Generating a little electricity

Drascomber

Drascomber

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As there seem to be a lot of Electrical gurus on the forum . .

I should like to make a generator that is smaller by a factor of 10
than those commercially available.

Is there advantage of an alternator over a dynamo for a dedicated generator? I always understood that the only advantage for cars was just that you could run them faster so it was safe to gear them up so that they worked at low engine rpm.

How much advantage does a dynamo have over a electric motor externally driven?
 
I've also been looking for a cheaper alternative to assist in battery charging, not wanting to run the main engine to the charge the batteries. AC requirements being looked after by an inverter.

It makes sense to me to run a dedicated 12v high output (say 100amp alternator with a small, say 50 - 85cc engine. Whether this is feasible or not the gurus on this forum may have some ideas on the subject.

Such a genny would be hopefully quieter, lighter, smaller and more efficient than commercial offerings, that all seem to be based on 240v and diesel motors.
 
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It makes sense to me to run a dedicated 12v high output (say 100amp alternator with a small, say 50 - 85cc engine.

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Of course, it's feasible.

Honda has done it. There is even a "battery charging cable" available.

It fits pretty well your specs : 12V - 1.25 kVA (that's 12V - 100A) 50 cc. engine.
And its very low noise : mine runs on the after deck, and I happen to forget it running even going to bed! ... Really. Just place it on a 1" foam sheet.

Of course, you can assemble one from a scooter engine, a second hand alternator, etc. I doubt the result will be any better, even if cheaper ... may be?
 
Very cute site. A bracket out of 1/8" steel is woefully underdesigned or every car manufacturer is doing it wrong and they still break! Spinning a 65 amp generator at full output is going to require a lot more horsepower than the engines shown. I know that 65 amp at 12 volts is only prox 800 watts or 1 hp BUT the 1 hp doesn't come a 100% efficiency. Likely 20% on a good day. Now that is 5 hp. Next on the agenda is mechanical loss of the alternator. Throw in a few more hp. Alternator runs about 95% +- and now we are up to 10 hp. Now you cannot run the engine against the wall for long ever. A safety factor would dictate 15 hp at a minimum. Now we have a non water cooled, hot, CO generating machine making a racket. That sure will be popular!

This is not fiction. Grabbing 40 amps off of your engine mounted generators is common.

Lastly, hi all. Nice forum!

Ted
 
Drascomber, What's going to turn your genny? Wind, Sea, old outboard head?
If you get an alternator from the local scrappy, and disconnect the regulator, replacing it with a rheostat, you should be able to balance the output current to the motive force available. So, if you only have a small motor, you turn up the amps until the motor is under load but not overloaded, and accept whatever comes out. If you want to go really small, bolt a couple or three dyno hubs (from bicycles) together and spin them (connected in series). If you have an outboard, does it have a lighting coil? If not, try fitting one and a bridge rectifier. (I made one for a seagull silver century in the 60's and it popped a little into the battery) You haven't really set the parameters.
 
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Spinning a 65 amp generator at full output is going to require a lot more horsepower than the engines shown. I know that 65 amp at 12 volts is only prox 800 watts or 1 hp BUT the 1 hp doesn't come a 100% efficiency. Likely 20% on a good day. Now that is 5 hp. Next on the agenda is mechanical loss of the alternator. Throw in a few more hp. Alternator runs about 95% +- and now we are up to 10 hp. Now you cannot run the engine against the wall for long ever. A safety factor would dictate 15 hp at a minimum.

[/ QUOTE ]Welcome to the forum. Your first post, sadly, is both inaccurate and misleading. No way do you need a 15hp engine to run a 65A alternator! If you're interested in how little power is actually needed, take a look at the specs of a few DC generators. The HFL Traveller uses a 4hp engine for a 12v 100A alternator. The Fischer Panda uses a 7hp engine for a 12v 220A alternator.
 
Firstly we need to reiterate that the amp rating of an alternator is that max current that it can provide (with regulator flat out and high rotation speed) that will provide about 12 volts. Few alternators will provide 14 volts needed for battery charging at anything like full current. ie the rating is capability to supply a 12v system not to charge a battery.

As suggested using a car alternator a rheostat (variable resitor) in place of a regulator can set the field current to near max and the engine speed and power then will determine how much goes in the battery.

You need an amp meter to tell you how much is going in the battery. I suspect you would be very disapointed at just how much charge you can get from a Honda gen with "12v" o/p. The best current would comn from a charger running on the 240V o/p.

Back to the original question. A car type alternator has small brushes which transfer the field current only to the rotating field coil. The brushes are in constant connection and carry about 1 amp. The actual power is taken from a 3 phase pick up coil arrangement around the outside (fixed) and this is rectified by 3 phase rectifier. So actual generated current comes via the rectifiers. This physical arrangement allows much higher speeds before the rotating parts disintegrate.

In a generator (dynamo as in cars of old) the field coils are on the outside with much more iron and more residual magnetic field for self starting. The rotating coils generate the power (AC) which is collected and rectified by the switching action of the brushes on individual segments. Hence brushes carry all the current and have a fairly finite life and can carry far less current for their size. The armature is complex and more liable to disintegrate from excessive speed so speed range is a lot less. Hence on a car they usually run so slow at idle they will not generate. (gearing is such that they can handle the highest engine speed but not work at the lowest)

Now to build a small current generator. (presumably from wind or water flow) you need to find a suitable generator or preferably alternator. It needs to be able to work at low speeds unless you have a gearing up of speed. (with loss of power)

Bicycle generators gave a few watts but the type I am familiar with ran at high speed off the side of the tire with a small wheel on the gen.. They generate AC but this can be rectified with diodes.

I think the next easiest source of gen of small size is in fact an electric motor. You need one with permanent magnet field ie you can feel the pull of the magnets in an uneven manner as you rotate the motor. If you can find a suitable sized 24 volt motor (or higher) this will have more chance of producing 12v when driven at a reasonable speed.

If you have a large battery to charge then you won't need a regulator the load of the battery will drag the motor output down to the battery voltage and the current will not cook the battery. However if you have a small battery and the motor delivers some current you will need something like a solar panel regulator of suitable current rating.

Good luck you will need an ampmeter to determine charge current and a lot of patience. olewill
 
And a lot of small boats only have a 10hp engine (e.g. a Bukh DV10), and that needs to drive the prop as well.

I have read somewhere that 1hp for every 25A generated is a reasonable "rule of thumb" for a 12V system.

J
 
What a question to put on here but the simple answer by reason of cheap assed cruisers using the machine is HONDA, HONDA or HONDA little red suitcase gennys.
Cheapish, very quiet, fairly lite and easy to maintain.
Or we can open the Solar and or Windgen bebate for laughs, just for general info our 36 cat has 2x 85 watt solar panels and 1 KISS 300watt windgen and we are going to add more solar(2 x75watt) and upgrade the battery bank to 800amp(8 x trojan 6v 125amp).
 
Hi,

What I am trying to achieve is a charging current of about 4 amps at 14v to charge the boat's battery which comprises a pair of parallel 26AH wheelchair batteries. There is no engine starting requirement. A light and compact package that can run for a few hours a day is the target.

Available alternators are way too big in both capacity and physical size but small electric motors are available in a wide range of sizes and specs.

I have charging current from my outboard. My ammeter shows that it produces 2amps at 3,500 rpm and only a little more flat out, but since the outboard typically runs for 30 minutes a day if there is any wind, it does not provide enough juice for a Waeco compressor cool box. I do not have enough deck space for a solar panel big enough for the job unless the sails are down and the sun is out. Available wind generators are designed for much larger yachts than mine.

So I want to play - experimenting with water drive, a small internal combustion engine and possibly wind.

I have to start somewhere and have decided to start by choosing one of the devices that will generate electricity if its shaft is rotated at an appropriate speed - and before someone suggests motorcycle alternators they appear to also have a greater output than I need.

Richard & Bill - Is this enough info?
Thanks William_H. Most helpful response so far.
More responses invited.
 
You don't tell us which Waeco unit you are wanting to run. Presumably its the CF18 as that's the smallest I can find. It is difficult to figure out how much power it uses but I think I have discovered out that when running it consumes 35watts or 2.9 amps (call it 3 amps) and I think I have deduced that in maintaining an internal temperature of 5C when the ambient is 20C it will run for approx 10% of the time. Therefore to keep pace with its current consumption you will need something that produces about 0.3 amps if running continuously and proportionally more for shorter periods of operation. Something producing the 4 amps you specify will only have to run for about 7.5% of the time.

You have ruled out a solar panel and a conventional wind generator for the fairly obvious reasons. I looked at the vertical axis Forgen machines. The larger of the two would satisfy the requirement to produce 0.3 amps continuously in the sort of wind strengths you are likely to be sailing in, while the smaller one would probably fall short. It would be genrating all the time there is wind regardless of whether you were sailing or not and on a windless day you would be getting power from your outboard. The trouble is that they are very expensive for their outputs. £575 for the larger one.

Another alternative is a towed water driven generator such as the Aquagens. The Aqua4gen will generate 1.5 amps at 3.5 knots and 2 amps at 4 knots. Provided you spend a significant proportion of your day actually sailing ie 15 to 20%, as opposed to on the beach or at anchor, that would just about do but at £599 is slightly more expensive than the Forgen although in terms of output better value.

That brings us back to your idea of a small DC generator driven by a small engine but so far no one has come up with any practical suggestions.

A small motor run as a dynamo may seem like the basis for something compact but you then have to consider an engine to drive it, and an adequate enclosure for sound proofing. or a wind turbine or a water turbine. You are unlikely to produce anything to rival the commercially available wind turbines, which you have already dismissed or the Aquagens which I have discussed above so, unless you have some idea for an acceptably quiet and small petrol engine, I don't see where that idea will take you.

You do not want a portable generator on the grounds of size. The smallest Honda weighs in at 13.5kg, measures 450mm x 240mm x 380mm, is reasonably quiet at 52db and at around £570 it costs a similar amount to the other methods considered In terms of power output per £ is by far a better buy. There are of course cheaper models about such as the Wolf ones, the smallest of which is a similar size to the Honda, a little heavier and rather noisier but at £90 much cheaper.

That has got us nowhere but it has been an interesting exercise digging out all the facts.

I think I'll stick to my current arrangements. Some freezer packs and an insulated box!
 
Hi Vic,

Very interesting. That is similar to the thought process that brought me to where I started this thread (and will eventually result in development of the solution).
 
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and will eventually result in development of the solution

[/ QUOTE ] <u>Will,</u> I notice. <u>When</u> it does you will share the solution with us won't you.
 
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