# Alternator Torque Requirements

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Threefingeredjack, May 26, 2009.

1. ### ThreefingeredjackNew Member

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Hi everyone, I'm new to the forums and happened upon this site when trying to find the answer to this question:

How much torque, in Nm, is required to turn a variety of alternator sizes? Say from 5Kw to 50Kw? Is there any kind of rule or formula I can apply? For arguments sake, 50Hz, brushless, three phase. The kind you'd use to generate household power.

I'm trying to build a micro-hydro project and have some ideas, done the maths so I know how much torque I have, but I need to know how much I can do with it. Any help would be very much appreciated, thanks guys.

Great site, glad I found it. Keep up the good work.

2. ### tcmtechWell-Known MemberMost Helpful Member

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torque time RPM divided by 5252 will give you horsepower.

So just work it in reverse. Generators are typically 2 pole 4 pole or 6 pole. that equates to 3000, 1500 and 1000 rpm at 50 hz.
if you had a 4 pole 10 kw generator you would need roughly 18 hp to drive it after factoring in efficiency losses.

5252 x 18 = 94536 94536 / 1500 = 63 ft pounds of torque or about 85.5 NM

Check out calculatoredge.com they have calculators for every thing you will need!

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3. ### MikeMlWell-Known MemberMost Helpful Member

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Good info tcmtech. Just for a point of reference, a standard automotive alternator puts out 60A @ 14V = 840W. 1hp = 746W, so it takes over one hp to turn your car's alternator to full output, not counting losses in heat or the belts...

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5. ### tcmtechWell-Known MemberMost Helpful Member

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Try arguing that with some car guys!
I got few locals that sware a stock 63 amp gm alternator takes at least 10 hp to run!
And any air conditioner compressor is well over 25 hp!
(both instances they are off by factors of ten or better.)

6. ### ThreefingeredjackNew Member

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Cheers tcmtech, thats exactly the kind of thing I was looking for, you've helped me out massively.

[Who 8 Ω Π ?]

7. ### lockelokiNew Member

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Hey guys,

My wave-power device produces 690 N.m. What is my best generator choice?

I've been up all night trying to learn the math for this project, but now I'm just plain burnt out.

8. ### tcmtechWell-Known MemberMost Helpful Member

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What RPM can you produce while maintaining the 690 NM?

Torque x RPM = power.

9. ### SABornNew Member

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Yes i will disagree with that.

Try running a 60 amp alt off a 3 Hp petrol motor and see how much current you draw before you load the motor down.

By memory i think around 15 amp would be about the limit.

10. ### tcmtechWell-Known MemberMost Helpful Member

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I have done that before and 3 Hp lawn mower engine is more than enough to power a 63 amp Delco alternator even with the voltage regulator bypassed in order to make a home made DC arc welder.

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11. ### ronvWell-Known MemberMost Helpful Member

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The problem with the gas engine is that the rated horsepower isn't developed until high RPM so the gear ratio is very important.

12. ### ReloadronWell-Known MemberMost Helpful Member

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My understanding on automotive alternators is they typically run off the belt around 2X or 3X engine speed (crankshaft speed). Also on a good day they are about 60% efficient and the more power they deliver they really fall off as to efficiency. I think the bottom end to get much of anything is about 2,000 RPM. Then too, this is really getting off topic but I am curious as this would run with tcmtech using a 3 hp motor. Makes sense anyway. Assuming a respectable speed.

Ron

13. ### tcmtechWell-Known MemberMost Helpful Member

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I built it in my late teens out of an old B & S push mower engine. I used a 5 inch cast iron pulley on the engine and the alternator had a roughly 3 inch pulley.

It would weld but not all that great being it was maybe 70 amps at short circuit and 50 volts open circuit. Engine wise it would pull down noticeably but it still held its RPM's well enough.

There are plenty of small portable generators out there that only have a 3 HP engine on a 1500 watt system and they work well enough. I see a few old ones every once in a while at work.

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14. ### ReloadronWell-Known MemberMost Helpful Member

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I think my old gasoline generator is 4 KW using an 8 HP B&S engine (3600 RPM). Pretty much runs with what you explained. Your pulley ratio makes good sense. Well I guess we wait for the return of the OP and see where it goes.

Later
Ron

15. ### lockelokiNew Member

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The RPMs will vary since the alternator will be driven by the rise in water level caused by waves at the beach. I am new to generators. Do I need a constant level of RPMs? All I know is that the device will produce 690 nm (which i JUST learned how to calculate last night ) worth of force to turn the pulley on the alternator whenever there is a change in the water level. I know that "torque = radius of pulley X force applied" and so my torque with a 3cm radius pulley will be 2070? I'm really getting into uncharted territory here guys...

16. ### crutschowWell-Known MemberMost Helpful Member

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Torque in nm is the torque in newtons at a one meter radius. Since there are 100cm in one meter, the torque-force at 3cm would be (690 x 100)/3 = 23,000 newtons.

17. ### tcmtechWell-Known MemberMost Helpful Member

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Well 23,000 anything at zero RPM is zero power if that gives you an idea of what you get without sustaining constant motion while also providing steady torque.

18. ### SABornNew Member

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A car alternator is a poor choice for a hydro/wave system and are way to low in efficency to be of practical use, you should look towards a large permenant magnet motor, or research some wind or hydro generator designs to best use the energy source you have.

19. ### lockelokiNew Member

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I know that the belt attached to the generator will produce this 23,000 torque at 22.4 meters of movement/minute. With a radius of 3cm, the circumference of the pulley will be 18.85 cm. This gives about 119 RPMs without a gear ratio. With a 1/17 gear ratio the device could accomplish 2020 RPMs at 1353 Nm. What is a normal level of RPMs needed for a generator? How are these figures compared with that of a lawn mower engine?

20. ### ronvWell-Known MemberMost Helpful Member

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I'm with SA. Waves are slow. Google up PM alternators.
Tell us some more about your gadget. Wave by wave? Charge a battery?

21. ### shortbus=Well-Known Member

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You guys are just part of the conspiracy to keep cheap power from the hands of the public!