Q: Track conductivity at extremely low voltages

[mmagliaro]

Something I have observed for quite some time is that when I power a loco with a coreless motor/gearhead, the engine
will start moving at 1v or even less.   The super-smooth creep speeds happen at around 0.8v.
(This is all straight DC)

It may be my imagination, but I also think that this makes them fussier with regards to electrical pickup.  The track has
to be cleaner to sustain this kind of performance.

So here's the question.
At only 1 volt, does track dirt or oxidation have a much more dramatic effect on conductivity than, say, at 3 volts?
Is there any scientific way to back this up?  Any of you folks know enough about the science of the metals
to explain this?

Or is it pretty much true that if the wheels can't conduct at 1 volt, they aren't going to conduct at 5 volts and I
just plain ol' need to clean the track?

Since the engines typically still have more top end speed than they need, my inclination would be to wire a resistor
in there (or a couple of small diodes) so that the track voltage has to be raised higher before the motor starts to turn.
What I'm wondering is, what would a good "higher" point be?   2 volts?  3 volts?

Thanks.

[kalbert]

I believe what you're getting at is Ohm's Law

[mmagliaro]

I believe what you're getting at is Ohm's Law

Not so much Ohm's Law as what the actual resistance is of typical dirt and oxide build-up on our rails.
Yes, Ohm's Law would tell me how much voltage drop would be across the "dirt".    But the crux of
the matter is:

If the resistance is so high that it doesn't matter whether the voltage is 1 or 10, then raising the starting
voltage from 1 to 3 isn't going to matter.   There is also the physical phenomenon of bridging across the
dirt.  In extreme cases, we've all seen a little spark from dirty wheels or track (although this is rare in N Scale).
But is that happening all the time, although not visible, with dirty track --- tiny little arcs jumping across the
dirt to the wheels?  Does it happen more easily at 3v than at 1v?

It's a matter of where the magic "breakover" point is (if there even is one that matters).

[mionerr]

The higher voltage allows the current to "jump" a gap between the two current carrying metallic do dahs. The higher the voltage the further thae distance that can be "jumped".
That said, I don't know if it holds true for that low a voltage (EE theory was 45 years ago) but it might. The distance involved is very small as well. I think this is the principle behind that device that helps break though the crud on wheels and/or track. It's running at over 20 volts.

[randgust]

I'm neither physicist nor electrician.    Just reach my own conclusions.

Higher voltage leads to more potential between two points; along with the carbonization and sparking.   It's been my observation, primarily working with our storefront window HO layout, that running faster (both with higher voltages and higher current) dirties up the wheels and rail significantly faster.   So we slow everything down so that it is steady, and the track requires much less if any cleaning.  Where it does carbon up is any place where rail-to-wheel contact is poor.   You can see it.

I deliberately lower the input voltage to my DC transistor throttles, primarily as a control override but also to minimize motor noise and indirectly to lower the sparking.    I really notice the sparking buildup on the end-axle pickups on Kato critters.  If they hit high voltage, those cups carbon up pretty fast.   That's also offset if they've been hit with Conductalube, which appears to act as an oxidation barrier as oxygen can't get into the carbon formation.

On the other hand, I've seen some of the DCC layouts I've viewed have less problems with carbon buildup than what I have.  Not sure why.  There's enough power on the rails to melt the trucks, but the stalling is negligible.   And I can't explain it.

[PAL_Houston]

The basic reason you observe no motion below some voltage is because the motor has to overcome the "stall torque" of the motor plus attached mechanical stuff.  Now if you add external electrical resistance (owing to track distance, or dirty track or whatever), you (effectively) can reduce the motor voltage at the same current to a value below the "stall voltage".

This would probably be a lot clearer with a couple of diagrams, but I couldn't quickly find the ones I was thinking of online -- but they were standard fair in 1970's vintage EE texts on electromechanical devices.

[PAL_Houston]

The basic reason you observe no motion below some voltage is because the motor has to overcome the "stall torque" of the motor plus attached mechanical stuff.  Now if you add external electrical resistance (owing to track distance, or dirty track or whatever), you (effectively) can reduce the motor voltage at the same current to a value below the "stall voltage".

This would probably be a lot clearer with a couple of diagrams, but I couldn't quickly find the ones I was thinking of online -- but they were standard fair in 1970's vintage EE texts on electromechanical devices.

Oops!  In the above, where I said "...you (effectively) can reduce the motor voltage at the same current to a value below the "stall voltage".",  I meant to say " ...you (effectively) can reduce the motor voltage at the same Applied (external) Voltage to a value below the "stall voltage".

[jagged ben]

Ohms law certainly plays a role in all this, but I think the reason that performance at low voltage suffers is more basic: you're going slow.  The loco has little to no momentum for rolling past dirtier spots on rail or wheels.  So it stops at those spots instead of rolling past them.  At higher speeds, a train has enough momentum to just keep rolling through the dead spots to where it conducts again, and you just don't notice the very momentary loss of power.

The same logic applies to running a DC train with metal wheels through a reversing section that is too short (or through any other section of track that has a very small section that shorts the wheels, for whatever reason).   Go fast, and the momentum of the train will put it past the point where it shorts, and it may get jerky but it will keep going.  Go too slow, and the wheels will lodge on the gap where the short is, and the train will stop (and the trucks will melt, if you don't catch it right away). 

[VonRyan]

The basic reason you observe no motion below some voltage is because the motor has to overcome the "stall torque" of the motor plus attached mechanical stuff.  Now if you add external electrical resistance (owing to track distance, or dirty track or whatever), you (effectively) can reduce the motor voltage at the same current to a value below the "stall voltage".

This would probably be a lot clearer with a couple of diagrams, but I couldn't quickly find the ones I was thinking of online -- but they were standard fair in 1970's vintage EE texts on electromechanical devices.

My H10 must have a crap-ton of stall torque.
Even on freshly cleaned track, it doesn't like to start up on its own, usually requiring a bit of a nudge to get rolling.
Of course the decoder is beside the gear-tower which has resulted in a slight tilt to the left, but with no noticeable difference in the running quality as far as I can tell.

-Cody F.

[peteski]

My H10 must have a crap-ton of stall torque.
Even on freshly cleaned track, it doesn't like to start up on its own, usually requiring a bit of a nudge to get rolling.
Of course the decoder is beside the gear-tower which has resulted in a slight tilt to the left, but with no noticeable difference in the running quality as far as I can tell.

-Cody F.

Let me see if I understand this correctly.  You have a DCC controlled model which, even on clean track, needs to be nudged to start going?  Even if you crank the speed to max?  The track voltage on DCC is always high. Max was talking about DC power and low voltage behavior of his DC locomotive.

[VonRyan]

Let me see if I understand this correctly.  You have a DCC controlled model which, even on clean track, needs to be nudged to start going?  Even if you crank the speed to max?  The track voltage on DCC is always high. Max was talking about DC power and low voltage behavior of his DC locomotive.

Yes. There have been instances where even with the throttle all the way up, the engine required a slight poke to get it to actually start moving.
My comment was directed mostly towards the 'stall torque' factor since I always assumed that if the engine isn't moving of its own accord, then there must be an electrical issue. However, after having placed my H10 on spotless track (standard Samhongsa motor in it) and turning up the throttle beyond where I would position it for regular operation, it still required a very slight nudge with my finger before it started moving.

-Cody F.

[mmagliaro]

Yes. There have been instances where even with the throttle all the way up, the engine required a slight poke to get it to actually start moving.
My comment was directed mostly towards the 'stall torque' factor since I always assumed that if the engine isn't moving of its own accord, then there must be an electrical issue. However, after having placed my H10 on spotless track (standard Samhongsa motor in it) and turning up the throttle beyond where I would position it for regular operation, it still required a very slight nudge with my finger before it started moving.

-Cody F.

Cody:
You either have loss of contact or a problem in the motor.  My bet would be contact, since the engine runs fine otherwise.
Are you still using the standard tender pickup and drawbar in that thing, or have you swapped it out to
use Spectrum or Kato tender trucks with all-wheel pickup.    With the stock pickup, yes, I don't care how clean
your track is, it is going to be fussy until it runs for a minute.

[VonRyan]

Drawbar is standard and tender trucks are standard. No running issues. It even does well over PECO insulfrogs.
The only modification of the drawbar wire is that it now has an upward spring to it so that it also makes contact with the flange-like bit of the contact post. When I assemble the engine. I always check the drawbar wire and the contact post goes into the closer-coupling hole on the side of the wire that results in more pressure. Periodically the wire is sanded with 1000 grit sandpaper and wiped with a piece of corduroy soaked in a mix of 70% and 90% rubbing alcohol.
I have not changed the trucks as there is no substitue for the PRR type trucks.
I have not changed the drawbar because I put the engine in its original box since it lives in my briefcase between train shows, and it hasn't given me any grief as long as I make sure that the contact post goes on the side of the wire that results in more pressure on said post.
I know the engine is making contact as the engine makes an almost un-noticeable slight-movement when the throttle is turned up to a certain point.
The engine doesn't always require a nudge, but I would say that at least 50% of the time it does.


-Cody F.

[peteski]

Yes. There have been instances where even with the throttle all the way up, the engine required a slight poke to get it to actually start moving.
My comment was directed mostly towards the 'stall torque' factor since I always assumed that if the engine isn't moving of its own accord, then there must be an electrical issue. However, after having placed my H10 on spotless track (standard Samhongsa motor in it) and turning up the throttle beyond where I would position it for regular operation, it still required a very slight nudge with my finger before it started moving.

-Cody F.

I would agree that your orriginal assumption (that the problem is electrical contact) is correct.  If you crank the DCC throttle to the max and the loco is not moving then I suspect that the power is not being delivered to the loco rather then a stalled motor. Unless there is some serious bind, a 12V motor with 12V applied to it is generating maximum torque it is capable. Remember, permanent-magnet based DC motors generate maximum torque when stalled (at full supply voltage).

Since you have a DCC locomotive, assuming it has a working headlight, you can easily see whether the problem is electrical or if the motor is stalled.   When your loco doesn't move while the throttle is all the way up, see if you can turn the headlight onn and off. If not, then the decoder is not getting any power from the track.

From all the explanations so fay I thin that jagged ben's makes most sense.  At slow speeds there is very little inertia in the gear train to allow the model to coast through the dirty spots in the track.

[peteski]

I know the engine is making contact as the engine makes an almost un-noticeable slight-movement when the throttle is turned up to a certain point.
The engine doesn't always require a nudge, but I would say that at least 50% of the time it does.


-Cody F.

That sounds like you have dead pole in the motor.  If the motor stops with the brushes contacting the commutator feeding the dead pole, the motor will not move. until the motor shaft is slightly nudged so the brushes contact the next commutator segment.  In 3-pole motors, because of the way the poles are wired, the motor can sometimes move if powered with higher voltage.