'Variac' power transformer?

Well, a properly designed, for the purpose, motor speed control is pretty much what most of the modern electronic train power supplies ARE.

A 'Variac' (autoformer, or a true variable transformer, both produce pure AC (sine wave), just a modified, variable version of the input voltage.

Lamp dimmers and motor speed controls generally produce a modulated square wave or 'chopped' AC wave output. Often this is some form of pulse-width modulation. When the pulses get narrow, the result is a "spike' waveform. This can cause pure hell with an inductor. Inductors do strange things, especially when subjected to square waves and voltage spikes.

The big difference between a lamp dimmer (for purely resistive loads only) and a motor speed control is that the speed control is designed to drive an inductive load (the motor).

You generally can't use a lamp dimmer or speed control with much success ahead of a step down transformer ... the waveforms are not right to 'transform' properly. Also, the transformer may 'kick-back' a voltage spike at the lamp dimmer, and damage it (less likely to happen with a speed control).

And, the common lamp dimmer electronics may not work properly if the step down transform is placed ahead of it.

Either way, while it may work, sort of, it's not a good idea, and may damage either the components or the train. The whole circuit needs to be designed to work together for the desired output.

Dan Mitchell ==========

railrider wrote:

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Daniel A. Mitchell
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Actually the lower the armature resistance is the higher the efficiency. An ideal motor has zero armature resistance.

Reply to
Clark Martin

And may overheat the transformer.

Reply to
Clark Martin

While it's true that pure DC resistance in the armature IS UN-desirable, it's also unavoidable in normal practice. The only way to eliminate it is to use superconducting wire and run the motor with cryogenic cooling. That IS done, in labs and for very limited applications. One of the 'holy grails' of physics is to find a "room temperature superconductor". No luck yet, though it's a lot closer than it once was. Anyway, it's not even remotely practical for model railroading uses ... yet! :-)

DC resistance is only PART of the armature current story, however. Inductive reactance produces reverse EMF in the armature. This must be overcome by increased voltage. The result behaves much like an effective 'resistance'.

Ideally, you would like ZERO resistance in the armature, and have ONLY reverse EMF limiting your current. The magnet strength would play an even bigger part than it does with conventional motors. Of course, once the motor is superconducting, you can also use that effect for electromagnetic field magnets. This provides the maximum efficiency, with NO resistive losses. There are still limits that cannot be exceeded (current densities, field saturation) that will cause a loss of superconductivity. And, sudden loss of superconductivity at very high current levels can be catastrophic.

For model railroading motor purposes, it is desirable to limit CURRENT ... that means a higher resistance armature is usually desirable, as is a stronger magnet. POWER efficiency is not really the issue.

And, lower current draw means LESS voltage drop everywhere in the layout's wiring ... also a desirable situation.

Dan Mitchell ==========

Clark Mart>

Reply to
Daniel A. Mitchell

Do you think we can expect to see model train locomotives that will not have to get their power from the rails? I realize it can be done now to a limited extent using onboard batteries but battery technology is really not sufficiently advanced for acceptable running times. I'm thinking about some kind of fuel cell that produces electricity.

Reply to
railrider

Sure. Hornby is offering an HO live steam model already! :-)

Aside from improved batteries, I don't see any other possibilities as anywhere near to being practical for a long time. I suppose a fuel cell COULD work, but hydrocarbon fuel cells are still troublesome. Getting one that small to work dependably could be very costly. There's also the problem of removing the combustion byproducts (sludge). A pure hydrogen-oxygen fuel cell is far better behaved, but then there's the problem of providing pressurized H and O in a model loco.

I suppose you could also make a working atomic powered model loco ... using a sub critical 'reactor' or similar. Such have been used to power spacecraft. I don't know how small it might be possible to build one, so that might be limited to the larger scales. Wouldn't THAT be a can of worms?

Maybe a tiny MHD (Magneto Hydrodynamic Generator) is a good option? Where did THAT technology go to? A few years ago it was touted as the solution to most of our energy needs ... just like fuel cells are today. It worked, some were built. Apparently it was another attractive idea that came to a dead end.

MANY things are possible that are not even remotely practical.

Dan Mitchell ==========

railrider wrote:

Reply to
Daniel A. Mitchell

I got it, beam the power to the engine! Seriously, how about a micro turbine that powers a generator for the electric drive motor(s). Might not sound too authentic for a steamer.

If only cold fusion had worked out. :-)

Reply to
railrider

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