Locos run slower on DCC than on DC. Why?

[jpendle]

I am in the process of measuring the scale speed of all my locos as I am automating my layout using iTrain.

 

I model in N Gauge and the majority of my locos have Zimo MX series decoders. Loco wise I am talking about Farish and Dapol CL66's, CL60's, and CL68's.

 

Almost without exception the same thing happens when measuring scale speed on any of these loco's fitted with a Zimo decoder.

 

First on my DC test track, with a Dapol CL68 fitted with a Zimo MX618N18 at firmware rev 40.1 I measure a top scale speed of ~150mph.

 

With exact same loco on my layout controlled by my Z21 the loco only gets to a top speed of ~75mph.

 

And, when I measure the speed of the loco using iTrain every 5 speed steps from 126 down to 1 the loco speed maxes out at 75mph at speed step 60. The speed then  wobbles around 75mph all the way up to speed step 126.

 

I have also discovered that I can set the max voltage delivered to the motor to ~6V (CV57 set to 60), with no effect on max speed. The speed curve looks better, but that's not what I'm after.

 

Now, and this is important, I don't care what the measured speed profile looks like in relative terms, I can change CV's 2,5,&6 and mess with a 28 step curve if I need to.

 

I have 3 questions.

 

1) Has anyone else noticed this kind of behaviour?

2) Does anyone know why this happens?

3) Has anyone been able to fix the issue?

 

A few other random thoughts, the same happens with my ESU Loksound 5 equipped Farish CL90's. I'm using mine on intermodal trains so 75mph is just about good enough, but I wouldn't be happy if I were running with rakes of MK3 coaches.

 

My Pendolino's with both Zimo and ESU decoders achieve prototypical high speeds, but as a result I haven't ever measured their speeds on DC.

 

A user on the iTrain forum reported the exact same issue in 2021, but as this isn't an iTrain issue no conclusion was reached.

 

Maybe this is just an N Gauge problem and HO or OO models with the same chips, but presumably bigger and more powerful motors don't have this issue.

 

Regards,

 

John P

Edited January 2 by jpendle

[bandmbill]

What voltage does your DC controller deliver?

 

What is your DCC system and what is it set to deliver?

 

 

[peach james]

Several possible answers 

 

1.  Track voltage (rms) is low, so the decoder cannot supply enough v to the motor to run “fast”

2.  Decoder settings (which you seem to have in general ruled out)

remember that the decoder eats about 4v as a minimum due to H bridge ect.  So, if you have track voltage at 12v, only 8v is available to spin the motor.   Unless you are using decoders that are specifically designed for low voltages. (Typically Z sized decoders..), then they are designed for track voltage of up to 22v service.  (and not less than 28v before damage). (16v is the low voltage standard)

My first guess is low track voltage.

[Dagworth]

Agree, track voltage. 

 

I found that trains ran significantly slower using ECoS than with my Lenz set 100 on my layouts that use a diode drop track circuit module, ECoS just didn't have the output voltage to get the speeds I wanted

 

Andi

[WIMorrison]

John,

 

I am very aware of this issue, however as I mention below I don't see the DCC speed as the issue, rather the converse.

 

6 hours ago, jpendle said:

1) Has anyone else noticed this kind of behaviour?

 

Yes, on almost all models, irrespective of scale.

 

6 hours ago, jpendle said:

2) Does anyone know why this happens?

 

There are various reasons, none of which concern me because the DC scale speeds are ridiculously high and the

needs tuning to bring the max speed down to a sensible, prototypical value.

 

A common reason is that the DC voltage output by most controllers is around 16v, not the mythical 12v that everyone quotes.  Another reason is the universal adoption of can motors which are stock items with very high rotational speeds, allied with incorrect gearing.

 

7 hours ago, jpendle said:

3) Has anyone been able to fix the issue?

 

 

The MS and MN versions of decoders allow for the DCC settings to be applied to the DC and that makes the speeds much more comparable between DC and DCC, however I generally switch this off because I use DC to clean the wheels whilst the loco is in a servicing cradle and I want the highest wheel speed possible for cleaning.

 

7 hours ago, jpendle said:

With exact same loco on my layout controlled by my Z21 the loco only gets to a top speed of ~75mph.

 

 

The default track voltage for a Z21 is 18v to allow for the 1.5 to 2v lost due to the operational features of a decoder, (not the 4v suggested earlier) taking the default voltage to much the same as a DC controller, however if you have reduced the Z21 output voltage then this will also reduce the max speed. When you reduce CV57 you are simply reducing the apparent voltage presented to the motor. 

 

Another feature that will impact the performance under DCC are the back EMF setting that you have set because they cause the decoder to modify the apparent motor voltage. There is no back EMF control in DC, excepting for limit at max motor revolution and some motors produce little real back EMF therefore the motors will 'overspeed' under DC control.

 

The bigger and more important issue is not why there is a difference in DC and DCC operation, but why the manufacturers create models that break the sound barrier on DC. They should be geared to create a semi-realistic max speed when at full DC voltage (perhaps the mythical 12v that everyone seems to believe exists), and not require CV57 reducing to very low values, often accompanied by reductions in CV5.

 

A realistic DC top speed may also result in realistic operation by DC exhibition layouts where the max speed is often far above what the loco ever performed at - though the current preference for 'shunting planks' with trains running slower than real life has created the opposite problem.

[Pete the Elaner]

6 hours ago, bandmbill said:

What voltage does your DC controller deliver?

 

What is your DCC system and what is it set to deliver?

 

 

 

This would need to be measured with the loco running at full speed. The 'open' voltage will be more.

[RAF96]

A similar disparity has been noted with the Hornby HM7000 series decoders twixt Bluetooth running and DCC direct running.

[Michael Hodgson]

Speed is the distance covered in a given time.  "Scale speed" is rather meaningless in model railway practice, because we take all sorts of liberties with both time and distance to make it work in a way which "looks right".

 

For example a carriage is scaled at the correct length, but the platform is shortened and the distance between stations is massively shortened, our curves are much tighter than reality and in OO even the distance between the rails is wrong.  Even if you try to scale accurately it's not possible - a OO loco scales a 1/76 its length but 1/438976 its volume - and therefore also of its mass.  The thickness of the body would be too slight to pick it up without damging it.  Futhermore if we run to a timetable, we also distort time by using a speeded up clock, and because nobody wants to watch a station with nothing moving most of the time (the reality of railways) we run a frequency of service that the London Underground would struggle to achieve.

 

So we aim to run our models "looking right" whatever that means.  Fast vs slow is probably what we have all become accustomed to by playing with train sets as a child or attending model railway exhibition and watching other people's layouts.  I would suggest that what matters is the relative speed of different types of train - an express passenger should be moving faster than a coal train and very much faster than an engine shunting in the goods yard.  That is more important than how long it takes to travel to/from our storage sidings = another fiddle!

[34theletterbetweenB&D]

The notion of 'scale speed' is very much down to personal preference, no point arguing about 'correctness'. I happen to be of the scale linear distance per unit time school, but if others want different that's their choice to make. With a 90mph steam period mainline, I will have the contrast between Doncaster's finest racing past, and the pre-group 0-6-0 struggling for 15mph with a heavy load on.

 

2 hours ago, WIMorrison said:

The bigger and more important issue is not why there is a difference in DC and DCC operation, but why the manufacturers create models that break the sound barrier on DC.

There is no standard I am aware of in respect of this, the RTR designer has nothing constraining the choice of motor and effective gear reduction combination, and it shows! For completeness there are also specimens in RTR OO that won't achieve anything like maximum speed with the nominal 12V DC supplied at the motor terminals, and much the same may be the case in other scales.

 

9 hours ago, jpendle said:

3) Has anyone been able to fix the issue?

Yes, and it is so simple in DCC. I have increased the voltage output from the system so that there is 15V available at the motor terminals for my OO layout: that enables the most sluggish slugs to make their scale maximum speed the way I want it. Despite some direly pessimistic warnings, no harm has resulted in the dozen years since I embarked on this; and the system manufacturer (Lenz) and the brands of the decoders used (Lenz, Zimo, ESU) do actually acknowledge that their product is designed with this capability.

 

Every decoder is set up with maximum speed trimmed to scale. One of the oldest mechanisms which can achieve circa 400mph on 12V DC, has CV5 set at 30 from 255, and that delivers the 60mph maximum that it might just graft up to, on a good day with a following wind...

 

Both the DCC track voltage applied and the CV settings will vary widely dependent on the owner's tastes and the scale and products used. (It was the accepted noem that N gauge shouldn't have more than 9V DC applied at the motor terminals, but that may well be long out of date.)

[jpendle]

12 hours ago, bandmbill said:

What voltage does your DC controller deliver?

 

What is your DCC system and what is it set to deliver?

 

 

Hi and a Happy New year to all,

 

The maximum DC voltage with the CL68 running is slightly more than 14V, I’ve got a Z21 and it had been set to a track voltage of 15V, increasing this to 16V, which is as high as it will go, makes no difference.

The Z21 came with an 18V power brick by the way, although I will double check that.

 

Regards,

 

John P

 

 

Edited January 2 by jpendle

[MichaelE]

My trains mostly run very slow, especially the freight consists. IR and City Shuttles run a bit faster, but nothing runs so fast it looks like a train around the Christmas tree as is so often seen in O scale here in the States.

 

My layout straight sections are not long enough to run most express and passenger trains at more than about 45 MPH. Freight typically runs around 30 MPH. Sometimes much slower.

 

Now, if I had a layout with 20'+ straight sections, speed would be increased somewhat for both types of trains.

[WIMorrison]

46 minutes ago, jpendle said:

...Increasing this to 16V, which is as high as it will go ..

 

5.1. Can I set the output voltage on the track?

 

The output voltage can be set on the black Z21 between 12 and 24 V. However, the actual track voltage can only be a maximum of 1 V below the input voltage.

E.g. input voltage 20 V, set 18 V = track voltage 18 V

E.g. input voltage 20 V, set 24 V = track voltage 19 V 

E.g. input voltage 20 V, set 12 V = track voltage 12 V

On the white z21, however, the output voltage cannot be adjusted and is around 1 V below the input voltage. 

Please note that it is not possible to use a standard multimeter to measure the output voltage reliably. This applies both to the DCC signal and more specifically to the asymmetrical Motorola signal.

 

Taken from - Questions & answers - FAQs & Support - Roco z21

[Crosland]

14 hours ago, peach james said:

remember that the decoder eats about 4v as a minimum due to H bridge ect.  

 

It's 2 V, tops, in the bridge rectifier in the decoder. The H-bridges in the booster/command station and decoder will both be MOSFET based these days and very low resistance. If the booster/command station can be powered by AC then there will be another bridge rectifier and another 2V max drop. Some systems allow the track voltage to be adjusted down.

 

The loss in the decoder will be a lot better than 4 V.

Edited January 2 by Crosland

[jpendle]

1 hour ago, WIMorrison said:

 

5.1. Can I set the output voltage on the track?

 

The output voltage can be set on the black Z21 between 12 and 24 V. However, the actual track voltage can only be a maximum of 1 V below the input voltage.

E.g. input voltage 20 V, set 18 V = track voltage 18 V

E.g. input voltage 20 V, set 24 V = track voltage 19 V 

E.g. input voltage 20 V, set 12 V = track voltage 12 V

 

Hi Iain,

 

As I said, my Z21 came with an 18V DC power supply so I cannot get the track voltage any higher than 16V with my existing setup.

Roco themselves do not sell a PSU rated at more than 20V DC. I can get hold of a suitable 24V PSU for $20 from Amazon, or $40 from Digikey, but there's probably not a great deal of difference in quality. I can probably borrow a suitable bench supply from work and try it out with that.

 

With regards to designing drive trains to suit the loco, I have 7 Pendolinos, 4 have Loksound and 3 have Zimo MN180 decoders, all run quite happily at scale speeds.

I also have 3 CL92's, reviewing the speed profile for one of these locos shows a straight line from 0 to ~100mph, with default settings for the speed curves, all these models are designed by Rapido, again fitted with a Loksound decoder, so it does appear that this can be done right!

 

2 hours ago, MichaelE said:

My trains mostly run very slow, especially the freight consists. IR and City Shuttles run a bit faster, but nothing runs so fast it looks like a train around the Christmas tree as is so often seen in O scale here in the States.

 

My layout straight sections are not long enough to run most express and passenger trains at more than about 45 MPH. Freight typically runs around 30 MPH. Sometimes much slower.

 

Now, if I had a layout with 20'+ straight sections, speed would be increased somewhat for both types of trains.

 

I'm modelling in N Gauge and I do have 20' straight sections, so I expect my express passenger trains to run at 100mph or above, and for the fast freights to run at 75mph.

 

I think my next step is to source a 24V supply for my Z21 and then increase the track voltage to see what effect it has.

 

I am also sorely tempted to buy an ESU decoder tester so that I can actually measure the PWM waveform supplied by the decoder to the motor.

 

Regards,

 

John P

[spamcan61]

15 minutes ago, jpendle said:

 

 

I am also sorely tempted to buy an ESU decoder tester so that I can actually measure the PWM waveform supplied by the decoder to the motor.

 

Regards,

 

John P

I use one of these cheapo oscilloscopes for that kind of thing.

 

https://www.ebay.co.uk/itm/285367662920?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=4aHBSGodTHK&sssrc=4429486&ssuid=CUw97j2zRRy&var=&widget_ver=artemis&media=COPY

Edited January 2 by spamcan61

[WIMorrison]

@jpendle I use old laptop power supplies that I have lying around from what seems like a lifetime of using them. They range from 16v to 23v and 2.8A to 3.6A - strange values but there you go.

 

I just cut the ends of wire and use a screw connection barrel. They work a treat for all sorts of things 😊

[jpendle]

@spamcan61 The company I work for makes Oscilloscopes so there are plenty sitting around not being used in the office.

@WIMorrison I went through my box of redundant wall warts this morning and not one of them goes above 16V 👎

 

I'll have a poke around in the office, I've got a very large system supply that'll do 24V @ 4A, but someone will probably have a suitable bench supply lying around.

 

Regards,

 

John P

[WIMorrison]

1 minute ago, jpendle said:

The company I work for makes Oscilloscopes so there are plenty sitting around not being used in the office.

 

Want to send me one? happy to PM you my address 😂

[jpendle]

Found a 25V bench supply as well, so I just need to make up a suitable cable to start testing the track voltage.

 

1 hour ago, WIMorrison said:

 

Want to send me one? happy to PM you my address 😂

 

They are only "apparently" sitting idle. If one went walkies it would soon be noticed.

 

We also have lots of "apparently" empty cubicles and storage rooms, put something in one and you get an email within a week asking to get your junk out of someone else's space!

 

John P

[Crosland]

15 hours ago, WIMorrison said:

They range from 16v to 23v and 2.8A to 3.6A - strange values but there you go.

 

If they have a two pin power cord, then they are designed to be below 65 W. Anything above that almost always has an earth. Otherwise the filtering gets very difficult.

 

[Nigelcliffe]

I have a thought on measuring this stuff....   Requires minimum of a decent volt meter, but ideally an oscilloscope.  

 

Loco will need to be on bench in some sort of cradle (running on rollers might be good). 

Connect the measuring device to the motor wires inside the loco.   And then run it on the different power sources (DC, DCC, different track voltages).    See what happens to the applied volts at the motor at different throttle settings.   

 

Ideally this would also be coupled with some measure of RPM of the motor, or wheel RPM, which will equate to running speed.   

 

 

No idea if the above kit is available to do the testing.  

 

 

- Nigel

[jpendle]

So armed with a 25V 1A bench supply I did some experiments this morning.

 

First of all I disconnected the storage yards from the track bus, this reduced the current consumption from 2A to 0.5A.

I then hooked up the supply to the Z21 and set it to 24V.

 

With the Z21 track voltage set to 16V I ran a CL68 (Zimo MX618N18) over a fixed length of track and it took ~20 seconds.

With the track voltage increased to 20V the same distance took ~ 10 seconds!!

With track voltage reduced to 18V the loco still managed to do the distance in 14 seconds or about 40% faster than at 16V.

 

I couldn't measure any speed profiles as the measurement tracks are in the storage yard area.

 

However, it would seem that even with just a 2V change, a 42% speed increase should take my CL68 locos from 75mph to 105mph.

 

I think it's also interesting to note that Roco no longer supply the 18V adaptor with the Z21 they supply 20V adaptors instead.

 

I now need to get my hands on a 24V 4A adaptor or perhaps borrow another power supply from work.

 

This one will do the job, it'll do 35V @ 6A but it's a bit of a beast.

 

 

Regards,

 

John P

 

 

Edited January 3 by jpendle

[jpendle]

Hello,

 

My 24V 4A power brick arrived today and I duly set about measuring speed profiles for my CL68 68004 Rapid.

 

First is the measured speed with the track voltage set to 16V, the eagle eyed will notice that this is a different CL68 but the original speed profile for 68004 was almost identical.

No CV's have been altered so everything is set to Zimo defaults.

 

 

Now we have 68004 with the track voltage set to 18V

 

Top speed is now ~90mph but the curve still has far too much "wasted" at the top end.

 

Next track voltage set to 20V.

 

 

This is getting there, top speed in excess of 100mph, remember CL68's are rated for 100mph running.

 

And finally with CV57 set to 90 (9V max at the motor) and CV2 set to 20.

 

 

Still plenty of overhead with regards to max speed and a nice slow crawl at speed step one.

 

Once this curve is applied iTrain will smooth out the top end.

 

The only problem now is that I'll have to re-profile all my locos because they had their profiles captured with a 14V track voltage.

 

Regards,

 

John P

[WIMorrison]

John,

 

When I see this sort of curve  I reduce CV5 and/or CV57 (but you have already reduced CV57) to shift the curve to the right and lose the bump. Ideally I like to get the top of the curve at SS 126.

FYI, I had a discussion a couple of years ago with Zimo about CV57 and asked them to clarify how it actually works. I was told that it is more relative than absolute.  I asked if a value of 100 meant 10v with a track voltage of 14v and it was still 10v at a track voltage of 18v and was told that I shouldn't consider it providing an exact voltage! The upshot seeming to be that your value of 90 may not be 9v, but it will certainly be less than the track voltage. I just use CV57 now on all my locos, with the lowest I have at CV57=75 on a track voltage of 17.5v (with 19v supply).

[jpendle]

Hi Iain,

 

Yes I still plan to tweek CV5, if I set CV57 any lower I still get the "bump" at the top end of the curve and top speed is lower.

 

I am still very tempted to buy a decoder tester to see what the motor voltage looks like on a scope as CV57 is adjusted, but for now I've got a lot of loco's to profile.

 

Plus, I was cutting twigs to 1" lengths to load my log train and after doing 24, worked out that I'll need more then 2000 to do the whole train, at least it'll give me something to do while I'm profiling locos.

 

Regards,

 

John P