Decoders and speed

[frazzle]

Hi

I have had a diesel loco running for a while with a ba***ann decoder. Max speed was scaled at 100mph, while was actually spot on to the original loco.
Due to CV6 not being programmable i decided to use a different brand of decoder (le*z)

as this has more flexibility. Strangely the top scale speed has dropped to 85mph?
Both decoders were set to full, so surely the limiting factor is the motor, and that hasnt changed?

Anyone able to explain?

Paul

[poliss]

Like motors, different decoders do things differently. CV5 (V-Max) is the CV that handles maximum speed. CV6 is (V-Mid).

[minoo]

Hi Paul,

If you can read back the CV values of L**z decoder then read the value of CV 94. Try and set this higher than the set value, if already not set at maximum. CV 67 to 94 are of individual speed steps of the speed curve. If you set the value

of CV 94 at the maximum, it should give your loco a full speed as per the gearing.
I had tried the same and was sucessful in reducing the top speed. Hence viceversa should work.
NOTE : please read and the values before changeing to revert back, if need

be.

minoo

[96RAF]

@Paul
As you have seen not all decoders behave the same using the same controller.
minoo is correct in that you can set a custom speed profile - decoder type permitting - and this may solve your problem, which seems to be decoder output dependant

rather than a motor issue.
This is done by way of changing values within CV29. The Elite menu makes this easier by allowing you to choose speed table 2#5#6 (CVs for Vstart, Vhigh, Vmid) or speed table 67#94 (CVs for custom speed curve)for the decoder. Via

Menu/Loco/Direct/Address/Config////Sp.Table.
Once selected you can then amend the particular CV.
You can also use DecoderPro and (I think) Railmaster to read/amend speed curves rather than altering each CV by hand if you are pc capable.
I am presuming

that your Elite is at the latest firmware revision state.
Rob

[frazzle]

Thakns for the input guys

CV94 helped a little, but as it was already at 254, all i could up it was 1.
I guess as you guys have rightly said, the decoders have nore to do with the way the trains run that i thought. I literally thought that the

motors dictated the top speed, slow running potential etc, but it appears not.

Never mind, 10mph below proto speed wont matter

Paul

[Gregd99]

interesting problem.

indeed the top speed depends on the decoder and the CVs and the motor and probably a few more things beside:-)

My experience has been that with hornby locos/decoders the max speed achievable is around 1.5-2x the prototype

speed. I typically do not set a throttle much above 60%.

You seem to have found an exception.

[96RAF]

It has been said on the forums that back emf has an effect on top speed on some setups.
You could try disabling that to see if it makes a difference.
Rob

[poliss]

Don't see how bck EMF could effect top speed. See http://www.loystoys.com/info/back-emf-control.html for what BEMF does.

[hosh]

poliss said:

Don't see how bck EMF could effect top speed. See http://www.loystoys.com/info/back-emf-control.html for what BEMF does.

From that very article -

"Digitrax decoders can be set up to run at 100% Back-EMF

for switching purposes, but does not automatically cut out at higher speeds."

If bemf is still active at max speed then the motor will only get whatever the decoder is set up to feed it instead of all the motor can take. Or do they all cut bemf at 100%

throttle?

[poliss]

Back EMF does not determine the maximum speed of a loco.
With Back EMF off: If a loco is moving along a piece of track at maximum speed it will slow down when the loco goes up an incline.
With Back EMF on: If a loco is moving along a piece of track

at maximum speed it will stay at maximum speed when the loco goes up an incline.

[hosh]

The OP has not said he has disabled bemf. I am wondering if the decoder may be setup in such a manner that even at full speed bemf is still enabled and so the motor is not ever getting all the amps it wants.

[hosh]

What is the value of cv10?

Also, can anyone explain the difference between cv5 and cv66?

[hosh]

http://www.dccwiki.com/Configuration_variable

[poliss]

Back EMF has nothing to do with maximum speed. Why do you think it does?

[hosh]

If there is no entry for cv10 or it is set to 255, what happens?

I was told in another thread that bemf is typically turned off by default for the top speed setting (128) - is that the same for all decoders?

Btw, are the speed tables (cv67-94)

only in effect when bemf is turned on?

After reading here - http://www.dccwiki.com/Speed_Table

.....I still don't know whether the speed table stuff is a raw voltage or a bemf related thing. For example, what is the difference between cv5 (Vmax)

and cv66 (Forward trim)? It appears that cv65-95 might be measures of bemf (rather than raw voltage) and thus only in play when bemf is enabled? So if bemf is disabled then everything is dictated by cv2-6?

So is the OP's problem related to cv66?

[poliss]

Forget BEMF. It has nothing to do with maximum speed. It can help a loco crawl by altering the power pulses.
Unlike analogue, where you change the speed by altering the voltage, DCC always gives the full 12v to the motor in pulses. The speed is governed

by altering the amount of power pulses given out per second. Loy's Toys puts it better, "Consider this: to make a loco run at 30% throttle with DC analog, you will be providing 30% of the available voltage all the time. But with DCC, you provide full voltage

30% of the time."
Read the whole guide here. http://www.loystoys.com/info/how-dcc-works.html
Loy's Toys also has a guide to the most used CVs here. http://www.loystoys.com/info/configuration-variables.html

Don Fiehmann explains about BEMF and

PIDs here. http://www.amhobby.com/download/BEMF_PID_Intro_AHD.pdf

[hosh]

It is my understanding that there are 2 ways to control the decoder/motor - by a raw voltage value or by a bemf value. Is this not correct?

Again, what is the difference between cv5 (Vmax) and cv66 (Forward trim).

I want to know if CVs 65-95

specify a voltage or a target bemf (cruise control). Or is this dictated by cv10? (ie the values below cv10 are bemf values and those above are voltage.)

[poliss]

No, it's not correct.
See the section on the Loy's Toys website entitled Forward/Reverse Trim to see what CV66 and CV95 do.
CV10 has nothing to do with CV2, CV5, CV6, CV65 or CV95.

[hosh]

poliss said:

No, it's not correct.
See the section on the Loy's Toys website entitled Forward/Reverse Trim to see what CV66 and CV95 do.
CV10 has nothing to do with CV2, CV5, CV6, CV65 or CV95.

So are you trying to say

that the speed table (cv67-94) is not a set of voltages?

If you agree that they are (which they are) then you must surely see that if bemf is enabled for a given range of speed steps, then that over rides that table for those speed steps with the manufacturer

defined table for bemf (cv49-64, which is presently, afaik, non user definable for the most part).

Is this not correct?

[poliss]

No it's not correct. The voltage never changes on DCC. It's the amount of voltage pulses per second that changes.

[Fishmanoz]

It seems to me some of this discussion is becoming a little at cross purposes, rather than shedding light on the issue. My understanding is:
- DCC voltage to the motor is constant but duty cycle (ratio of time on to time off) varies with throttle setting,

with the result that average voltage varies and the motor responds to this average
- speed tables define voltages for each speed step, hence the speed of the motor
- bemf (voltage produced by the generator feature of a motor and so dependent on motor

speed)is a feedback loop - with a constant throttle setting, bemf feature keeps the motor speed constant by increasing the voltage to the motor under load until bemf (and so motor speed) is constant
- bemf doesn't operate until a throttle setting is made.

So if throttle is set to maximum, bemf will keep the speed constant at that defined for maximum throttle (always assuming bemf is operational on maximum throttle)
Consequently, bemf will not affect maximum speed beyond maitaining it under load, if operational

at maximum speed. If not operational, it doesn't affect maximum speed at all.
Do I have it right?

[hosh]

Fishmanoz said:
Consequently, bemf will not affect maximum speed beyond maitaining it under load, if operational at maximum speed. If not operational, it doesn't affect maximum speed at all.

What I think some people are missing

is that there is a seperate speed table for bemf defined by the manufacturers (cv 49-64) - in fact it is a bemf table rather than a speed table (which itself is actually a voltage/power table). The difference being that, unlike the user definable speed table,

the values are bemf voltages seen by the decoder from the motor (generator, as you put it) rather than voltages (average voltages for the pedants) fed to the motor by the decoder.

So the bemf values in cv 49-64 are cruise control settings and the speed

table defined by cv 67-94 are power values (average voltage). In the latter case, speed can vary due to grade and acceleration/deceleration, and in the former speed is held constant by feeding in varying voltages.

So, if bemf is enabled for top speed

(cv10 = 128), then cv64 is in play and, as far as I know, nothing else.

In other words, for any throttle setting that bemf is enabled for, the only speed table in play is cv 49-64. V max, mid and min are probably also over ridden by this. And I now

wonder about momentum as well. All of these are voltage settings. Perhaps the bemf only really takes over when in the vicinity of it's target bemf voltage? I read about the sensitivity of the bemf setting with xxxxxxxx decoders.

[hosh]

Btw, I am wondering if the OP set cv29 bit 4 to "1" (turn on speed table) otherwise changing cv94 to 255 was futile.

I just went over the pdf for the elite and there is no mention of CV29. Adjusting the speed table is in there but nothing about

enabling/disabling it??? It would be off by default on most decoders I'd imagine. Is it turned on automatically if the speed table is edited? If so, how does one turn it back off if so desired so as to re-enable the default table?

And btw poliss, bemf

must dictate top speed if cv10 is set to 128. Otherwise, what does?

[hosh]

PS - disreguard all previous references that I made about cv 49-64 - got that wrong.

But there is obviously an inherent bemf speed table that gives the decoder it's bemf target for any given speed step - that was the point. So all speed tables must

be out of play bar this one when a speed step that uses bemf is selected. It would seem the momentum CVs will bring the engine to that bemf target and then bemf takes over fully thereafter.

[hosh]

It is interesting to see, as one gets deeper into this, how non standardized the decoder industry is. So let's be L..z specific (I DLed a few PDFs from their site).

frazzle set cv 94 to 255 but did not say whether he had already made the appropriate

adjustment to cv 29 to actually enable the user deifined speed table (it is off by default). I wonder if he understands how to adjust such a binary byte to get the 8 bits correct (and, in particular, bit 5(4) = 1)?

My guess is that the factory speed

table is still in play and the CV value for speed step 28 (max) is a lot less than 255.

Also, but much less likely of course, is the possibility that bemf (Control) is enabled for the entire speed range, unless frazzle has played with CV50, bit 7(6)

(again requiring him to understand how to come up with the correct CV value to get the 8 bits all correct). It seems there are ways to alter the bemf behaviour, depending on what pdf one is looking at, but it is vague. It appears different motor types (emf)

can be selected, giving different results and that also CV113 and 114 can be further manipulated. This appears to be for very advanced users only.

I have looked at a few specific PDFs from varoius manufacturers and CV10 seems to not be in play with

any of them as per the DCCWicki page. It seems bemf (Control) is either on for the entire speed range or not on at all.