Scratch building a small but powerful generator.

[BenPeake]

Hi All,

Just wondering if anyone knows of any resources that might be useful in designing a small generator that is suitable for use with a model steam engine. By that I mean, it must have a high output at lows revs.

Very much appreciative of your help,

Ben

 

[b.lindsey]

Not my area of experptise by any means Ben, but the more output you desire means the more drag it will put on whatever engine it is attached to, so I would assume its kind of a balanceing act between size/output of the generator and the size of the engine. I am sure some of the more electrical savvy members can direct you to the proper resourses. If you haven't already, check out this PMR Dyno at the link below, though the specs are probably at higher revs than you are shooting for. I have one of these attached to a very free running Tiny Power #104 engine and even lighting a flashlight bulb makes a noticable and audible difference in the engines performance and thats a minimal load. Best wishes on the project however.

http://www.pmresearchinc.com/store/product.php?productid=3609&cat=6&page=1

Regards,
Bill

 

[kellswaterri]

Hi Ben, over on u/tube, there is an interesting video from a bloke by the name of ''James Petts2''. He is using a steam powered ''Stuart Score'' engine to drive a little dynamo, might be of some help...could well be quite a few more vids out there.
All the best for now,
John.

 

[Engine maker]

I built one of the first PM research generator form the kit. It's a nice generator but lacks power unless you spin it very fast. I was spinning mine at about 500 rpm and only got 5-6 volts out of it. I then stumbled across this site
Website:
http://www.modelenginegenerators.com
The guy's name is Paul Santamaria and he makes a generators from stepper motors complete with a rectifier (spin it in either direction and only Positive volts come out) and a meter. Spinning mine at 470 rpm I get almost 19 volts output. Top output is
35 -37 volts. His new models max output is 50+ volts. The price is quite reasonable, unless you really want to build one. You may just want to have a look. I use mine to run a z-gauge train at engine shows. There's a link on the website to see it running. See subpage #3

 

[Cheshire Steve]

Just remember that Volts isn't power, and you asked for 'powerful'. Power is Volts times amps, and of course power is what your engine produces.

So don't be misled by people quoting volts. In any steam engine the efficiency is low, but if you measure the flywheel output using a brake you can work out the output power (that is a very interesting exercise in itself). A half-decent generator will allow you to turn almost all that output BHP into energy, but you will never get more than 100% so you can work out the max you might get. 10Volts: n amps, 20 volts: n/2 amps, 100Volts:n/10 amps etc - the power is the same.

As for the best hardware to choose, I leave that to the leccy types. It might depend if you want a single fixed speed power generation, or the ability to vary the power delivery.

Steve

 

[Troutsqueezer]

Ben specified high output, now that can mean high voltage or high current or both. One or the other is far easier than both. You'll need an engine that delivers substantial torque if low revs are a priority.

-T

 

[mu38&Bg#]

How much power do you have available (shaft power)? How much electrical power are you trying to get ( Voltage x Current)? Accounting for losses in the generator you will get less electrical power output than you have shaft power input. If the load is trivial, like I small light bulb to shows it makes power, it's a simple task. If you have a specific load you are trying to operate, this will have a big impact on the selection of the generator.

 

[BenPeake]

Thanks for all your input!

I really like the generators that you suggested Engine Maker, however, I would still like to make my own from scratch so I can alter the aesthetic as I like older styled machinery. When I say high output I mean that I want to maximize V x I (Voltage x Current) given the available energy (output by the an average model steam engine). I think I'm getting some idea of what is required, and I will let you all know how it goes when I get back to my workshop (a month and a hlaf) as I'm on holidays at the moment.

Thanks again,
Ben

 

[Tin Falcon]

There are sources of surplus motors how about experimenting with, say the motor from that old drill driver, that the batteries have long since quit keeping a charge.
a dc motor run on an engine will produce electricity.
Tin

 

[kf2qd]

You might try building the generator into the flywheel. I saw a generator built that way up in Iowa a few years back. Was used as teh power plant there some years ago.

 

[Ripcrow]

I think from memory 1 hp is required to produce 25 amps at 12 volts.my theory is you should check the available torque and then decide what gearing you can run to get your generator to the right speed. I failed physics so all my work is experimental but Ian never bored and learn more now then I ever did.

 

[Tin Falcon]

Your theoretical numbers are a bit of but practical numbers may be about right.

1 HP is approx 745 watts.
watt = volts * amps
so in a system that is 85 % efficient that would be about right.
And a quick google shows only the best generators 80% efficient and as low as 60
Tin

 

[MCRIPPPer]

ac, or dc? permanent magnet, or field windings? if ac, how many phases? do you need a specific frequency? what voltage? how much power do you have from the engine?


PM dc motors are good sources of dc. brushless dc motors will make ac 3 phase when driven. auto alternators will do the same if you take the rectifier/diodes out.

stepper motors will make various forms of ac depending on how it is wired and how many steps per turn, how many phases it has.

 

[dnalot]

ou might try building the generator into the flywheel. I saw a generator built that way up in Iowa a few years back. Was used as teh power plant there some years ago.

You might take a look at how outboard motors generate, they all use flywheel alternators.

 

[Entropy455]

If you are using a low rpm steam engine as a power source, I recommend using a reduction gear to step-up the generator speed. Compare the physical size of a 10 HP single-phase motor built to run on 240 volts at 400 Hz, with a 10 HP single-phase motor built to run on 240 volts at 60 Hz - and the advantages of increased generator rpm become visually clear.

Here are some general equations to help with sizing a generator:
Horsepower = (torque * rpm)/5252
Where torque is in foot-pounds-of-force. The 5252 is actually a rounded-off conversion factor. The actual value is 33000/(2*pi)which is the conversion of linear horsepower into rotational horsepower (in radians).
One Horsepower = 746 watts
Typical electrical generator efficiency is 70% (mechanical power to electrical power)

Example problem: I have a steam engine that turns at 250 rpm. I want to generate 800 watts of electrical power. How much engine torque is required?

Solution: mechanical power input to the generator required to produce 800 watts: 800/.70 = 1143 watts of generator input power. Which is equal to 1143/746 = 1.532 horsepower. The steam engine torque required to produce 1.532 horsepower at 250 rpm is: (1.532*5252)/250 = 32.184 foot-pounds-force.

Taking it to the next level. How much boiler heat input is required?

Solution: A typical piston steam engine will achieve about 5% thermodynamic efficiency. Thus the boiler must provide 1.532/.05 = 30.64 horsepower of heat input. There are 746 watts in a horsepower, and 1055 Joules per BTU – thus the boiler must be fed with 21.67 BTU per second, or 77,998 BTU per hour. Assuming the boiler has an efficiency of 60% (where 40% of the combustion heat is lost out the exhaust), you must feed the combustion chamber with 104,000 BTU per hour. If you were running propane as a heat source, which has a BTU content of 91,690 per gallon, you would need to burn 1.134 gallons of propane per hour, to generate 800 watts of electrical power, using the steam engine. . . .

Cost comparisons:

Expensive electricity is about 10 cents per kWhr. Thus taking 800 watts off the grid will cost you 8 cents per hour of use. However in a steam engine, using Propane at 2.60 a gallon, it will cost you $2.95 per hour to make 800 watts. . . . . .Thus you better be doing this for the hobby aspect, and not for any attempted cost savings. . . . .

 

[Ripcrow]

Well entropy455 you certainly know your stuff,always reckoned there was a way to determine hp from torque that's why I suggested finding out the available torque and then gearing up to the generator. Does anyone have a way of measuring the hp put out by the models like a home made devise,I don't understand dynometers but from my understanding originally hp was measured by lifting a weight a certain height over time and than working the hp out from these numbers. Love this forum as I never stop learning from it

 

[JAndrew]

BenPeake,

If you're looking for something a bit more "nuts and bolts" here's a link:

http://otherpower.com/pmg2.html

This guy made a pretty ingenious permanent magnet alternator out of rare earth magnets, plywood and epoxy. With his design he was able to get 12-14volts (unsure of possible amperage) at 120-320rpm.

The guy does a good job explaining coil rotation directions required and magnet orientations.

It's interesting stuff even if its not exactly what you're looking for.

If you start building anything please post pictures. I'm anxious to try some coil winding myself.

-J.Andrew

P.S. I believe the guy could have gotten a much more effective generator if he had used metal for the stator instead of plywood assuming you have a surplus of torque to overcome any cogging.

 

[Jyman]

If you have a steam engine already you can calculate how much horse power you get from it by using the formula "PLAN/33,000" I found the formula on the Internet someplace a long time ago and don't have a link to the site

P = pressure
L = stroke of piston in feet
A = surface area of piston in inchs
N = number of rotations

Then divide by 33,000

So for example at 24 psi a 1 inch piston with a stroke of 2 inchs at 400 rpm would generate about 0.038 hp. So at 746 watts per hp that with those specs it would only make about 28.348 watts of power. And to run that engine at 400 rpm's you would need 628 cubic inchs of steam/air.

Now that engine could generate more or less hp depending on how much pressure or rpm's you put to it. Hope this helps

 

[Jyman]

I forgot to mention that the calculation isn't perfect but for a simple formula it would get you fairly close.

 

[MachineTom]

Imagine a single loop of wire formed into a U shape, each leg of the u is 1" long, the bottom length doesn't matter, a bar magnet rotating in the center of that loop first has the north pole swing past the right leg as the south pole swings past the left leg, the right leg would pulse a positive .02 V, and the left leg would pulse a negative .02V.
now you take the 1" leg and run it around a second time, same magnet runs past and the voltage now is .02 +.02v=.04V. Now if you make the leg longer the voltage will remain the same but the amount amps produced is increased.

A voltmeter is really an ampere meter that has a fixed resistor, and the meter is scaled such that the output is called as volts. So when you have these very low power sources the voltmeter sucks most the power in amps available just to move the scale.

Put a lightbulb as a load and see how many volts you have then. A generator is at its best 60% efficient, and a home made version its not near that good.