# Alternator Torque Requirements

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

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You figured out that I own stock in First Energy? Damn!

Ron

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2. ### tcmtechWell-Known MemberMost Helpful Member

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Typical lawn mower engines run at 3600 RPM as do most small portable generators. A 2.6 Kw (3.5 Hp) push mower produces about 7 NM of torque at 3600 RPM.

Alternators are cheap and easy to locate and modify to a PM rotor design as well and as far as efficiency goes they are surprisingly efficient given the output capacity Vs their physical size.
I am not sure how someone can see something the size of a coffee can that can out at least 14 volts at over 100+ amps while still being air cooled inefficient.

My math says a 3 CM pulley is 9.42 cm in circumference and that a surface speed of 22.4 meters a minute would be 237.67 RPM not 119.

23,000 NM at 237.67 RPM is roughly 575 kilowatts or about 780 horsepower! Unless this thing is the size of a large building complex I have serious doubts you are producing that much energy.

Unfortunately I suspect you have no real idea about what you are doing or working with being everything you have posted is basic physics that gets covered in early high school plus is easy to find on line as well. Just a guess at this point though.

3. ### SABornNew Member

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To run a car alternator off a petrol motor is one thing, to use it as a hydro, wind, or wave, generator is a completely different thing.

Every time you you convert the ratio of 1:1 to anything else you loose efficiency, its a slippery down hill slide.

There is far better alternator designs than the car alternator for use in RE situations where a 1:1 ratio can be maintained.

Secondly any alternator that requires power supplied to it, to excite it to start with can not be called efficient, a little loss under the bonet of a car dont matter much but in a RE situation its a huge difference.

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5. ### tcmtechWell-Known MemberMost Helpful Member

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I guess thats all relative to the type of alternator and the power source.
The commercial Leese Neville alternators I use typically self excite at around 1500 - 2000 RPM depending on what voltage its set up for in stock mode.

With a little reconfiguring of the windings they can easily self excite at below 500 RPM. After that its just simple PWM to limit the standby power that the rotor needs to keep it charged up which is only a couple of watts at the low end.

If I am running a AE power source that can handle a 24 volt 180 amp alternator supporting that additional 15 - 20 watts it takes to drive the rotor field at peak saturation is nothing nor is providing the two or three watts it takes at minimum is not an issue either.

Efficiency and output are highly relative to the power source and the load.

6. ### Integr8erNew Member

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"I am not sure how someone can see something the size of a coffee can that can out at least 14 volts at over 100+ amps while still being air cooled inefficient."

The efficiency he was referring too is not the relationship of space to power output, but rather the relationship of input power to output power, which is in fact very poor, probably because the auto companies are more concerned with low cost than high efficiency, and they have an excess of power available. I have a setup with an older GM alternator driven by a 3 HP B&S horizontal shaft eng with a pulley ratio of about 2 to 1 (on the vehicle I took the alt off of the ratio was about 3:1 but my 3 hp lacks enough torque to drive it like that) So at cruising speed the alt was turning about 7500 rpm and I figured their eff curve was based around that RPM. Anyway 3 Hp petrol engine will only output about 20 amps and then it loads the engine down and kills it. I have used two different engines one an OHV engine, and one a flat head, and both are about the same, but as expected the OHV would do a little better, but not much. I am an electrical engineer and I fully understand the Physics behind 746 Watts = 1 HP, but based on empirical data it appears that Alternators are only about 10% eff, however I suspect also that the engine manufacturers are also fibbing a bit, and you also have losses in the belt pulley system, so I would say maybe 25% overall eff on an alternator.

Last edited: Dec 29, 2011
7. ### alec_tWell-Known MemberMost Helpful Member

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How about the fall in level as the waves recede? Is there some mechanism to ensure the torque is always in the same direction, or is the torque (at least approximately) sinusoidal with reversing direction?

8. ### tcmtechWell-Known MemberMost Helpful Member

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For those wondering about automotive alternator efficiencies, http://en.wikipedia.org/wiki/Alternator

60% is considered a typical average but it can vary greatly. Also if external mechanical parasitic losses from needlessly over spinning an alternator that does not need that high of RPM to get full output, along with over tight belts are factored in I can see where it could come out to be well less than 60% in a poorly engineered home built design and a 3 Hp rated gas engine would be less than sufficient.

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10. ### Akshay@336New Member

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have 98cc engine .it has max power 7.1ps@8500 RPM. max torque 8.1 Nm@5500 RPM . so which king of altarnator required to genrat max power on such cases

11. ### specWell-Known MemberMost Helpful Member

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This is just off the top of my head, but I understand that V belt drives are inefficient and that a flat belt is better and better still a chain drive.

spec

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December of 2011 thread? Do we want to resurrect it? Akshay@336 you may do better with a fresh post and thread.

Ron

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13. ### dr pepperWell-Known Member

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Ashkay.
I built a battery charger using a 60 amp alty and a 5hp engine, in theory 60a at 12v is 1 hp, however the 5hp just managed it, the engine bogged down on full load.
So I'd say for your 7hp m motor not much more than this, maybe 80a max.
Car altys usually produce max current around 6k rpm, so 1:1 ratio would probably work fine.

14. ### shortbus=Well-Known Member

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Most small engines are governed to run at 3600RPM. So that would be ~ 1.6:1 ratio on the pulleys, with the large one on the engine.

15. ### dr pepperWell-Known Member

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That makes sense, I have a couple of brigg's, one 1 hp cast iron block and the other ally, both do about that, the 5hp was a tecumseh and that did around 4.5k, typical import.
There seems to be a diffrent rating with engines and electric motors, either that or the ones I've tried have been worn out.
Ashkay says his engine puts out max hp at 8.1k, sounds like it might be a 2 stroke.

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16. ### tcmtechWell-Known MemberMost Helpful Member

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Comparing non-specific alternators being driven by non-specific engines is pointless.

Not all alternators are built to be as cheap as the manufacturer or rebuilder can get away with just as not all small engines are built that way either.

Anyone who has been around enough small engines knows how one manufactures rated HP either walks all over anouther or falls way short. I have noticed over the years that it takes a 12 HP Briggs and Stratton to replace a 10 HP Koller or Honda or a 9 Hp 40-year-old Wisconsin.
Same with alternators. The operationg specs and efficency for some POS cheapo rebuild are not the same for a commercial unit.

It's similar to comparing the old Motorola units Ford used to the old Delco SI units that Chevy used. On the test bench the factory new Motorola that was speced at 12 volts 55 amps barely held 12 volts and 55 amps at 8 - 9000 RPM whereas the 12-volt 63 amp speced Delco SI held 14+ volts and 63 amps at 4000 - 5000 RPM and both took similar HP to turn them despite vastly different outputs.

Given that many of the commercial units are speced at self-excite speeds of 1000 - 1500 RPM and rated output at 3500 - 4000 RPM all while having efficiencies of 80 - 90% or better.

So yea I can totally see where a crappy poorly designed and built cheapo alternator running of a 5 HP rated B&S with a 2:1 gear up might be all that engine can do to make 1000 electrical watts whereas a commercial built Leece Neville running at direct drive off of a 5 HP rated Honda might be putting out 3000 electrical watts with relative ease.

Cheap low efficiency alternator on a cheap engine VS a quality high-efficiency alternator on a quality engine. No fair comparisons can be made between the two.

17. ### MeryNew Member

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Hello everyone.

Suppose a generator with following characteristics.....

Don't double post and don't hijack old threads - Moderator.

Last edited by a moderator: Aug 16, 2016
18. ### MeryNew Member

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Ok understand.

Thanks