Can DCC keep a constant speed, up hill and down?
I was at a LHS a week or so ago, and they had a display running, and it dawned on me, that up and down hill action of model trains, is not exactly proto-typical, because a proto-type will add power, when going up hill, and take power away or even brake on the way down, to keep speed in a given range.
DCC can do this. It is a decoder function. Some have it and some don't, it is called Back EMF. It uses the fact that the motor acts as a generator when it si turning withough being driven. By adjusting the input to keep the back voltage constant, the speed can be held.
Jeff But it's a dry heat!
Lionel collector, stuck in an N scaler's modelling space.
Just you you CAN do it with DCC doesn;'t mean you have to. In fact whwn consisting, I think it usually works better not to. Anyway....here's a link to Digitrax's app notes for its implementation. Others probably do it differently...
Oops!
http://digitrax.com/appnote_bemf.php
Hudson wrote:Is that similiar to how dynamic breaking works?
Hmmm, well, yes and no! Yes, dynamic breaking works by taking advantage of the fact that the motor is a generator when it is not powered. In the dynamic breaking case the electric motors are disconnected from the generator (that runs off of the diesel) and hooked to a bif resistive grid, essentially a big heater, to get rid of the energy. There are fans to cool this, to keep things from burning up. Hybrid cars (and Green Goats, for that matter) use the enrgy from breaking to charge batteries. I would not be too suprised if modern locomotive control systems are using some type of back EMF monitoring to control the motors in normal operation, but I don't know this for a fact.
TrainManTy wrote:That sounds great! How do you figure out what to set the CVs to?
If you buy a decoder with it, it's explained in the instructions for your decoder. It's really like an "on and off" thing. Digitrax decoders come with Back EMF "cruise control" activated and set at I think 9 (out of 15) for intensity. You can adjust the CV up or down if you want more or less compensation, or turn it off.
QSI systems have BEMF and what I think they call "sound of power" or something(??) so that when a train is going uphill, or just starting out, the engine noise becomes louder like it's working harder.
Yes, constant speed is a wonderful addition to the loco control tool box. Some of my locos have it and I think it's pretty neat.
Of course, I don't notice too many proto-type trains maintaining the same speed up hill and down. Few consists have the necessary power to speed up a grade of any consequence and there are stringent speed restrictions on downgrades.
So, after marveling at the constant speed for a while, I'm thinking slower up the grade than down is more prototypical. I've also noticed if you really load up the train, it slows a bit up grade, even with BEMF.
Tilden
Tyler,
Back EMF only comes with certain decoders. You just have to make sure that you purchase one that has it.
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
Usually what went up must come down at the same gear or less depending on weight and grade.
I watched a train come down off Cajun at what seemed to be draconian restricted speed but when you think of the total weight shoving against those engines, it's a miracle.
So if it's a matter of decoders, what decoders, have the most flexability for things like BEMF, and other controls?
I like the idea that some at least make it sound like the engine is working harder, when the proto-type should be working harder - but given I tend to N scale, I'm also thinking about under table sound - at least for sounds ( you know maybe a floor speaker to make the floor vibrate when the engine starts up/works harder ).
I think my DC operation is more proto-typical. It slows down going uphill and speeds up going downhill. With more cars producing more load, as with a real operation, it often is slow going downhill until most of the train is over the hill, then it picks up speed. Of course, the motor sound is not there, but I still like it.
Steve
As Safety Valve said, coming down the Cajon, trains are strictly limited on speed. Mostly due to a couple of very nasty runaway train wrecks. The after action investigations showed, due to a several factors, the trains did not have enough total braking power (engine brakes, dymnamic brakes and train brakes) to limit speeds to the then current speed restrictions. This resulted in further limits on down hill speeds.
As far as up hill speeds go, usually the engines are at Run 8 and their speed is still limited.
so can I sell bemf back to the power grid so to speak
or is it just a setting that slows to simulate
steel wrote:I think my DC operation is more proto-typical. It slows down going uphill and speeds up going downhill. With more cars producing more load, as with a real operation, it often is slow going downhill until most of the train is over the hill, then it picks up speed. Of course, the motor sound is not there, but I still like it. Steve
It's not a question of DC vs. DCC, it's whether or not you want Back EMF "cruise control". Many DCC decoders don't have BEMF.
It depends a lot on how you operate your layout. Back EMF is nice if you want to just let a train run - like say have a passenger train running on a continous mainline while you switch in the yards, or to just let a train or two run at an open house. It's also nice if you have a helix in your trackplan. Without it, you have to set the train's speed to be fast enough to climb the steepest grade, so it runs nicely uphill, runs fast on level ground, and races like a slotcar going downgrade. With 'cruise control' the train can crawl along slowly and still make the grade.
Of course, if you run one train at a time by yourself, you probably don't need BEMF, you're doing what a real engineer would do by adding power going uphill and easing off going down. It can also depend on the engine. Atlas or Kato powered engines usually don't get BEMF decoders on my layout, because they run so well as is they don't really speed up or slow down on grades that much. However, I have an old Athearn F7 that used to be greatly affected by grades; since I put a Digitrax decoder with BEMF in it, it runs very smoothly uphill and down and gets more use than it did before because of it's improved running qualities.