12 v dc dynamo
 

Has anyone out there any ideas on how or where I can get a double ended dc dynamo or motor for the front dynamo of our 4 inch scale Burrell showmans engine?
It has the casing and just a plain shaft at the moment but it would be nice if it actually worked and generated .
Any thoughts?

It's been a long time but I seem to recall dynamos only generate one way unless you muck about with the (I think) field windings. Can you fit a small alternator in? Another advantage being self governing at 12 (or 24)V.

The alternator idea is probably the easiest route to take, alternators need to spin fairly fast, so could be 'geared up' from the through shaft within a 4" scale body, but would probably need a battery to 'excite' the alternator. As Tufty pointed out the voltage is self regulated, so the engine revs could rise and fall a bit without blowing bulbs, there is also a pamphlet available describing various modifications to car alternators to perform different duties. If the engine is a scenic it would be fairly easy to arrange 'gearing up' via belts putting a car alternator on the exciter platform and using the existing 'casing and shaft' as a countershaft, worth a thought?

yes plenty to think about as there wqs an alternator in the exciter body but it didn't work so I put in a landroverr dynamo
I dont think it would matter too much about which way it spun if it had a shaft either end you could swap the dynamo round.
I thought about a dc motor but cant think of any double shaft ones

Front mounted vehicle engines usually run clockwise looking at the front so the Dynamo would also run in that direction.


What is wrong with crossing the drive belt ? excuse my ignorance.

If things are symmetrical at the commutator end of the dynamo then it would likely run either way . Whichever way it runs then you will still have a + and a - , just that they will not be the same way around . The two wires to the dynamo are one of the main output wires , and the other is one end of the field circuit . In both cases , output and field ( marked D and F by Lucas ) , the earth connection completes the circuit .

On an automotive dynamo you will need a CVC box to control the voltage , otherwise it will rise too high if you over rev the engine , and then it will blow the bulbs . There are two coils and some points in the CVC box . The coil with the points on top cuts in when the dynamo voltage reaches charging speed , about 12+ volts . This can be jammed closed with a matchstick if you are not connected to a battery . If you are , then the battery will try to drive the dynamo as a motor if you hold the points closed when not up to charging speed .

The other coil will start to vibrate when the voltage rises too high , This intermittently puts a resistance in the field circuit to reduce the voltage and hold it steady at about 14 and a bit volts .

If your dynamo will not charge then you may have it connected with the wrong polarity . The simple answer to this used to be to touch the cutout points together momentarily , when the battery will reverse the polarity of the dynamo . The same applies if the dynamo has lost all its residual magnetism , which will mean it will not charge until you energise the field coils by touching the points in with a battery in circuit .

This is from memory of old Lucas Magdyno sets on motorcycles , and Morris minor electrics .

No responsibility accepted !

I seem to remember, long ago someone was selling end castings for car dynamos to make them look more realistic on miniatures.

Might be worth contacting STW as they had castings and had working dynamos on their kits.

VW aircooled beetle you can get 6v or 12v dynamo's the shaft is long enough to put a pulley on either end. they do need a voltage regulator though.
Mike

Thanks for that the dynamo from a vw beetle seems the way to go as you say just run it through a voltage regulator
Can you tell me is that how the full size things work?
Never really thought too much about it until now

You don't need a voltage regulator with a conventional car dynamo. Just place a wire wound variable resistance in series with the field connection and adjust as necessary {as you would on a full size dynamo}.

Worth remembering that the output voltage (without regulator) will depend on engine rpm, if you are looking at a steady load then simply setting the regulator and keeping steam constant will probably suffice, if your not to worried about lamps dimming when steam and revs drop a bit, if the load is variable, then you'll be relying on engine governors, which in 4" are probably not too reliable, so could lead to blown bulbs etc.

Yes , but the car type regulator does that automatically .

The original regulator if it is available would be the easiest solution, a variable resistor the cheapest, but one of these controlling the Field would be a modern efficient solution.

Or do it electronically, depends how much the load current is but you can get some fairly cheap small electronic units to give you a constant 12v output regardless of engine speed once its generating.
If I owned a 4" Showmans I'd also look at including a small'ish battery to enable the lights to run for a short time without having to belt up, also would allow the lights to run while travelling. Once belted up this would then recharge the battery.
Once you have a stable power source it then allows all kinds of modern paraphernalia to be fitted, how about a showmans with USB phone charging ports, charge your phone while at the beer tent thumbsup

My link above is to an "electronic unit" LiveSteam you must have missed it.


http://uk.farnell.com/linear-technol...9&CMP=KNC-GUK-

Apologies, yep missed it totally :o

Although 5A isnt a huge amount, around 60w, not knowing what the lamps would be on a 4" showmans but lets say each one is 2w that is only 30 lamps, might be enough though as thats 10 per side and 5 front and back ? doesnt leave anything in reserve, I'd be happier with something around 10amps in a TO3 package, may be a LM396K ?
http://pdf.datasheetcatalog.com/data...tor/LM396K.pdf

LiveSteam and anyone else who is interested.

The voltage regulator would not be directly connected to the output it would be connected in series with and control the field coil only, that is how the original type regulators are connected, the full output of 20 - 40 Amps at 12 Volts would then be available .

I have guessed the current requirements of the field coil, the first thing I would do if I had a suitable dynamo is to check the resistance of the coil to enable me to select a suitable electronic regulator.

I would also set the dynamo up on a Lathe or Drilling machine to get an idea about the speed needed for 12 Volts.
A quick Google suggests a speed of 1500 - 2000 RPM is needed for maximum output.



Below is a diagram of the original setup.





http://www.tractiontalkforum.com/pic...ictureid=40355

So are you saying if you 'clamp' the field windings at 12v you will never get more that 12v output no matter what speed you run at? :confused:

No, while Steamy1 is kind of correct with their comment connecting the electronic voltage reg to the field, this wont regulate the output, my assumption was for not using a battery, and thus connecting everything up in series and hoping for enough residual magnetisium to start the generation process.
If you are adding a battery then stick with the standard mechanical regulator.

You wont find automatic voltage control on a full size engine, just a variable field resistance and a attentive engine driver.

An electronic regulator and the original mechanical regulator will both hold a pre-set voltage provided that the input voltage is higher.

So.......................... you won't get the pre-set voltage if the speed is too low.

That end of things I understand the regulator cannot 'create' voltage.. I was more confused about the other end of the 'graph'....

Once the Dynamo is generating over 12v are you saying if the field winding is then kept at 12v the generated voltage cannot rise above this no matter if the speed is increased?

Yes but the Regulator itself has an Input Voltage limit rating see "Absolute Maximum Ratings" on the PDF link below.

In theory the Regulator would fry if the maximum voltage was exceeded but would a miniature drive the dynamo fast enough? .


http://www.farnell.com/datasheets/19...508.1502905985

As i posted on this forum on the same subject but for a 2 ins Showmans engine , I found the easiest route was a 12/24 volt wind turbine

Hi,
I don't think your getting what I'm asking. I fully understand how voltage regulators work.


The thing I don't know about is the Dynamo itself.

Does holding the field windings at 12v hold the generated output at 12v and it makes no difference if the speed is then increased?

No , voltage will increase with speed . The idea is to control the voltage by varying the strength of the magnetic field . The field is , most of it , produced by the current in the field windings which surround the iron pole pieces . So you can vary the strength of the field by varying the current through the field windings .

All the regulators work this way . The old CVC boxes have a make and break mechanism operated by an electromagnet taking the dynamo voltage . This operates against a spring to break the field circuit at a set voltage . The voltage then drops , and the circuit is reconnected by the spring . This sequence is then repeated , in the same way that a bell buzzer works . Otherwise one could have a variable resistance in the field circuit which could be operated manually .

The whole idea is that it is easier to control the voltage by controlling the small current through the field windings than to put a heavy resistance in the main circuit .

You have explained correctly how regulators work but surely you mean, The dynamo voltage will increase with speed but the over voltage will be dissipated by the regulator ?

PS. 12 Volts is just a nominal figure, the old type regulators were usually set to cut out at around 16V.

No , there will be no overvoltage , because the power of the magnetic field will be reduced by the regulator , and therefore the voltage produced by rotating the armature within this field will be reduced .

A dynamo works by rotating a coil of wire - the armature - between the poles of a magnet . This induces a flow of electricity through the armature . In order to produce a decent magnet the pole pieces of the magnet are themselves wound with wire . Feed a current through these windings - the field windings - and you have an electro-magnet . The strength of the magnetic field between these pole pieces depends on the strength of the current fed through field windings . So - vary the field current - vary the field strength - vary the output of the dynamo .

Have you got that down ?

Now , for your homework ............. . .

I haven't made it clear that the electronic regulator would need "tweeking" and some more components to monitor the output and regulate the field accordingly.

If there was no overvoltage the regulator contacts would never operate, they reduce the voltage to a preset threshold.

Now we are arguing over semantics . The regulator will work when the voltage reaches a set limit . whether you want to call this an overvoltage is up to you . Qn a 12v setup the CVC is set at about 14 and a bit volts , so as to charge the 12v battery .

My safety valves lift at a set pressure in the same way , but I don't call it an overpressure , unless they are set wrong .

Looks like an awful lot of trouble to reinvent the wheel. Just a variable wire wound resistance is all that is needed in the field and keep your eye on what's going on.

Except that Uncle Joe Lucas , Prince of Darkness , made millions of the things , and modern , smaller solid state equivalents are also available . So perhaps not so much reinventing the wheel , as using one off the shelf .

I think I would start with the basic unregulated circuit first to see what is attainable.

I would connect terminals F and D together so full potential should be available between terminals F/D and the body of the dynamo. (see circuit post 18)

I think using a variable wire wound resistance might be much the same as controlling the engine speed manually.

I would also check out bulb availability and investigate the dynamo’s output at 6 Volts.

use a bike reg from john goff cant remember his website small solid state and reliable he does ones for dynamos and alternators inc dynamos with no battery