DCC - Scale running speed?

[Noel]

How do folk calibrate their DCC locos to run at maximum scale speeds?

 

I spent some time this evening using a stop watch timing locos as various CV5 settings through a 3 meter long section of the layout and trying to scale up the measurements to calculate speeds in MPH.

 

If my calculations were correct with a Lenz silver out of the box (i.e. CV5=255) I was getting a scale speed of between 64 and 70mph on the 141/181 locos I tested. Out of curiosity I ran at the seven available speed steps in 1-28 step mode to see what scale speed was achieved at each throttle setting in steps of 4 (i.e. 0,4,8,12,16,20,24,28). I think the prototypical 141/181 may have only had eight throttle settings. Also compared with an MM0071.

 

 

The MM locos seem to run out of the box at excellent scale speeds and also excellent low speeds on startup. The old generation Hornby guff I was used to 30/40 years ago started off at scale speeds of 25mph!!!

[irishthump]

Very useful info, Noel. Thanks for posting your results!

 

Funny, I always thought the 141's/071's ran way too fast with TV set to 255. I usually set my locos top speed around 120!

[Noel]

Very useful info, Noel. Thanks for posting your results!

 

Funny, I always thought the 141's/071's ran way too fast with TV set to 255. I usually set my locos top speed around 120!

 

I was surprised at the results myself. I just needed to know for sure what the models actual scale speed was. So I measured out 3m of straight track on the layout and put two posts at each end for stop watch timing gates. I timed three sample runs and averaged the times. The MM 071 models cannot achieve their scale top speed of 90mph.

Edited November 19, 2014 by Noel

[Dhu Varren]

It has maybe something to do with the voltage at the track. Different control equipment/transformer combinations can sometimes give different track voltages.

[DiveController]

I was surprised at the results myself. I just needed to know for sure what the models actual scale speed was. So I measured out 3m of straight track on the layout and put two posts at each end for stop watch timing gates. I timed three sample runs and averaged the times. The MM 071 models cannot achieve their scale top speed of 90mph.

 

Interesting! This is a point I had been meaning to ask, i.e. whether the motors were geared and sufficiently wound to achieve they top speed of the real locomotive. For instance 141 might be scaled to top out at 70 mph but a 071 would be differently geared or have additional copper wound on the engine core to enable a scale top speed of 90 mph

Noel, you ran the locomotive without load which might limit its speed even further:confused:

That being said, I do not know how much load a rake of coaches or freight would provide (if much at all, especially on the level) since they'd be the correct scale size but not necessarily the correct scale mass/weight

[Dhu Varren]

Just carried out a similar test over 3m with 071 and 5 bogies. Using Noel's calculation figures, averaged times gave a speed of 79mph with CV5 at 255. Track voltage reading 18.25v.

[Flying Scotsman 4472]

Just carried out a similar test over 3m with 071 and 5 bogies. Using Noel's calculation figures, averaged times gave a speed of 79mph with CV5 at 255. Track voltage reading 18.25v.

 

If you want realistic running freight trains would be limited to 50mph not 79mph

[Dhu Varren]

If you want realistic running freight trains would be limited to 50mph not 79mph

 

Who mentioned anything about freight trains?

[Noel]

I noticed my MM141/181 locos had significantly greater top speed when run on DC rather than DCC. The DCC max speed was closer to scale, but I'd prefer to be able to achieve this with CV5. I suspected my DCC track voltage was a little low, so I read my NCE manual which suggested increasing the VREG voltage on the controller mother board from 14.1v to 15.5v (via adjustable pot) which would cause a corresponding increase in track voltage. I have no way of correctly measuring track voltage as I only have a basic ammeter.

 

I had timed my MM locos before changing the controller voltage, and afterwards some of them had higher top speeds, but not the Lenz nor LokSound decoder powered models. I'm guess their circuits must produce specific DC voltages rather than just rectify, irrespective of track voltage (i.e. constant DC out irrespective of AC in). This DCC malarky has a lot to teach me yet.

[irishthump]

If you want realistic running freight trains would be limited to 50mph not 79mph

 

I think the OP is just trying to get the locos to run at a prototypical top speed, pulling a train is another issue.

[Noel]

I think the OP is just trying to get the locos to run at a prototypical top speed, pulling a train is another issue.

 

Yes and the older loose coupled unbraked wagons and vans I prefer would have run closer to 30-40 mph. 181 hauled passenger trains would have operated on good mainline sections at speeds of 60-70mph.

 

PS: I don't do freight without a break van at the end! It's a bit like a blunt pencil - pointless without the need for a guards van!

Edited November 19, 2014 by Noel

[Flying Scotsman 4472]

Just carried out a similar test over 3m with 071 and 5 bogies.

 

 

Who mentioned anything about freight trains?

 

 

I assumed when you said 5 bogies you were talking about wagons. Didn't realise you pulled the bogies of your wagons and just coupled them on their own behind your 071.

Edited November 19, 2014 by Flying Scotsman 4472

[Dhu Varren]

I assumed when you said 5 bogies you were talking about wagons. Didn't realise you pulled the bogies of your wagons and just coupled them on their own behind your 071.

 

I apologise, but my statement has obviously been misunderstood. The phrase '071 and 5 bogies' is railway speak for an 071 and five bogie coaches.

Edited November 19, 2014 by Dhu Varren

[Noel]

Very useful info, Noel. Thanks for posting your results!

 

Funny, I always thought the 141's/071's ran way too fast with TV set to 255. I usually set my locos top speed around 120!

 

Hi IT.

 

Well that may be correct for your DCC system and your decoders. I discovered that DCC track voltages are not quite standardised across vendors. This evening I ran extensive tests and observed 4 different MM 141/181 locos that all ran at significantly higher top speeds on DC (ie no decoder with 21pin loco supplied backing plate). These DC top speeds were 90mph which was 20% higher then the same locos running DCC, which yielded scale top speeds of 70mph unladen at CV5=255 (i.e. closer to prototype max of 75mph).

 

I then adjusted my NCE track voltage using their recommended procedure from internal 14.1vdc to 15.5vdc on the motherboard (ie using their recommended VREG test point on pcb). Btw, this is NOT the same as the output track voltage but has a corresponding effect on it.

 

After this adjustment, the Zimo decider 141 was achieving a scale top speed of 75mph with CV5 reduced to 210. BUT here's the odd thing, my Lenz decoders on 141/181 remained at the same scale top speeds of 64mph with CV5 still up at 255. In other words the Lenz decoders appear to be delivering the same DC voltage to the motors even though the DCC track voltage was increased, wheras the Zimo was delivering higher DC voltage to the motor!

 

The two questions are why do decoders run locos 20% slower on DCC than the same loco on 12v DC? And why do some decoders output higher DC when DCC track voltage is slightly increased, and others do not (eg Lenz)?

 

Good night

[irishthump]

The two questions are why do decoders run locos 20% slower on DCC than the same loco on 12v DC? And why do some decoders output higher DC when DCC track voltage is slightly increased, and others do not (eg Lenz)?

 

Good night

 

There's a couple of reasons DCC runs slower than DC.

Firstly, the DCC signal is modulated AC so it can never deliver the same voltage as a similarly rated DC controller, it loses a bit more as it's run through the rectifiers in the decoder. Also, the decoder itself needs a certain amount of power just to keep it "ticking over". Typically when you run a DCC equipped loco on a DC layout you have to up the start voltage to get it moving. (I tried it with a Zimo sound decoder on DC and the loco drew 7 volts before the sound came on.)

 

 

Can't answer your second question, maybe it's just a manufacturer thing....

[Noel]

There's a couple of reasons DCC runs slower than DC.

Firstly, the DCC signal is modulated AC so it can never deliver the same voltage as a similarly rated DC controller, it loses a bit more as it's run through the rectifiers in the decoder. Also, the decoder itself needs a certain amount of power just to keep it "ticking over". Typically when you run a DCC equipped loco on a DC layout you have to up the start voltage to get it moving. (I tried it with a Zimo sound decoder on DC and the loco drew 7 volts before the sound came on.)

 

 

Can't answer your second question, maybe it's just a manufacturer thing....

 

Thanks. I would have hoped DCC standards were designed to take into account the voltage drop in the decoders bridge rectifier ensuring a max of 12v DC was available on output to the motor. But obviously not.

 

Ps. What are we both doing up at this hour! good night

[Junctionmad]

I suspect you have back emf sensing on those decoders

[Noel]

As I have been tinkering around with DCC sound and notching levels on MM CIE 141/181 locos, I have had to do quite a bit of trail and error testing, and time trials at different CV settings and different speed curves to find the optimal for scale speeds. The time trials were done on a 3m segment of straight level track using two timing gates.

 

The end results below. I have two speed step tables showing the scale speeds in MPH for each throttle speed step. 1-28 and 1-128 tables included. The 141 now continues progressive notching up, slowly accelerating to a target speed for example of 60mph and then notches down after attaining cruise speed. The only way to stop effectively is using brakes (F4) as the loco will coast for a very long time at idle if throttle set to zero. It's a whole new way of driving but now I just love it. It is so prototypical requiring cab operator to plan and think ahead just like the real thing. The CV3+4 inertia values are set at large values.