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Motor as Dynamo/Generator

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Alan Worland05/07/2011 22:27:42
247 forum posts
21 photos
I had a little experiment tonight using an electric motor from a radio controlled model car as a generator - to be driven from my steam engine - and was most disappointed!
The motor itself is about 35mm dia by 55mm long and when in the car runs off 7.2v and is capable of some real torque/rpm. (a nice size)
With it being driven at about 1000rpm it would supply about a volt (I was hoping for more!)
Looking inside it the windings appear quite heavy, about 25 thou with 3 segments on the commutator.
My question is, is it worth considering rewinding the armature with more turns of a finer wire to get the voltage up? or look for another motor that might be more suitable?
I am after a generator that doesn't have to spin at thousands of revs.
Guidance/advice required please!
 
Alan
Clive Hartland06/07/2011 07:48:44
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2929 forum posts
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Your best bet would be an old bicycle dynamo, they worked very well at low rpm.
I think they put out about 3volts?
 
Clive
John Stevenson06/07/2011 08:13:56
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5068 forum posts
3 photos
What about using a stepper motor?
Never tried using one as a generator but if you connect two steppers together wire for wire, as you turn one the other turns in time with it so they must output something useful.
 
Might stuff one in the lathe later on today if I get a minute and see
Rob keeves06/07/2011 08:40:22
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5 photos
An electric motor as a dynamo dosn't work that well, the magnets are set slightly off bonk with the armiture this works wonderfull as a motor but as a dynamo not very well as you have found. Bicycle dynamos are a good bet, the stepper motor route will work very well and gives phases to get D.C. voltage you will need to rectify the O/P from the connector / wires but larger stepper motors require quite some force to get the armiture rotating.
 
David Clark 106/07/2011 08:50:49
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3357 forum posts
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Hi There
I used to use Como motor as a generator.
It produced about 9 volts at 1000 or so revs I think.
It was driven by a four stroke OS engine.
regards David
 
Les Jones 106/07/2011 08:51:12
2292 forum posts
159 photos
Hi Alan,
These comments refer to permanent magnet motors. (Which is the type you will probably use.) First choose a motor with a rated current equal to or greater than the current you require when used as a generator. The voltage output when used as a generator will be slightly less than the voltage required to drive the motor at a given speed. So if you chose a motor rated at 1000 RPM at 12 volts input I would only expect about 10 volts when used as a generator. As most small motors rotate at much more than 1000 RPM at their rated voltage you would probably need to select one of a much higher voltage rating. So if the motor was rated at 8000 RPM then you would need a motor rated at about 96 volts times about 1.2 which would be about 115 volts. To give the output you require do not expect the motor to be all that small. Even if you use a stepper motor an rectifier as John Stevenson suggests you will probably need a reasonable size motor. Wind up torches normally use a stepper motor but are geared to run at a very high speed.
Les.
Richard Parsons06/07/2011 09:13:04
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645 forum posts
33 photos

Alternators (your bicycle generator) have a better output at low revs than DC dynamos. This is why car makers changed to them when the reliable solid state rectifiers became available. What do you want to do with your steam generator set? Power some lights? Then use LEDS, no rectifier is needed.

Ian S C06/07/2011 11:31:31
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7468 forum posts
230 photos
Alan, I use motors out of vidio and tape machines, try and find a low revving motor. Just last year I got a 200V , 500W DC motor from a food mixer ( one of those stick ones by Braun), with that on my Ross Yoke hot air engine, I get about 20V with out a load, at about 1000 rpm, I have had around 5 watts from it at 10 V. These motors are nice and free running. I,v housed it inside a steel tube to hide its origin. Ian S C
Baldric06/07/2011 12:38:01
195 forum posts
32 photos


Posted by MICHAEL WILLIAMS on 06/07/2011 10:07:14:
Have a look at the Stuart Turner dynamo . Not available anymore (?) but very easy to make something similar and this design gave a useful output at relatively low rpm . The main reason for its sucess is that it's armature and magnetic circuit are relatively large diameter and with a lot of iron in them . Another attraction of this design is that it looks something like a real generator of early days and its size is in proportion to the engines likely to drive it .

 

Look at Forest Classics, they sell a dynamo that looks like the Stuart Turner one.
http://www.forest-classics.co.uk/pm_research_dyn-1m.htm

Edited By Baldric on 06/07/2011 12:46:24

Alan Worland06/07/2011 22:33:51
247 forum posts
21 photos
Thanks for all the responses. I think being realistic I will only be running a few bulbs/leds but I wanted to get the engine working at low revs to enjoy the exhaust!
I see bike dynamos on Ebay are very cheap and I found one that would give 12v at 6w but whatever I used would need to be diguised in some sort of casing to resemble a proper generator. My memory seems to recollect that hair dryer type motors have a permanent magnet type motor in, wired through a diode or two to obtain the DC required - might have the wifes dryer undone when she goes out!
The Forest generator looks great and has a decent output but I wonder what it would give at much lower revs? Like the idea of building it though.
I guess a rubber belt would be used for the drive - the Forest generator looks like it might be a flat belt? Has anyone got one?
I do like this part of a project, almost as much as building it all! Boys and their toys!
 
Alan
Stub Mandrel07/07/2011 20:48:08
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4318 forum posts
291 photos
1 articles
Try a stepper motor out of an old printer.
 
Neil
Ian S C08/07/2011 10:22:18
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7468 forum posts
230 photos
Heres one using a motor from a cassett tape recorder.

The little motor is my version of one by Justin Jones,  The origional motor won the Gnat power Competion in 1996, and it appeared in ME in March 1997.
        The generator powers the 3 V radio  Ian S C

Edited By Ian S C on 08/07/2011 10:31:01

Alan Worland08/07/2011 20:39:22
247 forum posts
21 photos
That looks a proper generator shape Ian - is thet a flat belt driving it?
 
Alan
JasonB08/07/2011 20:57:28
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25215 forum posts
3105 photos
1 articles
Cotswold do a nice littel generator kit
Ian S C09/07/2011 12:09:03
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7468 forum posts
230 photos
Alan, yes it's a flat belt, only thing is that the motor pully in that pic is a V pully, the one on the generator is.
 Just a note, to the left of the finned bit above the cylinder is the water jacket, in this are 4 aluminium fins to further disipate heat from the water. One thing I did find was that between the aluminium fins and the copper water tank there is a .5 volt current, the ally's going to get eaten!
  I have a generator based on thedesign of a Canning electroplating generator, it has six wound poles for the stator, but I'm thinking of removing those and replacing them with permanant magnets.  As it is now it generates very littlevoltage, and less current. The outside diameter of the field is about 3", and length over all  about 4".  Sorry no photos at the moment.   Ian S C
 

Edited By Ian S C on 09/07/2011 12:39:26

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