Train speed

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 Deepwoods Software -- Linux Installation and Administration

-- Web Hosting, with CGI and Database snipped-for-privacy@deepsoft.com -- Contract Programming: C/C++, Tcl/Tk

The loco will love you for the lower speeds as the mechanism won't be screaming along and the dynamic forces on the wheels will be less. The power pack really won't care as long as you don't overload it.

-- Why do penguins walk so far to get to their nesting grounds?

8> ~Brad H. :) Not knowing what you really mean by slow speeds, I'm not sure how to answer. But any electric motor turning at extremely slow speeds can cause the motor to run hotter than designed. This can shorten it's life.

The reason is this: In an electrical circuit, the windings of a motor is just a wire, which results in a short circuit. However, with the motor turning, it actually also becomes a generator in reverse direction w/ a low charge. This causes the 'resistace' in an electrical motor circuit. The term is counter-electromotive force (CEMF)if you wish to research the topic further.

-- dw

Thanks for the responses so far. They seem mixed but I'm getting a general idea: slow is O.K. but not too slow.

It will be a DC setup but I'm not sure about the electronics of the transformer. I'm wanting to buy the basic one Bachmann makes. I trust the brand and it's not too expensive.

Now as to defining what "slow" is. Using a watch with a second hand, a tape measure and an imaginary loco, I'm guessing the speed I'd like to run it is about 12" per 10 seconds. I'd actually have to see it to be sure. A little faster might be O.K. Is there any official way to judge safe slow speeds? A certain number on the transformer dial or by timing it by distance per second?

As I type this, I'm recalling the Bachmann site has a question feature. Maybe I'll pop over there and see what they say since it's their brand I'll be using. Will let everyone know what they say.

linxlvr spake thus:

You're unnecessarily clouding the issue. Yes, there is back EMF at work, but as with all other forces, it's less the slower the motor goes, so nothing to worry about running a motor at slow speed.

snipped-for-privacy@webtv.net spake thus:

Wow; that's slooooow. If you have a loco that will run that slow reliably, you've got a pretty damn good runner there.

Yeah, it would be kinda like watching the minute hand on a watch go around :) I think you'd need to seriously gear down the motor output speed, like maybe 4 to 1 (and do it quietly).

Dale

The lowend Bachman controler should be avoided except as a last resort. The lowend MRC or Atlas will work much better for you.

One think to keep in mind, and just as important, is the need for exceptionally clean wheels and track. At higher speeds the momenteum of the loco will allow it to "coast" over dirty spots but at super slow speeds, even a tiny dead spot can stop the unit in its' tracks.

Carter Braxton

That's really not true as the current of a motor stays pretty constant as the voltage changes with any load. If anything, the current goes up as the voltage does by a small amount due to increased frictional losses at the higher voltages.

-- Why do penguins walk so far to get to their nesting grounds?

A good slow speed is about 3-4 times the starting speed of the loco as this is fast enough to not run into load problems stopping the train yet low enough to look good.

-- Why do penguins walk so far to get to their nesting grounds?

Thanks again for these responses. Lots of interesting points. I posted over there on the Bachmann board and got a few replies too:

The Bach-Man himself said they have run demos for 8-10 hours straight at "very realistically slow speeds" with no problems. However, I'm still not sure what that speed exactly is or how to gauge it.

Another fellow said as long as it's all DC, it should be O.K. A follow-up to that note explained something interesting: If a DC engine is left standing still on a track using a DCC controller, it sends some type of unusual AC signal that can heat up the motor. Weird. And another person said if I leave the throttle at zero it will extend the life of the motor. :)

Charles mentioned avoiding the low-end Bachmann controller (or Power Pack -when I was a kid we called them transformers I think). Probably good advice. The standard Spectrum is only about $10 more so may go with that or see what Atlas makes.

connections for accessories, and I do plan to have accessories. Eventually I hope to have lots of low voltage lighting across the layout (since it's mostly a night scene). Would the 44281 Spectrum handle that O.K. or is it better to have some separate power source if the accessories get large in number (that would be at a much later time but it would be good to plan ahead). I'll also be working in some village accessory electronics but those will be on a completely separate system with their own power packs.

David and Dale frowned on my low speed plans. :( It does sound too slow (and by the time I actually see it running I might agree and speed it up) but my thoughts are based on thus: Maximum length of the layout will be 6' at most and probably less than 3' deep. There will be lots of curves, some possibly as tight as 9" radius. Going too fast in such a small scene would not only look funny to me but might be rough on the train (even though some guy did run this same type On30 0-4-0 engine on a 6" radius test circle at full speed with no problems).

I also want the kids to be able to get a good look at the train at one point where I'll have it get about 1" away from the plexiglass for a short section. Hopefully the rest of the scene will be interesting enough that the train creeping past old structures and terrain (and through a graveyard at one point) will still be fun to watch and keep with the atmosphere of the setting. At least that's what the plan is and why I'd like to keep it slow.

~Brad H.

I recomend a seprate power source for accessories. A 9 or 12 volt wall wart works great and is cheep.

Brad, keep in mind that you don't want the train to go so slow that it will be boring for the little ones. I'd throw out a guess as to about 1/2" per second as being about right for attention spans and motor life.

fl@liner

Good afternoon Brad;

This might be a bit late, but here is a table which may be of help to you. As for the running of the train at slowest speed, a compromise will be reached on what runs best. My only suggestion is to make sure the engine/cars and trackwork are in good working order, start it off and read a thick book or watch movies while the train is doing its thing. Periodically check it. If you're happy, it's a go.

Speed - O Scale

MPH Inches/second

1 3/8 2 3/4 3 1 1/8 4 1 7/16 5 1 13/16 6 2 3/16 7 2 9/16 8 2 15/16 9 3 5/16 10 3 11/16

11 4 1/16

12 4 3/8 13 4 3/4 14 5 1/8 15 5 1/2

16 5 7/8

17 6 1/4 18 6 5/8 19 6 15/16 20 7 5/16

Cheers, John

More interesting responses! Thanks!

Charles posted:

? I've never heard of this but will do a Google and check it out.

Fl@tliner posted:

You sure about that speed? I calculate that to be 24 seconds to travel

12". That's twice as slow as what I was figuring. :)

Keeping the scene interesting is actually something I thought over during some early sketches; In fact I was designing it backwards for this particular project and trying to plan structures and features around the train. What worked better was to lay out the scene to be interesting by itself and incorporate the train into that. Certainly when the train is visible (there will be a tunnel in the back portion) it will command all the attention even though there most likely will be some other animated effects going on as well (more so as I add to it each year). But I will speed it up if needed. The 0-4-0 has a headlight and cab light, and I need to look into what's involved in lighting the interior of the caboose (it will be a kit I've been eyeing at the hobby shop) to make sure it shows up well.

John posted:

Not late at all John. This chart will help a lot, and test running for extended periods ahead of time to make sure everything runs steady is a good idea. I'll do it.

Now if I knew what Bachmann meant by "realistic speeds" or what is realistic for a real 1:1 Porter in general I'd be able to see if the speed I'm thinking of running on is really realistic. :)

Thanks again everyone. ~Brad H.

Maybe he meant "1 to 2" - the high side of that would seem more appropriate.

Yup, my bad!

Memo to self: Never post anything when tired!

fl@liner

FD64:

Seeing as a Porter is an industrial engine, I would think walking to running speeds should be realistic. It certainly won't go 88 mph while pushing a De Lorean.

I think your 1.2"/sec speed should be okay. That speed scales to about

6 mph in HO scale, and I have several engines that will do that smoothly without stalling out, either on my cheapo $30 MRC pack (excellent control, no guts) or my $2 variable transformer antique (plenty of power, fairly good control, smell of frying selenium when run for more than 5 min. I need to get some diodes.)

Turning the motor slowly shouldn't hurt it...it's geared down enough that it will spin quite fast enough for cooling, I am sure. You don't want to stall. When a motor stalls, it isn't generating back emf, and its resistance goes way down, so current through it goes way up. I melted a plastic wheel center on a Mantua Plymouth that way. This, by the way, is the alleged 'unusual signal' problem mentioned earlier. The AC can't turn the motor, but it can heat it up and make it buzz. Sizzle.

Cordially yours: Gerard P.