DIY 12V generator

[pieter]

Inspired by a thread on a German motorhome forum (link, 34 pages when I last looked!), I decided to have a go at building a 12V petrol engine powered generator when I saw a suitable Honda engine on sale for below half price.

 

I am a motorhomer, normally just spying on you lot for interesting technical bits, but thought maybe someone would find this of interest. Obviously, for those of you with big (or even smaller) generators to run mains equipment like washing machines, this has no relevance, as it is only able to charge batteries, not generate mains.

 

The basic setup is very simple:

- a small petrol engine (normally used in strimmers)

- linked to a brushless motor as used in RC models; these work as generators when driven

- a 3 phase rectifier

- I added a voltmeter, but that is not strictly necessary

 

 

This gives a fairly light generator capable of I would guess 30A, although so far only tried it on slightly discharged batteries and got 15A.

 

 

The clutch is removed from the strimmer engine, and replaced by a flexible coupling.

 

 

A mounting plate is used to fix the motor.

 

The recitfier can be seen here.

 

 

It gives me a bigger charge current than a 230V generator driving the van's battery charger would. Obviously it doesn't give me mains electricity, but the main reason for me to have a generator was to charge the batteries, and this does that with a quality Honda engine, for a fraction of the price of a Honda generator.

 

As mentioned at the beginning, not my idea, but maybe slightly different implementation. You could do the same by coupling an engine to an alternator, but this would be less efficient and heavier (admittedly more an issue for a motorhome than on a canal boat).

 

Plus it was an enjoyable project.

 

Pieter

Edited February 28, 2011 by pieter

[grahoom]

Hello Pieter, can you let me know what the brushless motor you used was?

 

nice post by the way.

[deletedaccount]

Nice

 

Although I wouldn't know where to start tbh.

[blodger]

Yes. I think that doing the DC directly is far more effficient than generating AC to power a charger.

 

I have built a few Suffolk Punch driving Lucas Dynamos for 15 amp output and a couple of B&S 3.75 driving alternators for 30 amp output.

 

One problem is that the DIY efforts are much noisier than the suitcase jobbies and sound like lawnmowers.

 

I did not think that anything smaller would put out enough output and/or overcome the initial start up load but it seems you have.

 

I have looked at RC PM motors before, probably for wind charging use, but came to the conclusion they were too small so am likewise interested in the type of motor you have used.

 

I look forward to any further postings

[grahoom]

I have looked at RC PM motors before, probably for wind charging use, but came to the conclusion they were too small so am likewise interested in the type of motor you have used.

 

similar, I am planning a very simple micro-hydro experiment and am looking for a suitable motor - the example that is used by the OP seems like it ticks the boxes for my little experiment.

Edited February 28, 2011 by grahoom

[pieter]

Thanks for your replies.

 

The motor I used is a Turnigy 5065 Brushless 270kv, but I suppose any make will do. You would have to match the kv value (not kV kilovolt, but stands for rpm/Volt) to the rpm your engine does. I went for something that generated the required voltage for my batteries (14.4 V) at near the max torque rpm of the engine, assuming that it didn't need the (higher) max power rpm to generate the kind of currents I was after, which worked well.

 

It is indeed quite noisy. I don't know how noisy or quitet the suitcase generators are, but this might indeed be noisier. Maybe the next step would be some sort of sound absorbing housing?

 

Pieter

[Robin2]

Very interesting.

 

Have you any fuel consumption figures, and if so how accurate are they?

[FadeToScarlet]

I want a nice little Lister A with a dynamo...

[onionbargee]

There are much better units out there than this, an un sheilded 5000+ rpm strimmer engine will be horrendously noisy, and a permanent magnet multipole alternator like a motorbike unit would give a much better output.

[Sir Nibble]

I want a nice little Lister A with a dynamo...

I used to have one, the dynamo came out of an AEC regent bus (24V 55A) and is now on a lister D.

 

Back on topic, how much did that motor cost? My understanding is that these things are quite pricey. I note also that you need (and by the look of the pictures have) a rectifier/regulator.

[blodger]

I want a nice little Lister A with a dynamo...

 

Not very portable which makes it a bit theft resistant. I presume you keep it on the bank beside your mooring?

[Sir Nibble]

Not very portable which makes it a bit theft resistant. I presume you keep it on the bank beside your mooring?

Has the advantage that you can make tea out of it.

[blodger]

There are much better units out there than this, an un sheilded 5000+ rpm strimmer engine will be horrendously noisy, and a permanent magnet multipole alternator like a motorbike unit would give a much better output.

 

One of the problems with ordinary alts and I presume motorbike PM multipole ones is the torque needed especially at start up with discharged batts sop necessitating a more powerful unit.

 

I think it is quite an achievement to get 15amps plus out of a strimmer motor, the sound of which may not be music to everyone's ears.

 

I think I'll have a look at the engine on some hedgetrimmers that I never use to see if it is similarly adaptable.

 

It's some time since I had a motorbike but then the alts were very low output. These RC motors are quite compact and powerful and are mostly built for high revs, the opposite to what you want for generation. I have seen 200 rpm/volt for about 30.000; I'll see if anythbing suitable comes up 2nd hand for a bit.

 

I used to have one, the dynamo came out of an AEC regent bus (24V 55A) and is now on a lister D.

 

Back on topic, how much did that motor cost? My understanding is that these things are quite pricey. I note also that you need (and by the look of the pictures have) a rectifier/regulator.

 

I think a similar rectifier needed as per driving stepper motors to generate lecky.

 

I also wonder around cooling needs. These motors are used for short periods powering helicopter/boat models. I should think that a cooling fan would be a must for tankfuls of power generation

[pistnbroke]

I recently purchased a new strimmer from the rejects at a k mart( $30) ..26cc two stroke and have a motor like yours rated at 3200w ( 4 hp ) Where did you get the coupling ? Was yours a 4 stroke ? What rpm was it run at or limited to ? Did you regulate the output with the voltmeter and revs?

 

I have large electric outboard for my canal MC wanted to have generator to charge battery/run outboard if the battery goes down before I get home ...Advantage of yours its that if you rate the diodes high enough for voltage then you dont need to worry about always having a battery connected.

 

Also have 6.5hp Honda 4 st and 65 A alternator which I had intended to use ...perhaps heavier quieter and more robust . I understand the bearings on some of the aero motors are not good long term ....

[FadeToScarlet]

Not very portable which makes it a bit theft resistant. I presume you keep it on the bank beside your mooring?

 

Not currently got one and despite spending time lusting on eBay, I can't get one right now. I wouldn't be able to keep it on the bank because that's against the mooring licence conditions set by the landowner, and lugging on and off the boat is impractical!

 

Otherwise there's a fair few items online that would have quite a few bids!

[pieter]

Very interesting.

 

Have you any fuel consumption figures, and if so how accurate are they?

The only figure I have so far is Honda's quoted consumption of 340 kg/kWh, which is just under half a liter per kWh.

 

Haven't used it enough to have my own figures.

 

There are much better units out there than this, an un sheilded 5000+ rpm strimmer engine will be horrendously noisy, and a permanent magnet multipole alternator like a motorbike unit would give a much better output.

Agree about the noise. Not convinced about the alternator giving a better output.

 

I used to have one, the dynamo came out of an AEC regent bus (24V 55A) and is now on a lister D.

 

Back on topic, how much did that motor cost? My understanding is that these things are quite pricey. I note also that you need (and by the look of the pictures have) a rectifier/regulator.

Hi,

 

The motor was £57 inc postage, the rectifier £6 plus £5 shipping from Hong Kong.

 

I recently purchased a new strimmer from the rejects at a k mart( $30) ..26cc two stroke and have a motor like yours rated at 3200w ( 4 hp ) Where did you get the coupling ? Was yours a 4 stroke ? What rpm was it run at or limited to ? Did you regulate the output with the voltmeter and revs?

 

I have large electric outboard for my canal MC wanted to have generator to charge battery/run outboard if the battery goes down before I get home ...Advantage of yours its that if you rate the diodes high enough for voltage then you dont need to worry about always having a battery connected.

 

Also have 6.5hp Honda 4 st and 65 A alternator which I had intended to use ...perhaps heavier quieter and more robust . I understand the bearings on some of the aero motors are not good long term ....

Yes, it is a four stroke, so noise a bit more bearable than two stroke.

 

The coupling was off Ebay,but you can get them here as well.

 

The reason I went for a flexible coupling is exactely because of the point you make about the bearings on these little motors possibly not being very robust. I cannot comment on long term reliability yet, but on the forum I got the idea off one guy has been using his (slightly different construction) for I think hundreds of hours.

 

6.5 hp is much more than you need for 65A.

 

 

I do agree that if you live on board fulltime and a generator is your sole or main source of recharging the batteries than this is not the solution for you. For me it is ideal because it is only for occasional use in addition to solar and mains hookup.

Edited March 1, 2011 by pieter

[pistnbroke]

What is not said in this is how to regulate the output ....the governed speed of the 4 stroke should be set so that when a fully charged battery reaches 14.4v the charge current is very low say below 0.5A . Pushing current into a battery to raise it over 14.4v is not good .

 

a series regulator using a MOSFET is a good idea or incorperate 3 into the three phase bridge. this will make the engine speed non critical.

 

The idea and construction is superb but without automatic regulaton the voltmeter is essential .

[Gibbo]

a series regulator using a MOSFET is a good idea or incorperate 3 into the three phase bridge. this will make the engine speed non critical.

 

If you can find a mosfet the size of a housebrick and mount in the Atlantic ocean for a heatsink.

[Chalky]

If you can find a mosfet the size of a housebrick and mount in the Atlantic ocean for a heatsink.

Got one on my desk!

[Sir Nibble]

The regulator/rectifier deals with it. They used to consist of meaty stud mounted zeners in the +ve side of the bridge, these days I believe they do use FETs, this is why the case is a finned cast lump.

[Gibbo]

The regulator/rectifier deals with it. They used to consist of meaty stud mounted zeners in the +ve side of the bridge, these days I believe they do use FETs, this is why the case is a finned cast lump.

 

The one in the photos is a 30 amp unit. Assume it regulates at 14.4 volts. When the battery is fully charged the reg somehow has to get rid of 430 watts (and that's a best case scenario). There is no way on this planet that heatsink will do that. It's not even close to being big enough.

[larkshall]

If you can find a mosfet the size of a housebrick and mount in the Atlantic ocean for a heatsink.

 

Ever heard of Ocean Warming?

[Sir Nibble]

The one in the photos is a 30 amp unit. Assume it regulates at 14.4 volts. When the battery is fully charged the reg somehow has to get rid of 430 watts (and that's a best case scenario). There is no way on this planet that heatsink will do that. It's not even close to being big enough.

Fine, there are loads of these rectifier/regulators available, they are about the size of a packet of fags and work by dissipating the unwanted energy. I'm inclined to agree with your snapshot appraisal but nonetheless they do work. 30A is optomistic though, I think 10-12 is more like it.

[Gibbo]

Fine, there are loads of these rectifier/regulators available, they are about the size of a packet of fags and work by dissipating the unwanted energy. I'm inclined to agree with your snapshot appraisal but nonetheless they do work. 30A is optomistic though, I think 10-12 is more like it.

 

That heatsink looks to be about 20cm long, would you agree?

 

If it is that sort of size, then even at 10 amps, it's going to get to at least 125 degrees C above ambient.

[pieter]

The heatsink you see on the pic is the rectifier, it has no regulator (as yet).

 

I regulate by manually adjusting rpm to give me the voltage I need.

 

I think effective automatic regulation would need to include a servo to adjust the throttle. It seems very ineffective to have the engine run at full power all the time and just burn up the excess energy as heat.

 

Pieter

Edited March 8, 2011 by pieter