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Low rev. dynamo

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Johan van Zanten09/06/2010 13:26:25
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Hello everyone,
 
I am currently building a 3 cylinder Fairbanks- Morse type RE engine which is about ready to test.
 

The prototype engine was intended to generate electricity so I am looking for a suitable generator. As the generator  is direct coupled to the engine crankshaft it only makes 500-600 RPM. I tested serveral DC moters but the best only generated 6 Volts and 2.5 Amps.  I also tested a motorcar alternator, which fits the engine perfect, but it starts to generate some power at 2000RPM.  I carefully removed the stator windings and replaced them with more turns of thinner wire. The result was that it self exited at 1000 RPM (still much to high ) give an exellent 12 Volts but hardley enough power to  light a small bulb.
Is there anybody out there with some expierence?  Is it usefull to test a wheelchair motor or a big steppermotor?
The ideal generator should give 12-14 Volts AC or DC and about 3 Amps. at 500-600 RPM
 

Johan van Zanten,
 
The Netherlands
Ian S C09/06/2010 14:39:17
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Hellow Johan,I think my motors are a bit smaller than yours' I build hot air engines(addicted),and run small DC motors as generators, the biggest is the 220V 500watt motor from a Braun food mixer,it gives about 14V at about 300rpm. I don't know what power you expect to get from your engine, but consider getting about half that out of the generator and you will be doing very well. The motor from a tread mill(the local gym might have a warn out one), is a good one, the revs at full power arn't too high, they are rated at over one HP ie., 1100/1200 Watts, and you get out what you put in, and its a bit moer efficient than the smaller motors. Direct drive should give quite a reasonable out put.Ian S C
Alan Worland09/06/2010 21:40:46
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Just a thought (not sure how good) but could you use an inline epicyclic gearbox to put the revs up to what's required?
Roger Hart10/06/2010 15:37:28
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Hi Johan,  The problem is the low revs/min.  From my old "Dynamos & Electric Motors" book of 1902 I reckon you need an armature about 120mm dia and 100mm long to get the sort of power you seek from a conventional dc dynamo at say 500rpm.  Designs of the period are the Manchester design or the Siemens design.  You might look at the generators used for wind generators - static coils and rotating magnets - plenty of homebrew designs on the web.  The result is low frequency ac though.  Very impressive engine!
Les Jones 110/06/2010 20:28:27
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Hi Johan,
                   You could try to find a magnet that could be placed between the pole pieces of rotor of the car alternator. This would remove the need for it to generate the power to magnetize the rotor. Possibly a magnet from a loudspeaker would do if you can find one of the correct dimensions.  I have not tried this myself but it may work.
Les.
Ian S C11/06/2010 11:16:28
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Hi Les, I was going to do that a while back, but could not get the armature apart, even with a big hammer. May be a 20 ton press might move it, but large speaker or micro wave magnets would be ideal. Roger the low revs don't cause too much problem, wind the alternator 3 phase then rectify the output, this can be smoothed with a capacitor. The more poles used the smoother the output. Ian S C
DMB11/06/2010 11:41:24
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Hi all,
I noticed Roger Hart`s comment about low frequency ac. I do realise that oyu need pressure in the form of enough Volts to overcome resistance in the human body nad if Volts high enough can be dangerous BUT as I understand it, so can low frequency ac.
Possibly, lower volts than mains voltage, coupled with lower than mains frequency, could be a killer - be very careful!
Not sure, but I think thats how the electric chair used in USA works - gradual reduction of frequency.
John. 
Steve Garnett11/06/2010 12:54:18
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Posted by John Coleman 1 on 11/06/2010 11:41:24:
 
I do realise that oyu need pressure in the form of enough Volts to overcome resistance in the human body nad if Volts high enough can be dangerous BUT as I understand it, so can low frequency ac.
Possibly, lower volts than mains voltage, coupled with lower than mains frequency, could be a killer - be very careful!
Not sure, but I think thats how the electric chair used in USA works - gradual reduction of frequency.
John. 

 

Those of us who have been involved with large quantities of electricity and lived to tell the tale will tell you that there's a little rhyme that sums up the real situation quite well - "It's the volts that jolts, but the mills (milliamps) that kills".
 
A lot of volts on their own don't actually constitute a life threat, otherwise even electrostatic build-ups would be lethal. It's current that's the killer here, not voltage. And AC at any frequency is safer than DC, because at some point in its cycle it passes through zero, and because it keeps doing this in positive and negative directions, it has an average value of zero as a consequence. DC just keeps flowing, and if you are in the process of getting a shock you get no chance of letting go because your muscles won't relax. There is an important consequence to this, and it's worth remembering; if you are going to touch anything that may be capable of giving you a shock, use one hand only and make sure that you touch it first with the back of your fingers, not the front. The back of your hand is more sensitive, and if you get a shock and naturally pull away, you won't be gripping it more firmly instead - which is what is likely to happen if you touch it normally.
 
If you can find a way of passing 50 milliamps through a human body, chances are you'll kill it. Strangely enough, if you pass 5 amps through one, it might survive because bone conduction effects will kick in (depending...). My understanding of what happens with those barbaric American killing chairs (where they're still used) is that they go through a whole regime of different voltages and currents in a specified order, just to make thoroughly sure the recipient is non-resurrectable - and to make some smoke.
 
Oh, and the American electricity system is inherently more dangerous than ours as well. In order to get the same amount of energy (work) out of a 110v system as the uk nominal 240 volts you need more than twice as much of that dangerous current to flow, and at 110 volts, it's quite capable of doing significant human damage.
Ian S C11/06/2010 14:38:33
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We don't yet know the HP/ Watts of Johan's engine, so we can'tdo more than guess about what sort of power generation can be set up. One of the main problems with low frequency AC current (less than 50hz), is that to be efficient more iron is required in the core, and more copper in the field. Proberbly the best thing is to convert the AC to DC, then if AC is required, use an inverter and produce 50hz or 60hz at the voltage required. Ian S C
Steve Garnett11/06/2010 22:31:31
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In the shed I have a couple of (inherited) old 1930's house-generator sets to restore, and they are both low voltage ones, and were originally attached to two-stroke low-revving Stuart Turner engines. I'd thought from all the indications in the construction that the generators were direct-coupled to the engines, but having now looked at the rating plates in the light of this thread, I don't see how they could be - to get their rated outputs, they have to be spun at 3000 rpm. And since I don't think that these engines run at more than about 1000 rpm, even on a good day, that appears to be a bit of a mystery I'm going to have to get to the bottom of...
 
But I'm sure that Roger and Ian are right - in order to get more efficiency from a low speed generator, it needs to have a larger diameter armature, not just for the stated reasons, but also to get the flux coupling more efficient, simply on the basis that the larger the diameter, the faster it flys past the stator segments at any given speed.
 
In the picture of the original engine shown (that being the Mount Wilson Observatory one), the generator overall is about 2/3 the size of the flywheel. At normal running speed the system generates 40kW @ 125v.DC from a 50HP engine, so I'm presuming that Johan has scaled his generator expectations accordingly. So whilst we really do need to know what scale it is, we already know from looking at the pictures and just guessing, that it's going to require more than your average old car dynamo...
 
I'm also convinced that given Johan's undoubted skills, he could probably make one now he knows what the issues are - which may well be the best answer.


Ian S C12/06/2010 12:50:01
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One thing Johan won't get is scale out put from his generator, as the size reduces so does the efficiency,a small machine will be lucky to attain 50% of the in put power. Just looking at the photos I would immagine a car alternator would be a similar scale to the generator on the full size genset, but its a good idea when photographing models to have something in the photo for comparison ie., a 6" rule or similar. He would like 12/14V AC or DC at3 Amps, a hub dyno for a bike would do this with ease. Ian S C
Johan van Zanten12/06/2010 12:53:43
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Low rev. Dynamo update.
 

Thank you for all your help and good suggestions so far.

I have certainly not the intention to electrocute somebody so I opt for a low tension say 12-24 volts. I need only a small current;  about 0.3 Amp for the cooling fan, 0.5 Amp to charge the ignition battery and a few Amps for some lamps. Say a total of 3 Amps that makes about 40-80 Watts of usable power.
 The power of my engine is unknown but I won’t push it to the limit.
I have learned that a few things are important. 
1. The diameter must be as big as possible
2. There must be as many as possible poles, and 
3. The magnetic field must be strong ( preferable permanent magnets )

          In a motor cycle garage I saw a Honda motor cycle with a nice 3 phase dynamo with 18 poles and about 14 cm in diameter. The rotor is of a drum type with very strong permanent magnets (See photo )



Together with the engineer we tested the output, straight from the coils, witch was about 30Volts AC at 1000 RPM. ( equals 15 Volts at 500 RPM ? ) The coil resistance was very low ( less than 0.6 Ohms ) witch means a constant tension and low losses.  The max. power is 320 Watts. All to all this look promising.
After my holidays ( 2 weeks Ireland and Northern Ireland ) I test my own rotor and stator purchased  from a broker.
I keep you informed and suggestions are welcome!

Johan van Zanten. 

Edited By Johan van Zanten on 12/06/2010 12:56:51

Edited By Johan van Zanten on 12/06/2010 13:02:55

Steve Garnett12/06/2010 16:01:54
837 forum posts
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Posted by Ian S C on 12/06/2010 12:50:01:
One thing Johan won't get is scale out put from his generator, as the size reduces so does the efficiency,a small machine will be lucky to attain 50% of the in put power.

 Actually I'd sort-of mentally included that in 'accordingly' - which is why I wasn't even prepared to speculate in the tiniest about what he might actually end up with!

Ian S C13/06/2010 12:22:55
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That Honda dyno looks good, if you don't take it someone else would love to have it for a wind turbine.Ian S C
Roger Hart13/06/2010 15:02:42
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I too like the Honda generator!
 
As a design guide in 1902 the book says:- "In large machines,  carefully wound, an efficiency of 1 volt per foot of effective wire on the armature at a circumferential rate of 1250 feet/min has been obtained, but, as has been stated, 1 volt per yard is what may be expected from small machines".
 
Plus ca change...
Ian S C14/06/2010 16:01:34
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Been looking at the net for Fairbanks-Morse and found a few similar motors both full size and models.I noticed that some of them had a belt pully,it seemed to be about half the dia of the flywheels which would be about right to run a generator at say 2 or may be 3 times engine speed. A modern generator can be made to look like a vintage one. I see that you can get castings for models similar to this one in the USA, did you go this way Johan, or did you build it from scratch, its an interesting engine either way. The only Fairbanks- Morse engine I'v had anything to do with is a 3hp open crank hit and miss, I did'nt realise how big the company was/is ie., it was Americas' biggest manufacturer until Henry Ford came along, not just engines they started by building scales. Ian S C
Stub Mandrel23/06/2010 20:32:22
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I'll endorse what Steve says; my Dad drummed 'volts jolts, mills kills' into me as a kid, and I'm still here despite lots of volts...
 
The honda generator looks good, but don't trust off-load voltages. As soon as you add a load the internal resistance will suck away many of those volts!
 
I have heard tell that stepper motors work well as low speed generators/alternators as they have lots of poles passing each other quickly giving a magneto-like effect. Expect the output to be in some sort of wierd polyphase (two sine waves 90 degrees out of phase perhaps?) which might require a little ingenuity to get smooth DC out of it, and don't expect to just hook the two phases together and get good results.
 
Good luck
 
Neil
Rob Stevens24/06/2010 18:35:03
12 forum posts
Sorry to arrive late.
 
Try looking in the direction of wind generator alternators - work well at 300 - 600 rpm.
 
Several designs on the internet - would need modifying / scaling down eg.
 
 
 
Stepper motors do work but produce relatively low frequency output and are prone to 'cogging' (try spinning one on load).
 
Rob Stevens
Rob Stevens24/06/2010 19:26:06
12 forum posts
Further to my last post try:
 
for information on using stepper motors for electricity generation.
 
Rob Stevens
 
Johan van Zanten03/07/2010 16:01:24
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Low rev. dynamo update 2
 
 
Back in my workshop again I started to test the Honda generator.
I first made an axle and bearings.
 

I used my lath to drive the dynamo at different speeds and measured the rectified voltages under different loads.
 
 

At 500 rpm  (red line ) the no load voltage is 13 Volts which drops to 7 Volts at a load of 8 Amps.
At a speed of 900 rpm  ( black line ) the no load voltage is 25 Volts and loaded 18 Volts at 8 Amps. That's over 140 Watts.
 

I think it's a very useful result.
The next job is to improve the appearance of the generator. There fore I make the coil spider revolving and the flywheel static.  To pick- up the current from the coils I have addid 3 sliprings.
 

Edited By Johan van Zanten on 03/07/2010 16:10:12

Edited By Johan van Zanten on 03/07/2010 16:14:51

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