DC voltage for conventional HO trains

12VDC is the industry standard for non-DCC operation, but transformers can -and often do- put out a volt or two more.

I've never heard of anyone burning out a motor because of this, but that may be because it's very rare that you'd ever run a locomotive at full throttle anyway.

~Pete

Motors are fairly tolereant of voltage for their power. The motors used in model railroading locos are generally rated for 12VDC for their rated rpm speed. Like lightbulbs and so forth, you can apply higher or lower voltagee for particular purposes. Generally model train motors start tuning at about 2VDC where they just barely turn and increasing voltage will make them turn faster.

-- Bob May

rmay at nethere.com http: slash /nav.to slash bobmay http: slash /bobmay dot astronomy.net

The simple answer is 12VDC maximum, and you should not exceed this.

A more complicated answer is that most model locomotives will reach a scale speed close to the prototype's maximum at around 8 to 12VDC. Also, they are weighted so that the wheels will slip before a dangerous level of power consumption is reached. There is no need to exceed 12VDC.

There are some complex principles behind this simple answer, so if you really want to know, read on. Voltage determines speed, amperage determines torque, the two together determine power (rate at which energy is used.) Too much power can burn up the motor. Higher voltage means the motor can draw more amps, thus increasing its power, and risking burnout. Higher voltage may also weaken the magnet's field, which would result in a higher amperage draw for a given torque, thus increasing power. A stalled motor will draw all the amps the power pack can deliver, which in the short term (a second or two) could be enough to start a small fire. Hence the desirability of wheel slip well below maximum power.

DC power packs produce a no-load voltage of about 16VDC, but as soon as you put a load on them, the voltage drops to around 12VDC or less (depending on the load.) The 16VDC no-load voltage may have led some people to believe that model motors can tolerate more than 12VDC.

HTH

Thanks. The minimum voltage was also something I had been thinking about but forgot to ask. I appreciate your help. Mike

Mike Lawson skriver:

Yes from 0 to 12 volts

Klaus

The starting voltage may then also be important. Present day locos generally will start moving at around 2 to 3 volts, with a steady minimum speed of 2-10 scale mph. But each one is different, and as mechanisms wear in, the starting voltage will drop.

Of course, you could look into DCC.... ;-)

[...]

Have you ever heard of that actually happening, or is this another of those theory-rather-than-practice things?

On 11/8/2008 5:03 PM a_a_a spake thus:

Nothing theoretical about it.

Think about how a typical DC motor is built. When stalled, one set of windings is connected directly to the power pack through the brushes. This is only a relatively few turns of wire, a fairly low resistance which will draw pretty much as much current as the power supply can provide; more than enough to let the magic smoke out.

That does not answer the question of whether anyone has heard of such a situation (in a model locomotive) 'starting a fire', as distinct from smoking, getting hot or self-destructing by insulation breakdown?

It's one of those things you can prove to yourself quite easily.

Take a loco, sans superstructure, (so that you have access to the motor & gear train. Place on track, apply power, (the loco should move), Now grasp the motor shaft and squeeze, stopping the rotation of the motor. Now, turn up the speed control (while still holding the motor shaft to prevent rotation. SHORTLY, you will see 'magic smoke', and/or let go of the motor shaft because of heat from the motor.

Now, stop being so dis-believing of free information!!!

Chuck D.

On 11/8/2008 6:04 PM a_a_a spake thus:

Oh, I don't know; wasn't there an infamous incident years ago on John Allen's Gorre & Daphetid, a roaring inferno in one of the canyons caused by a stalled Pittman motor? Or perhaps I'm just making this up ...

Yeah, really, it's not like it's a Zenith TV or anything, right?

One interesting thing with DC motors is that they will run at slow speeds a lot better if the voltage is a pulse rather than a DC level. Go explore the various throttles that are on the web and you will see that most of them provide a varying voltage at the lower voltages at a minimum. Some of the throttles just use the pulse width method of speed control. Short sharp pulses will allow the motor to move a short amount of distance and the motors generally have no reason to go backwards again thus negating the movement. As a result, you can make a loco move in inches per minute if desired. DCC modules are one place that pulse width speed control is used. These are easily controlled by a computer as each loco is addressed with a unique address and then the speed control is sent.

-- Bob May

rmay at nethere.com http: slash /nav.to slash bobmay http: slash /bobmay dot astronomy.net

John Allen's layout and house wasn't burned down by an errant train motor. The layout was off at the time of the fire.

However, I have seen plentiful smoke from electric motors on occasion. A Pittmann DC-70 will draw about 6-7 amps of current when stalled. When running, that same motor will draw about 0.8 amps. A stalled motor will evenually cook the inner windings of the motor and thus short them gogether, dropping the resistance internally. As the resistance drops, the current goes up and eventually the wires connecting the armature to the windings will melt which will then turn off the circuit. Where the melted bits of copper (the interior of the motor supprisingly won't melt as the current density isn't that high) will fall on may catch fire if you have a really dusty loco and thus catch fire. The seperation is more on the order of a fuse going than a sagging melt of the copper. I will also note that it takes a while to get to this point and you do need a power supply that can supply a lot more than what a power pack can supply and the loco can't be too far from the power suppply or the resistance of the wire will cut the current to the point where it may never fully toast the interior of the motor.

-- Bob May

rmay at nethere.com http: slash /nav.to slash bobmay http: slash /bobmay dot astronomy.net

Pulsing the voltage will help to lower the voltage at which a motor will start to run. Remember that there is static friction which needs to be overcome before the motor will continue to turn.

-- Bob May

rmay at nethere.com http: slash /nav.to slash bobmay http: slash /bobmay dot astronomy.net

Exactly. That is not a "small fire".

You are actually proving my point.

a_a_a wrote in news:01262f59$0$20651$ snipped-for-privacy@news.astraweb.com:

This happened long before I joined the group, but a local Ntrak club tells stories about one locomotive that started smoking. Remember the old axiom, where there's smoke, there's fire.

I doubt a small fire would be started in seconds rather than minutes. It would seem to me that the heat required for combustion around mostly metal and carbon would be much greater than what could be generated at 60W (Assuming a worse case scenario of 12V at 5A.)

Puckdropper

You're mis-remembering it. The fire that destroyed the Gorre & Daphetid was caused by an improperly vented floor furnace. Ironically, John's railroad only survived him by ten days.

A few of the slightly (?) singed and water-damaged structures from the G&D are now on display at the San Diego Model Railroad Museum. I was looking at them just this afternoon.

~Pete