Noob speed matching question

Greetings,
I have a small oval layout, (12 straight, 12 curves 18” radius), I’ve purchased two identical Bachman diesel locos (I know this doesn’t guarantee success) a NCE Power cab controller, JMRI and a MRT Accutrack II speedometer using the JMRI basic speed settings.

My question, I’ve matched the locos to within 1 mile per hour, one running forwards and one in reverse but when I run the two together in a consist, one of them is overheating. Just how exact do the speeds need to be for this to work correctly?

thanks,
Santa

 

i cant answer that but 1st problem is bachmann locos. they should be able to run at that setting. do they run pretty close at all speeds ?

 

I know, I know, but Bachmann was a good way for me to start at this scale.
Yes, I tested at steps 4, 8, 12, 16, 20 & 24 they are no more than 1 mph difference at any of them.

 

yup great for just getting started. HUMMMM they should run fine together then. di ya break them in first by running them a few hrs. i think one should be pushing the other one a lil bit. i really dont know for sure on that. hopefully somebody with much more experience will chime in.

 

Yes, I've run both separately for some time, they are not new, the consist is.

 

Did you ever solve this?

Try disabling BEMF on both decoders.

Greg

 

ya would like to know if this was resolved and what the problem was.

 

I have never speed matched using the speedometers, I have always used my eyes. I feel there is just too much play in the speedos. So that being said I also will only ever use linear throttle profiles on locos that will be in a consist together.

I start with the decoders set to factory default first. Then I put them on the rails about 6 to 10 feet of pure straight rails, no cars behind them. I leave them about an inch or so apart, set up the consist, getting the correct directions and orientation and then see if they both start moving at the same point. The start voltage is the first thing I set. I want my locos moving as slow as possible on click 1 (128 speed steps as well) once they both move at 1% throttle I see if one gains on the other for the duration of the test track. If so I adjust as needed. Then I try at 5% and adjust if needed, usually the majority of the time the speeds will be closer as the speed increases. I keep this up for the whole speed set using 5% increments. JMRI is really handy in this process, as there are slide bars to make the needed adjustments to the decoders. Sometimes there are only one or 2 adjustments sometimes there are many. I also run the locos back and forth a couple times to gauge the discrepancies. Oh and I make the faster locomotive slow down to the slower one. Seems to work better that way. Also I will usually put the loco that ends up being slightly faster (I have never got them perfect but I have had as close as 1mm per 10 feet of travel which isn't bad) in the front to keep tension on the couplers.

Oh and I have Digitrax for my DCC system.

 

Hi,
Not exactly.
I spent two days documenting the speed of both locos at each of my 28 steps, both forward and backwards, then I selected one for the lead on the other to run in reverse.
with JMRI, I used a “user defined” speed table and modified the reverse loco to match the recoded speeds of the lead exactly.
Then I placed them both on the track in the appropriate orientation and ran them about 6” apart no cars, starting at step 1 for several minutes, eventually the rear loco caught up to and pushed the lead.
I don’t know what BEMF is or where to find it.

 

Another vote here for turning off the BEMF when running engines together.

BEMF is a decoder function that attempts to sense if the engine is going uphill or downhill or pulling a heavy load, and will change the speed of the engine to compensate.

Multiple engines can fight against each other if BEMF is turned on. If one decides it needs to go faster and the other decides it needs to go slower, not good.

Some decoders will allow the engines to work together, but only if they are programmed together in a consist on the DCC system.

Not knowing what BEMF is will not protect you from the problem!

Having said all of that, I am under the impression that Bachmann decoders do not usually have a BEMF function, so there may be no BEMF to turn off.

You using JRMI DecoderPro to access the decoder CVs? If you look through the tabs, especially the motor settings, and there is nothing there about BEMF, then the engines don't have it, and something else is causing the problem.

Jim

 

thanks for posting back. maybe a change in decoders is in order.

 

BEMF stands for Back Electro-motive Force

DC motors when pushed are actually generators (this is important later)
DCC decoders do not give the DC motor in the locomotive varied voltages (also important)

The Decoder makes a motor work by rapidly switching full track voltage on and off. For example when the train is moving at it's slowest the off pulse is longer than the on pulse. At half speed both pulses are the same length and at full speed the on pulse is longer (there is ALWAYS and off pulse even at full speed)

BEMF is the decoder's ability to read that small voltage generated by the spinning motor during the off phase of the pulse. The decoder is programed to know what that voltage is supposed to be with the locomotive running on level track. If the BEMF signal is higher than expected the decoder will calculate the needed reduction in "on" pulses to keep the locomotive running at the same speed as on level track. This would be because you were going down grade or being pushed by another engine. The flip side here is if the signal is lower than expected you will get longer "on" pulse as you are either climbing a grade or being held up by a slower engine.

Now put 2 or 3 locos with this system in place and you will get one being pushed so it will slow down, then it will slow the one behind it which will increase speed and push the other one again and back and forth. When this happens you will watch what looks like the engines bump and pull into each other constantly. When its really bad the engines will jerk and go, jerk and go over and over.

I hope this helps understand what BEMF is and why it messes up engines in a consist.

 

thanks, great explanation, however if I understand my research it does not appear that I can disable the function on my Soundtraxx Tsunami decoders.


“The SoundTraxx BEMF Variables:
1) CV 10 BEMF Cutoff Default Value: 0

This is used to gradually reduce the effect of the BEMF Control as locomotive speed is increased. When CV 10 is set to 0, the amount of load compensation set by CV 212 is constant over the entire speed range. There are two non-zero modes of CV10.
1) Setting CV 10 between 1 and 127 will determine the speed step at which the load compensation is reduced to zero. The decoder will calculate the values in between on the fly.
2) Setting CV 10 to a value from 128-255 will cause the BEMF intensity to decrease to a percentage between 0 and 50% of the BEMF intensity set by CV 212 at full speed such that the load compensation level at speed step 14/28/128 is:

% of CV212 at Full Speed Load Compensation = 100 x (CV 10 – 128) ÷ 256 [2]

This CV has nothing to do with the BEMF control in terms of tweaking for low speed but what to do with BEMF once you got it working. “

mine were not set to the default values but I will make it so.

 

The "strength" of the BEMF is set by CV212. If you lower the value of CV212, the effects from BEMF will be less. Decoder designers sometimes do extra things with "0" settings, so going all the way down to 0 for CV212 might not do what you expect, but if you set it to 1, that is almost as good as turning the BEMF off. Even lowering it to 128 if it is 255 by default may help a lot.

Jim