking forward to replacing my tire-driven trainer...
Not sure if it's helpful but as a data point, a couple years ago I picked up a recumbent exercise bike that wouldn't "power up". A bit of research showed that the small D.C. Generator which powered the display and control motor was actually powered by a small battery pack (which had gone bad).
Upshot is that D.C. Power is easy enough and "evening out" the supply is done with a continuously charging/discharging battery to prevent spikes and dips in the output.
Rufledt
UberDork
12/25/17 1:51 p.m.
Back when I was cycling a lot, a fellow club member had a rear hub that measured his output, about 250-300 watt average over a 3 hour ride was normal. No idea how much power would be lost through inefficiency of a generator and stuff though.
Robbie
PowerDork
12/25/17 2:18 p.m.
For fun, one hp is 746 watts. So interesting to know a professional cyclist can do 1/3 to 1/2 of that for a few hours. I bet for like a minute a human in really good shape could hit one hp of output. Just gotta lift 33,000 lbs one foot, and you have a whole minute!
If you are not a professional cyclist, hop on a statuonary bike and try it. I bet 40w is hard work for most Americans, and over an hour might be more like 20w.
pfffftt Chuck Norris could power New York City indefinitely.
Almost anyone could very quickly get to the point where they could maintain 100 watts for a few hours.
I hear the lower end stationary trainers cap out around 300 watts of resistance. I had to get rid of one because it wasn't any good for interval training. Based on my experience that cap was more like 200 watts though.
If you really have high output requirements, use a rowing machine instead of a bike.
