how many rpms needed for an alternator?

ihatenoiseFebruary 16, 2009

Like how many rpms would I need to get at least 25-30 amps out of a 60 amp alternator?

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solargary

Hi,
I suppose it depends on the alternator.

Is this a renewable energy question? For windmill? Or?

Gary

    Bookmark   February 17, 2009 at 10:00AM
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zl700

No easy answer
All are wound differently depending on desired output at different loads and RPM's.
The average 60a could be about 1,400 to 2,000 rpm at shaft to achieve 50% rated output, which on average consumes 3/4 hp.

    Bookmark   February 17, 2009 at 10:05AM
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ihatenoise

Like what if I put it on a pedal bike? Saw something like it on tv, but they never went into details about it.

    Bookmark   February 17, 2009 at 1:15PM
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garymunson-2008

Hmmm.. Sound like you're expecting to get 25-30 amps off a bicycle..not going to happen. You need between 12-15 HP to get 25-30 amps from a 110 volt generator. RPMs are not the only issue...when you talk 'amps', you should really talk 'watts' as that is the true measure of power (watts = amps X volts). Also, when you say 'alternator', are you thinking of using an automobile alternator? Those can provide 30 amps but at 12 volts, 1/10 the power of 30 amps in your house at 120 volts. In addition, the automotive unit includes diodes that rectify the AC to DC. DC isn't too handy, especially at 12 volts.

    Bookmark   February 19, 2009 at 5:58AM
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pjb999

Nonsense. For anything pedal powered you will NEED to make DC, because you'll need a battery arrangement to buffer/filter it. If you were looking at a direct generation to ac you would have to keep up a steady speed, not very practical.

There may be special alternators built now for alternate power generation, but I would think the vast majority are just repurposed automotive ones.

Looking at it roughly, most car engines idle around 750rpm or so, and idling is not considered adequate to charge a battery with the load of ignition, accessories etc, although I guess a car should be able to run at idle indefinitely, but there would be little or no current left over to charge the battery. A pedal arrangement is not going to be driving much else. So 750rpm is not looking quite so bad. I don't know what the op is looking at doing but say he/she wants to exercise whilst watching tv, and wants an incentive to do so - I've always thought this'd be the perfect system, as you'd need to charge your battery in order to watch.

Either use a 12v tv or a decent inverter that would connect to the battery. Again the battery operates as a buffer.

Looking at a car and most pulley arrangements, the driving pulley on the engine is a fair bit larger than the pulley on the alternator, which means the alternator gets more rpms, of course, at the cost to the engine of more hp needed (no free lunch)

I've attached a spec sheet for Bosch alternators, just the first thing I came across.

Again, don't know what the original poster wants to do. I think you'd be pedalling pretty hard and you'd need an arrangement with a flywheel to smooth out the rpms I guess, looks like you'd need to be pedalling pretty hard but depending on what you want to do, it's possible I guess. If you're looking at some sort of off-grid application, it could only be a supplement unless you're talking about charging a cellphone or laptop or something.

The best sorts of alternative energy setups that I know of are a bit diversified for stability, ie, a mixture of wind and solar, if possible. A bike could be a possibility but it'd be like chopping wood - it warms you at least twice ;)

Here is a link that might be useful: Bosch alternators

    Bookmark   February 19, 2009 at 12:11PM
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garymunson-2008

PJB... I was looking at both his posts where he mentioned 25-30 amps and the bicycle idea. Those together don't work... My point was the bike wasn't going to provide the power he seemed to be looking for...It takes a lot of work to turn a generator/alternator connected to a load. True you can use the bike to charge a battery then use an inverter but you are still talking about a very small amount of generated power. Right now any alternative source is still trumped by increasing how efficiently you use energy...you never realize how cheap the power company is providing power until you try to make it yourself.

    Bookmark   February 20, 2009 at 5:06AM
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pjb999

That is very true, and I guess the major reason change is slow to come.

    Bookmark   March 12, 2009 at 7:31PM
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tommey-reed

When generating any power from mechanical energy in to electrical energy, 1 horse power is 746 watts of power.
When turning ac generators at 110v at 30 amps, this is ((110*30)/746)=4.42 hp, but this is only the power you put in to get out. One thing people forget is the amount of power needed to get up to the 3600 rpm's needed to get 60hz (3600/60)=60hz.
A average 5kw will need about 10 hp to keep up with loads and rpm's.(5000/746)=6.7hp . It takes about 1.2hp just to turn a generator at the speed of 3600 rpm's, the extra hp is call peek output.

    Bookmark   May 24, 2009 at 9:31AM
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tommey-reed

30 amps alternator with a output say 13v is (30*13)=390 watts
390/746=.52hp at 1200 rpms you will need a constant torque of (5252/1200)*.52=2.28 foot pounds or 27.31in/lb at 1200 rpm's

    Bookmark   May 24, 2009 at 10:49AM
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garymunson_2009

Don't keep fixating on the theoretical by-the-book math. In the real world, you'll need a 12-15HP motor to provide a usable 30 amps since that 30 amp load is going to also need some more ummph to get whatever it is going. There is so much efficiency lost in all the converting, whether from gas engine to electric output or 12 volts DC from an alternator (one conversion there already) to an inverter to provide 110AC. Again, most activity directed at alternate energy would be put to better use tweaking the final load's efficiency. We need a real breakthru such as EEstor or NanoSolar before the alternate energy scenario becomes really cost effective. Irritates me so seeing reporting on some yahoo spending 100K and having an energy hog house completely offline. Why? He could have spent a fraction of that on practical, efficiency-boosting strategies, and paid the remaing power bill on just the interest of the money he had left.

    Bookmark   May 25, 2009 at 3:00AM
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seeklfn_yahoo_com

i know for a fact that if u give alittle back to the prosess thats giving it will continue to do so and ive built a happy all electric generator and i want to sell it

    Bookmark   April 18, 2011 at 6:53AM
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soma_koma2000_yahoo_com

Not less than 1500 rpms, but it's beter to be about 3000 rpms, the alternator generates power at any speed, even less than that .. OK

    Bookmark   June 9, 2011 at 4:24PM
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doug9694

On the average the alternator runs two to three times faster
then the engine. If you notice that the lights dim when idling
then you need a little more rpm to get the alternator to charge above the battery voltage. About 13.5 volts. The average alternator is now between 1,300 to 2,100 rpm.
I set up an old 35 amp alternator to run off a 1/3 hp 1,750 rpm washing machine motor at a 1:1 belt ratio. Using the stock regulator would stall the 1/3hp motor. I put a large adjustable resister on the field connection of the alternator. The output was adjusted by the field resister so the motor would start charging a low battery and not get too hot. This was about 20 amps in my case.
A human can only put out about 1/4 hp. So that would be about 14 amp. How long could the average humane do this?
I would guess about 5 minuets. 13.5v x 14 amp = 189 watts or about 1/4 hp.

    Bookmark   July 20, 2013 at 2:05PM
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