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As usual, most of the detail in what you said went right over my head. But the credible explanation from “the expert” confirming Blair’s recommendation is reassuring.
In my case the fixed segments are several feet long, but not the length of my longest train; however the AR-switched segments are comfortably longer than the longest train. Now I have a bunch of re-wiring to do to implement the change.
Thanks for the insight.
On May 23, 2021, at 2:20 PM, whmvd <vandoornw@...> wrote:
What you describe is safe. No question. But it will also, usually, mean that you make a longer intermediate section than necessary. The 'bridging' you describe is indeed the tricky bit, but: only when the AR action HAS NOT YET TAKEN PLACE.
Consider a loco that pushes a lighted long wheelbase 4-axle wagon through the reversing sections, and all axles of the wagon have power pick-ups. Also assume that the polarity on entering is the worst it can be.
A = fixed polarity section
B = AR section 1
C = fixed polarity intermediate section
D = AR section 2
E = fixed polarity section
| = isolators
L = loco
W = wagon, with P being power pickup
Initially B has opposite polarity/phase to A, D and E (which are fixed), but the same as C.
1. Pushing loco moving to the right.
+ - - + +
2. A<->B boundary bridged, AR B switches, so A, B, D and E now have the same polarity/phase.
3. Wagon spans the A<->B boundary. No AR action. All further bridging of A<->B now irrelevant.
4. B<->C bridged, AR B switches.
5. C<->D bridged, AR D switches, so B, C, D and E now have the same polarity.
6. C<->D still bridged, No AR action.
=============A===========|==============B=============|==C==|==================D=====================|==========E=========== + - - - +
7. Nothing bridged, wagon spans entire section C, but, crucially: there is no problem, because there is no short!.
and so on - the rest is trivial..
And this is why only the minimum size of a TRUCK counts. A longer size becomes necessary only when a truck is not really a truck but a single axle. In other words: the long wagon only has one power pickup at each end (never mind the number of wheels, the number of pickups is what counts). Theoretically, it should STILL work. But in practice, a short on a one-axle truck can be very, very easily missed as it would be just the wheelset closing the gap in the rails on its own. See what happens in the figures above when the short in step 5 is not seen. After that, spanning the C section (fig. 6) DOES mean trouble, because the B<->C short has not occurred and the reversers now fight - exactly what should be prevented from happening.
One solution (yours) is to make the C section long enough to span the entire wagon, which is fine and works. However, if that is not convenient (and the layout might well dictate that it isn't) then the other possibility is to stagger the cuts in the rails. That increases the chances of the one-wheel short occurring by a lot. This is the only advantage to staggering that I have ever been able to see. But I repeat: there is ONLY a problem in case of one-axle trucks (if you can even call them trucks).
On Sun, 23 May 2021 at 12:27, Blair <smithbr@...
Think of any single loco as a pair of wires hovering, one over each rail, drooping to touch the rail at each point where a loco's wheels do. In order for a short length between AR sections to do its job, it must be long enough to prevent the wire from bridging the two AR sections. So before the wire enters the second AR it must leave the first AR.
Now generalize this. The user must be aware of the longest electrically connected "thing" he runs. For example if you run a dummy loco but wire its wheelsets to an adjacent engine, then that becomes the longest "loco".
That defines the shortest non reversing section between AR sections.
Hope this helps.
Sent from his Galaxy
-------- Original message --------
Date: 2021-05-22 22:30 (GMT-05:00)
Subject: [w4dccqa] Minimum length on Non-reversing segment
I realize that we have addressed the question of Auto-Reverse (AR) segment wiring extensively in the past ( a discussion in which I even participated). I understand that the track of an AR-wired segment cannot be electrically connected with another AR-wired segment, as the two will go to war with one another, switching back and forth until something blows. And I also understand that for similar reasons, the minimum length of an AR-wired segment must be as long as the longest train that will run over it (or at least as long as the length of the train to the last car with either steel/metal wheels or an electrical pick-up that would cause a short if the "polarity" is not correct).
What I am unclear about is the minimum length of the fixed (non-reversing) "polarity" segment that separates AR-wired segments. I believe I recall a suggestion that the non-reversing segment be at least the length of a locomotive, but not sure why that would be required.
In my case, my layout is a basic figure-8 pattern with two direction reversing segments, each wired with an AR circuit. Problem is that one of those segments (about 6 feet long) was shorter than my nearly 7-foot long N&W passenger train (six 11+" MTH passenger cars plus a 13+" MTH N&W J loco). In reconfiguring, I can lengthen the AR-wired segments to include portions of the figure 8 (assuring that both AR-wired segments are long enough), particularly if I do not have to be concerned that the length of the non-switching "polarity" segment have a substantial minimum length, e.g., not at least as long as my longest train.
I also assume that I can have more than one non-reversing segment separating AR-wired segments without any problems as long the "polarity" of all of the non-reversing segments is the same.
Apologies if we have already covered this sufficiently in previous discussions; but I do not recall the definitive answer.
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