Re: Train speed

flyingdragon64@xxxxxxxxx,
In a message on Sun, 23 Apr 2006 10:19:25 -0700, wrote :

f> This is for future reference but when my layout is finished I'm planning
f> on running the train at a very slow continuous speed, letting it creep
f> through the scene for a period of 4-5 hours for one evening a year (this
f> is the Halloween layout) and at lesser lengths of time throughout the
f> year. Question is; is it O.K. to run at low speeds for extended periods
f> of time? Would it be hard on the loco or transformer or does it make any
f> difference?
f>
f> Keeping in mind I'll be using an On30 Bachmann 0-4-0 Porter and maybe a
f> few cars. No grades are planned (but it would be interesting to know if
f> I did put grades in, what effect that might have). I haven't got a good
f> transformer yet but will be testing with an old cheap one for now (don't
f> remember the brand -it's blue with a single knob).
f>
f> Any thoughts or advice would be appreciated.

I will assume that this is a DC system.

This is going to depend on how the power pack 'works'. There are three
main 'classes' of power pack:

The old (probably no longer made) variable voltage transformer, which
uses a bare tapped secondary winding and selenium rectifiers.

The somewhat less old (I guess common in even current model 'cheap'
power packs), with a variable *resistor* (potentiometer) after a
silicon rectifier with a fixed voltage transformer.

The modern 'high-tech' power packs, with either a variable voltage
solid state power supply OR a PWM (Pulse Width Modulation) circuit.

The variable voltage transformer, variable resistor, and variable
voltage solid state power supply power packs all behave much the same:
speed is a function of voltage across the rails. And the motor in the
loco behaves the same with all three variable voltage sources. A
normal DC PM 'toy' motor's torque (= power / tractive effort) is a
function input voltage -- the more the voltage the more 'tractive
effort' the motor develops, along with a higher rotational speed.
*Normally* this does not matter too much, except with higher loads at
slower speeds, since most of these motors are 'overbuilt'. The motor
needs a minimal amount of torque to overcome basic friction and
inertia. So, with a flat variable voltage power source, there is
a minimum starting speed. Below this point the engine won't move
(although the motor may hum / get hot / etc.).

With a PWM power pack, the voltage across the tracks is always 12 volts
(or whatever the nominal motor voltage -- 12 volts is normal for most
indoor style small scale, 24 volts for outdoor G scale) and the motor
operates at its full torque rating. Speed is a function of the pulse
width. The wider the pulses the faster. This overcomes the limitation
of minimal power requirements being tied to a minimum rate of travel.
The down side is that this constant on/off can be hard on some
*cheaper* 'toy' type motors -- they can run hot and/or burn out under
extended running at narrow pulse widths (slow speeds). The PWM systems
also generate lots of electrical 'noise' (high current on and off with
sharp edges = high frequency harmonics).

About grades: grades represent added load to the above considerations.


Robert Heller -- 978-544-6933
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