richhotrainOne thing worth mentioning is that rolling stock (passenger and freight cars) with metal wheels that are unpowered may not actually trigger the auto-reverser,
I don't understand why it should matter if the wheels are unpowered? Or do you mean that having a truck with pickups so that a short exists for as long as both wheels are on opposite sides of the gap?
on the other hand, like the trucker racing across the bridge, does it matter how long the short exists?
greg - Philadelphia & Reading / Reading
gregc richhotrain One thing worth mentioning is that rolling stock (passenger and freight cars) with metal wheels that are unpowered may not actually trigger the auto-reverser, I don't understand why it should matter if the wheels are unpowered? Or do you mean that having a truck with pickups so that a short exists for as long as both wheels are on opposite sides of the gap?
richhotrain One thing worth mentioning is that rolling stock (passenger and freight cars) with metal wheels that are unpowered may not actually trigger the auto-reverser,
Rich
Alton Junction
richhotrainBy unpowered metal wheels, I was referring to a freight car or a passenger car without lighting.
why should it matter?
the short is between the two rails being gapped.
greg, I think that you are missing my point.
What I am saying is that in some instances a locomotive pulling a string of cars with metal wheels can run through a very short reversing section, in which the train is longer than the reversing section, without causing a short.
richhotrainOne thing worth mentioning is that rolling stock (passenger and freight cars) with metal wheels that are unpowered may not actually trigger the auto-reverser, depending upon the trip level of current. So, in some situations, it may be possible to get away with running a train that is longer than the reversing section of track, while still using only one pair of gaps at either end of the reversing section.
getting a way with what?
my original post questioned whether a reverse section must be longer than the train and suggested that for a longer train, the auto-reverser will toggle multiple/many times as the train straddles the reverse section.
several people have said they've had no problems doing just that.
i don't believe you need to hope that the auto-reverser doesn't toggle when a metal wheel bridges the gap on a long train because it doesn't excede the trip current of the reverser.
So Rich made me think about cars with pickups (e.g. lighted passenger). The pickups in each truck typically make contact with each wheel in a truck. A car with pickups doesn't just possibly cause a short when a single wheel bridges the gap, but as long as both wheels straddle the gaps.
will there be a problem if a wheel without pickups bridges a gap at one end of a reverse section while a truck with pickups straddles a gap at the other end?
the wheel bridging the gap causes a short and the auto-reverser to toggle, but reversing the section polarity now causes the truck with pickups still straddling the gap to cause a short. will the auto-reverse quickly toggle a second time and everything is ok?
what would happen if trucks with pickups simultaneously straddle the gaps at both ends of the reverse section? Is this where the short isolated sections described in the PSX-AR manual are needed?
A train travelling 5 scale mph travels 1 in./sec. At 25 scale mph takes 200 msec/in. What does an auto-reverser do if sees a persistent short? for how long?
Not always. It SHOULD be a short, it's a piece of metal crossing a gap. But if there is a spacer in the gap and it's not flush with the railhead, the wheel can lift right over, like there was a ramp there, and never contact both sides of the gap at once. Atlas insulated joiners have a vertical bar that sticks up slightly above the railhead, and if you cut your own gaps and fill with a thin piece of styrene and then file to shape, you'll have much the same thing unless you file all the way down into contact with the railhead, which then introduces problems of its own with scratches in the rail surface.
Even if this is NOT the case, inless the wheel is squeakly clean and the car it is used on has some weight to it (like NMRA recommended weight) AND the rail at the gap is nice and clean, the 'short' may not be a very low resistence one. For example, when I make resistor wheelsets, I put a 10K resistor on each axle I am converting, then paint in the conductive silver paint to complete the circuit. It doesn;t really conduct until the paint dries (solvent suspending the silver particles evaporates) but even so, 9 time out of 10, if I take a wheel set with otherwise clean wheels, and set it on a clean piece of track connected to my multimeter, it still shows as an open circuit, until I apply some gentle pressure to the wheels. When I pop them back in the trucks and put the completed car, which is weighted to NMRA standards (some are heavier, like open hoppers with their loads) and put it back on the same track, I get the correct reading.
So it's both the speed of one tiny part of the wheel tread touching both sides of the gap as well as the quality of said contact that determines if the AR device even sees the mismatch to be able to take action.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
gregc richhotrain One thing worth mentioning is that rolling stock (passenger and freight cars) with metal wheels that are unpowered may not actually trigger the auto-reverser, depending upon the trip level of current. So, in some situations, it may be possible to get away with running a train that is longer than the reversing section of track, while still using only one pair of gaps at either end of the reversing section. my original post questioned whether a reverse section must be longer than the train and suggested that for a longer train, the auto-reverser will toggle multiple/many times as the train straddles the reverse section. several people have said they've had no problems doing just that.
richhotrain One thing worth mentioning is that rolling stock (passenger and freight cars) with metal wheels that are unpowered may not actually trigger the auto-reverser, depending upon the trip level of current. So, in some situations, it may be possible to get away with running a train that is longer than the reversing section of track, while still using only one pair of gaps at either end of the reversing section.
gregc i don't believe you need to hope that the auto-reverser doesn't toggle when a metal wheel bridges the gap on a long train because it doesn't excede the trip current of the reverser.
richhotrainI have seen situations where a train that exceeds the length of the reversing section trips the auto-reverser on some passes but not on others. So when you crearte such a situation on your layout, all you can do is hope that the auto-reverser will detect the short and correct it.
you can't rely on luck and I suggested that you don't have to.
for a long train straddling both ends of the section, the auto-reverser should alternately toggle the polarity in the reverse section as whees bridge the gaps at alternate ends of the section. I see nothing wrong when this happens.
If for whatever reasons one set of wheels bridging the gap doesn't cause the reverser to toggle, that's fine. Maybe the next one will. But you can't rely on that happening all the time.
gregc So Rich made me think about cars with pickups (e.g. lighted passenger). The pickups in each truck typically make contact with each wheel in a truck. A car with pickups doesn't just possibly cause a short when a single wheel bridges the gap, but as long as both wheels straddle the gaps. will there be a problem if a wheel without pickups bridges a gap at one end of a reverse section while a truck with pickups straddles a gap at the other end? the wheel bridging the gap causes a short and the auto-reverser to toggle, but reversing the section polarity now causes the truck with pickups still straddling the gap to cause a short. will the auto-reverse quickly toggle a second time and everything is ok? what would happen if trucks with pickups simultaneously straddle the gaps at both ends of the reverse section? Is this where the short isolated sections described in the PSX-AR manual are needed? A train travelling 5 scale mph travels 1 in./sec. At 25 scale mph takes 200 msec/in. What does an auto-reverser do if sees a persistent short? for how long?
A car with pickups doesn't just possibly cause a short when a single wheel bridges the gap, but as long as both wheels straddle the gaps.
Not sure what you mean by that statement. When a single metal wheel bridges the gap, a short is detected.
I don't have data on how quickly an auto-reverser detects and corrects a short, but not all auto-reversers are created equal. The PSX-AR is solid state and seems to detect and correct a short instantaneously. On the other hand, the Digital AR-1 uses a mechanical relay, and it takes a little longer to detect and correct a short. You can hear the mechanical relay clicking repeatedly if the short cannot be corrected.
gregc for a long train straddling both ends of the section, the auto-reverser should alternately toggle the polarity in the reverse section as whees bridge the gaps at alternate ends of the section. I see nothing wrong when this happens.
gregc If for whatever reasons one set of wheels bridging the gap doesn't cause the reverser to toggle, that's fine. Maybe the next one will. But you can't rely on that happening all the time.
But, what I am saying, is that if someone creates a reversing section that is shorter than the longest train running through it, that someone is taking a chance that dead shorts will not occur. Dead shorts just might occur.
i don't understand the distinction between a short and a "dead short" (trying to understand your perspective)
a short exists when the wheel bridges the gap (sits in the gap making contact with both rails) when a train enters a reverse section that doesn't match the polarity of the mainline tracks at that end. There is a totally metal path (not resistive) connecting the positive side to the negative side of the power supply through the rails. It's a short, a "dead short", not excessive current.
but the auto-reverser can detect the high level of current drawn through the tracks in the reverse section and quickly (1, 10, 100, 1000 usec?) reverse the polarity before the excessive current causes any heat damage and restores any momentary drop in voltage.
But what happens if two wheels bridge the gaps at both ends of the section? The wheel at one end causes a short while the wheel at the other end doesn't. When the reverser toggles the polarity, the other wheel will now cause the short.
but as long as the train is moving, the short will be momentary (1, 10, 100, 1000 usec ?) and if the auto-reverser is fast (uses mosfets instread of relays), it may toggle several times.
Is this a bad thing? how long does it take before the circuit breaker trips?
the pickups on trucks may connect both wheels together. When such a truck enters a reverse section, there is an electrical path between the rails on both sides of the gap as long as the two axles of the truck are on opposite sides of the gap. In this case, the short is not momentary (~100s msec).
I don't believe an auto-reverser can handle such a long short. I believe this is why the PSX-AR manual describes creating the short unpowered sections at each end of the reverse section. In other words, that short section lengthens the gap.
Well, I guess we need the input of an electronics expert here.
To me, a dead short, as opposed to a momentary short, is a short which stops the locomotive until the short is corrected.
When an auto-reverser cannot resolve a persistent short, it will keep on trying unless and until a circuit breaker or the booster shuts it down.
As far as "excessive current", that is not the issue. Reverse polarity is the issue. In some situations, the auto-reverser may not sense a short when unpowered metal wheels pass over the gap. An auto-reverser is designed to detect and correct a reverse polarity situation, not to detect excessive current. That is the job of a circuit breaker.
The amount of time needed for the short to be detected would require a circuit analysis, and since there is most likely a microcontroller involved, access to the source code to see how tight the timing and control loops are written - good luck with that one. The switching speed of the MOSFETs used can be determined from a data sheet, but you need to know the delay in the detection and analysis that there is indeed a short, and then the signal being sent to the MOSFET to turn it on or off.
Yes, if you have wheels crossing gaps at both ends, the reverser will flip back and forth. However - you ARE relying on luck at that point, because if there is a wheel crossing the exit gap exactly the same time one is crossing the entrance gap, you now have a conflict and no way for the reverser to fix it. Even worse, if the cars of the train are lighted or there are powered locos at both ends that pick up from both sides of both trucks, the chances of conflict are much greater, because the short isn't just one wheel rolling over the spot of the gap, it's now the whole car length, front and rear truck. Now the conflict exists until the entire car passes the gap. Even cars with metal wheels and metal trucks - all the insualted wheels have to be on the same side, so that "cross the gap" time is the entire uninsulated side of the truck, not just one wheelset. There are many possibilities - which is why NOT making the reverse section long enough might work perfectly today but then you go aquire some new equipment and suddenly any time that car is in the train you have problems.
My distinction between shorts is a dead short would be as close to 0 ohms as practical, whereas you can also have a short where there are a few hundred ohms which can result in say 4.5 amps flowing throught he point of contact, which would not trip a circuit breaker or booster that is set for 5 amps - yet 4.5 amps at 15 volts is quite a bit of heat, plenty to melt plastic sideframes. That's why the quarter test is just setting a quarter on the rails, NOT pressing it down. Between the random contact of the quarter witht he rails and any losses in the feeders and bus, you have anything but a 0 ohm short. If the feeders and bus are inadequate, enough voltage will be lost in the wiring to drop the short current below the trip point of the breaker, meaning if a train drerails and causes a short, it may also have the same problem if it isn;t really jammed into the rails. If the breaker trips with the quarter, you can be reasonably certain that even a minor derailment short will also be detected and power cut off.
richhotrainTo me, a dead short, as opposed to a momentary short, is a short which stops the locomotive until the short is corrected.
good. momentary short is much more descriptive. but momentary can range from usecs to 100s msec.
richhotrainWhen an auto-reverser cannot resolve a persistent short, it will keep on trying unless and until a circuit breaker or the booster shuts it down.
right, how long must the short exist before the circut breaker trips?
richhotrainAs far as "excessive current", that is not the issue. Reverse polarity is the issue. In some situations, the auto-reverser may not sense a short when unpowered metal wheels pass over the gap. An auto-reverser is designed to detect and correct a reverse polarity situation, not to detect excessive current.
The auto-reverser can't "see" the wheels bridging the gap. It doesn't know the wheels are crossing the gaps where the rail polarity is different across the gap.
The auto-reverser senses the short by monitoring the current and taking action when it excedes some limit (possibly for some period of time).
On the Pacific Southern, the auto-reversers started tripping when they began running 14 car passenger trains with powered A-B-B-A units. The auto-reverser didn't know the higher current drawn is because of four locomotives pulling a long train.
richhotrainThat is the job of a circuit breaker.
The circuit breaker monitors the current as well, except when the current excedes the limit, instead of reversing the polarity, it opens the circuit to the rails. If it's automatic, it will periodically close the circuit which may immediately open because the short persists.
PSX-AR manualIf your train is longer than your reverse block and has metal wheels, you may need to cut additional gaps into the reverse section.
So what happens when the loco/consist is straddling the gaps at one end of the reverse section and the first metal wheel bridges the gap at the other end causing the auto-reverser to toggle the reverse section polarity!?
That would be a short, not a reversable one, and the circuit breaker function of the PSX-AR would cut power to the reversing section completely. Since only a tiny distance is needed to get the wheel off the gap (jsut the slack in the train would be more than enough), the power would be turned back on after the short cleared and the train would continue on, with maybe a blink of the headlight or a glitch in the sound if it's a sound decoder.
with the loco straddling the one end of the reverse section and a wheel bridging the gap at the other end the auto reverser toggles the section polarity.
But the auto-reverser should immediately detect a short and toggle polarity because the loco straddling the the opposite end now causes a short.
After the 2nd polarity reversal there will no longer be a short if the wheel bridging the gap has moved off the gap.
can this happen quickly enough to avoid the circuit breaker from tripping?
is an auto-reverser designed to do this?
someone on another forum asked a similar question. They brought up a different situation.
On my steam locos, the loco picks up one rail and the tender, the other rail. I assume diesels are similar, each truck picks up power from opposite rails.
consider what happens when the loco has exited the reverse section but not the tender and a wheel on a freight car bridges the gap at the opposite end. The auto-reverser toggles polarity because of the short but now the both the loco and tender see the same polarity and the loco stops.
seems the only recourse is the PSX-AR suggestion of having the short dead section to prevent the short when the freight car enters the section.
gregc I assume diesels are similar, each truck picks up power from opposite rails.
Greg:
On each diesel truck the wheels one side picks up one power phase and the wheels on the other side pick up the other power phase.
Joe
4 wheel pickup diesels are pretty much a thing of the past. Even Athearn Blue Box locos were 8 wheel pickup. Any modern (at LEAST since the 80's) diesel picks up on all 8 wheels, even many train set quality locos.
Offset pickup in steam locos can cause problems just moving between two booster districts. Most any of the modern ones (BLI, etc) no longet use the spliut pickup system, both sides of the loco and both sides of the tender pick up power.
And actually, it's not a problem - unless the loop is too short and there are metal wheels/lighted cars crossing the input at the same time as the poutput. When the pilot wheels short the first gap, the reverser will reverse the loop, making it match the exit trackage - so the one side of the loco and the other side of the tender will have opposite polarity and the loco will keep moving just fine. The problem is if something shorts the input side of the reverse section before the tender has crossed the gap in its rail. The AR will flip the polarity and now the pickup side of the loco is the same polarity as the pickup side of the tender and the loco will stop. Stick with train-length reverse loops or use only plastic wheels and no lighted cars, and there never will be a problem.