Motor for use as a generator on miniature traction engine 2

[scalesteam]

Hello everyone. I've joined this forum as I'm in need of some help from a sparky!

I'm building a 3" scale Fowler Showmans road locomotive and I need to make a generator to go on the front to power 12 or 24V lighting etc.

I have been told that the best way to do this will be to use a 12 or 24V permenant magnet shunt-wound motor. Is this right and can anybody help me with one as I've looked without success. The motor needs to be about 6" diameter.

The below shows the sort of thing I'm looking for:

 

[ScottyUK]

3" scale is quarter size, right?

I'd look for a standard vehicle alternator and incorporate it into a custom housing to make it look like a period DC machine. Alternators for a modern vehicle have high output for their volume and have the regulator built in so the voltage is constant regardless of rotation speed. Size-wise they're about right too if you can fabricate a sheet metal casing to disguise it. Alternators for earlier vehicles tend to be a bit smaller than current ones because the electrical load of modern vehicles is considerably higher than in the old days.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

A shunt wound motor has a shunt wound field. A permanent magnet motor has a permanent magnet field. You may not have much luck looking for a shunt wound permanent motor.
BUT a permanent magnet motor will generat just fine.
I do like Scotty's suggestion though. It will save you a lot of burned out lights.
The output voltage of a PM motor or generator is directly proportional to the speed of rotation and hard to control.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scalesteam]

Thank you both for your quick replies. Doesn't a vehicle alternator need a supply to the alternator to excite it before it will put any voltage out?

 

[waross]

I haven't worked hands on on automotive alternators for a few decades. As I remember some would, some wouldn't. A trick from the old days was to drill or tap one of the rotor fingers and insert a hard steel screw or rivet. This would hold enough residual magnetism to bootstrap and start generating. It doesn't take much to start and once started, generation will generally continue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill that is some crazy hacking..

I have never seen an automotive alternator keep generating once the external battery gets below a certain point. The internal or external regulator chokes and the field is shut off. It has happened to me many times.

I would suggest that you do go the alternator way but that you add to the 'camouflage can' a small battery. A motorcycle battery or similar. This shouldn't be too hard as alternators have a radically different form factor than older generators. They are very short. This gives you the area behind the alternator which has to be be there anyway to provide a realistic exterior look.

BTW the scheme would free you to make the exterior look identical to your original model.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

You may be right Keith. My experience was the same as yours. I have heard of and seen some DIY magazine articles concerning alternators that would pick up without battery power. BUT I haven't seen one first hand.
I have found that jump starting with a completely dead battery, the alternator would run the ignition as long as the revs were high enough. If you let the revs drop before the battery had enough charge for the ignition,you were hooking up the jumpers again. (And again, and again, usually. Always had to try it too soon. LOL)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Yep..

I worked in an auto shop for a while and saw exactly that many times, and my Renault taught me that too.

I suspect that if you somehow ran with no regulator you would actually get something but being the regulators have integrated circuits or relays that need X voltage or Y current to get into their operating regions it's a no-go.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[scalesteam]

Thanks for all your posts, but it's still not clear! Do I go down the shunt wound motor road or a pm motor and where from, it needs to 12 or 24 volt and 6 to 7" dia. someone suggested from a electric fork lift as they use motors like this to drive the hydraulic pumps etc.
Thanks Trevor.

 

[MikeHalloran]

Forget the motor. It works just well enough to force us to admit that it works, but it needs a lot of outside help in the form of controls to make it do anything useful for you.

Go to a junkyard and pull the alternator from a Mazda Miata. All the engine accessories are cute as a button and just the right size. I don't know if it uses an external regulator or not; if you find one, pull that and the charging harness too. Conceal a 12V motorcycle battery somewhere, wire it up, and you've got a nice 12V lighting system.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Yeah, definitely go for a Jap alternator. Don't use anything that bloody GM fit to their engines, especially not anything from a recent Vauxhall Astra. They are of such sh;tty quality that the one on my wife's car packed in eight days out of warranty. I'm honestly not still raging about the £400 bill: dealer-only part, interchangeable with nothing else on this planet, and major dismantling to fit it even with full workshop facilities. Grrr...

Anyway! I had a quick look on eBay and there are a few permanent magnet DC machines intended for exercise treadmills which are good for 1HP at about 4000 rpm. The 180V output might be an issue but if you run it at a more reasonable speed of 500 rpm then you'll get 24V or so. The output will be very sensitive to speed and load changes, which is generally bad news for lighting circuits as they aren't very tolerant of over-voltages.

A small shunt wound DC machine will be an expensive thing to buy new, and there aren't that many around surplus simply because they aren't that common. You would need some form of field controller which you could do manually - pain in the ass - or get involved with some moderately complex auto controller. I still think the alternator is the best solution even if you have to conceal a small lead-acid battery. An alarm battery would be adequate - the field doesn't take much current.


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If we learn from our mistakes I'm getting a great education!

 

[ccjersey]

I don't know if the proportion is correct, but you could go to an automotive electric (generator and starter) shop and look at an older generator. A generator will start up with no battery, but the open circuit voltage might be high without the battery. While most generators were designed to use a battery, some of the very old machines like Caterpillar tractors had a "no bat" connection on the regulator to connect the electrical system to.

A quick search on ebay might show up a picture to look at.

 

[itsmoked]

Do not pay any attention to Scotty's DC treadmill suggestion!

He should've left it at his fine alternator suggestion as that is what you should do.

Forget the DC PM shunt-whatever solution. It's going to end up being a major undertaking for you. If you are serious about constructing this model you should spent your creative time and energy on other aspects.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The auto generator will work but they were replaced with alternators about 40 years ago. It may be difficult to find one. Actually they are a shunt wound machine. May be either generator or motor. The voltage regulator controlled the voltage by switching the field on and off. If the current went too high another circuit cut the field to drop the voltage to the point that the current was held at maximum.
But, having said that, go with an alternator and make a dummy case.
If you are really good in the fab shop, you may consider making a new drive end end bell that looks like the generator you are trying to copy. The new end bell could support the alternator and some dummy sheeting back to a similar ND end bell.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scalesteam]

The engine is 95% finished, it's the electrics thats holding me up.

 

[itsmoked]

Very nice!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[vinmann]

You could just go with a two bearing brushless 120 volt generator. Some thing like a Winco TB2400C may look more dimensionally correct. You could modify the rear cover for a more original look and you might find someone to modify the regulator for low voltage/low rpm use.

Or you might find an old 32 volt generator from the lightplant days.

 

[waross]

Bill that is some crazy hacking..

Comingfrom the friend who replaced the controls on his washing machine with a PC and an interface board, I'll take that as a complment. LOL
A couple of other hacks.
There used to be kits available to generate 120 volts from an alternator. There was a Voltmeter, a manual throttle pull and a switch that reconnected the output of the alternator to a receptacle and put unregulated 12 volts to the field. You pulled the throttle until you had about 90 or 100 Volts DC. That was enough for 120 VAC rated power tools. I guess someone discovered the high voltage ratings on the diodes and insulation needed to cope with load dump.
There were some good old boys making welders. They would install a 60 or 100 amp alternator in a pickup. They would buy a box of diodes and "match" the characteristics with an ohmmeter. They would then parallel enough "matched" diodes to carry the welding current. Of course they also used a throttle pull. At the engine RPMs needed to weld with these, cooling was not a problem. I met one of the fellows building the welding conversions. They were very popular in remote ranching operations.
Both these conversions were back in the days before solid state Voltage regulators and gas was cheap. It took a lot of Revs (and gasoline) to either weld, or run a power tool with these conversions.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scalesteam]

Thanks for your replies. This is the sort of thing I'm looking for:

http://cgi.ebay.co.uk/ws/eBayISAPI....m=230324394366&ssPageName=STRK:MEWA:IT&ih=013

Has anyone any ideas where I can get one from or can anybody help me with one? Thanks.

 

[ScottyUK]

DC machines turn up now and then on old variable-speed machine tools, but are getting rarer and rarer with the progress of time. A lot of people have removed the DC motors and replaced them with modern drives and cheap AC motors, putting the DC machines on eBay or offloading them to the rallys. Scouring the rallys or maybe visiting a machine tool breaker might be the best bet.

What's wrong with the one on eBay - £23 looks like bargain of the week!

I have an old 3HP AC motor made by BTH nearly seventy years ago lying in the garage - it's the original from my own lathe. No direct use as a generator but would make a lovely housing to hide something more modern inside to get a period look. You want it, you can have it: it's up in the North East.


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If we learn from our mistakes I'm getting a great education!