Bad performance at higher speed with DCC decoder

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denfin
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Bad performance at higher speed with DCC decoder

Postby denfin » Fri Oct 29, 2021 5:31 pm

I converted an old Hornby Dublo 2-rail A4 engine to DCC, but it seems to not work too well with DCC at higher speeds, cus when running, it like slows down for a second, then it picks up speed again and runs fine for a few seconds, then it slows down for a second again, and picks up speed again, and it happens over and over, so it runs kinda jerky. At a little slower speed it seems to perform better for the most part. Anyone know why this might happen? Is it maybe cus the decoder might not be strong enough or something for these older motors? The motor is re-magnetized at least. The decoder I use is a Lais DCC Kung Fu decoder, I do not have any other decoders to test with. I also seem to have similar problems with some Triang X03 motor locomotives if I remember correctly when I messed around trying to put a DCC chip in it, I guess it might be the same problem there.

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Ironduke
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Re: Bad performance at higher speed with DCC decoder

Postby Ironduke » Fri Oct 29, 2021 10:37 pm

denfin wrote:The decoder I use is a Lais DCC Kung Fu decoder


specifically what model?

denfin wrote:I do not have any other decoders to test with.


That would certainly prove something one way or the other.
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Rob

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Re: Bad performance at higher speed with DCC decoder

Postby Bigmet » Fri Oct 29, 2021 10:52 pm

Don't know this decoder at all, but it may have protection built in that limits current delivery when the decoder starts heating at high output. You need to read what is to be found in the decoder information to assess that. One possibility for the cyclic variation you describe when running fast.

I'd want to take the decoder out and test the mechanism on an adequate 12V DC supply with no feedback assistance, firstly to measure the stalled and running current draws to compare to the decoder's rated output; and also to see if the same cyclic variation shows up, in which case it is a mechanism problem. Motor shaft bearings can heat and bind; but the cyclic rate you describe strikes me as too fast for this, it's usually more like 30 seconds slow, 10 seconds fast on OO motors, or an even longer cycle time (rapid heating at full tilt, heat build up then takes longer to dissipate while running slower).

My experience with decoders is that best performance is obtained with a stalled current draw no more than a third of the decoder's rated sustained output. In this respect modern RTR OO mechanisms with can motors score, few draw more than 300mA so a 1A sustained output decoder is fine. To get good performance out of sixty plus year old open frame motors, sustained output capability of circa 2A was required. Did that with expensive Zimo decoders, until I had replaced all the older open frame motor locos with modern can motor mechanisms. (I whang my locos along at scale for 90-100mph on full size expresses, that's what I want to enjoy and the reason for having my mainline ECML layout!)

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Bufferstop
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Re: Bad performance at higher speed with DCC decoder

Postby Bufferstop » Sat Oct 30, 2021 11:36 am

You say the problem is present on a Hornby-Dublo locomotive and on ones fitted with the Triang XO3/4 motors. It's almost certain to be the case that the decoder cannot deliver the high output that both of these motor designs require. They are both power hungry designs in comparison to a modern motor. Both designs were almost unburstable and capable of long periods of high output, but it does come at the price of needing a high output power supply, most DCC power units should handle one of them but the decoder is likely to be the restricting factor in getting that power to the motor.
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denfin
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Re: Bad performance at higher speed with DCC decoder

Postby denfin » Sat Oct 30, 2021 11:55 am

Thanks for the replies!

The decoder name is LaisDcc KungFu 870021: https://laisdcc.com/LaisDcc_Decoders_Manual_V2.pdf

This is what it says about the decoder:
"4 Function decoders with NEM652 interface standard and with a fly out purple wire F2, Function Rating at 100 mA, total Rating at 1.0 Amp continuous, 2.0 Amp peak."

And it's a bit tricky to understand everything, but I disconected the decoder and measured the resitance on the locomotive now, (which i guess is good to do according to Ohm's law) and it seems it was 15-16 ohms, which I guess is kinda low? I also measured the stall current, and it seems to be over 900mA something, so kinda close to 1A. So yeah maybe it's recomended to have a better decoder?

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Re: Bad performance at higher speed with DCC decoder

Postby Bufferstop » Sat Oct 30, 2021 1:30 pm

The Resistance of the windings is around what I would expect from motor designs of that age. You cannot apply Ohm's Law to a motor winding whilst it is still in the motor. Flemming's left hand rule says that in the presence of a current the motor will turn in a predictable direction. Once it's turning you can also apply his right hand rule that a conductor turning in a magnetic field will generate a current in a predictable direction, which will be opposite and almost equal to the current applied in the left hand rule. That current gives you a back EMF which restricts the total current drawn, so OHM's only applies to the small difference in current due to applied voltage over that generated by the back EMF. If you want to know the current drawn by the motor you need to insert an ammeter in series with the motor (taking care to get it the right way round). The current drawn when the full voltage is applied (with the decoder out of circuit) and the motor physically restrained from turning, is known as the "stall current" and is useful to know when deciding what decoder is needed. It should be capable of withstanding the stall current for as long as the power supply takes to cut out.
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Re: Bad performance at higher speed with DCC decoder

Postby RAF96 » Sat Oct 30, 2021 2:11 pm

Likely a weak magnet is pushing the decoder towards its limits.
Try this trick - plonk a strong magnet onto the motor and see if it performs better. You will find a magnet somewhere indoors, magnetic pickup tool, torch with magnet base, fridge, old HDD (OK maybe over the top), etc.
If so then you need to get the motor re-magnetised or some motors will work OK with a neo magnet but others suffer and see bearing wear.
A reasonably strong magnet will allow a screwdriver to pickup the motor.
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Re: Bad performance at higher speed with DCC decoder

Postby Flashbang » Sat Oct 30, 2021 4:55 pm

I have to agree with RAF96. These older loco motors lose magnetic strength over the years and that in turn causes motor current to rise. Which is probably causing the decoders internal circuity to cut out intermittently

A remag. will improve the locos/motor performance no end, so I would get this done on both old locos.

Also ensure the loco is well maintained lubrication wise. Only use oils sold specifically for model railway use. Never use spray types of lubricant such as WD40 or thicker oil like 3 in 1 examples. Never over apply oil and if necessary blot surplus off with a tissue or kitchen paper. :D
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Peterm
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Re: Bad performance at higher speed with DCC decoder

Postby Peterm » Sat Oct 30, 2021 11:55 pm

He said the motor's been re magged.
Pete.

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Re: Bad performance at higher speed with DCC decoder

Postby Bigmet » Sun Oct 31, 2021 8:41 am

Peterm wrote:He said the motor's been re magged.

Indeed, and now we have a stall current measurement of 900mA. These old open frame motor designs are not that efficient, need a lot of current; based on my own now near twenty years past experience I wouldn't use anything less than a 2A continuous output decoder on a motor with that stall current.

Which is expensive, and led me to pretty quickly replace the old open frame motor mechanisms with more recent can motor powered mechanisms. You can not only use a cheaper decoder, but the running is significantly more refined all around, quieter and smoother thanks to a multistage drive rather than a worm on an axle. But this is very much a choice for the individual.

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Re: Bad performance at higher speed with DCC decoder

Postby Flashbang » Sun Oct 31, 2021 9:36 am

Peterm wrote:He said the motor's been re magged.

Oops! :oops: Missed those few words in the OP text. Sorry
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Peterm
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Re: Bad performance at higher speed with DCC decoder

Postby Peterm » Sun Oct 31, 2021 9:54 pm

Brian,
It wouldn't be the first time I've missed something either. :)

Bigmet,
I've got a Wrenn tank loco that'd be current hungry, but choose not to try fitting a decoder because of the current draw which is exacerbated by a dodgy magnet.
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Re: Bad performance at higher speed with DCC decoder

Postby RAF96 » Mon Nov 01, 2021 8:46 am

OOPs - There is none so blind as us who cannot see the printed word in front of them. It wasn’t a neo magnet was it as they can be more trouble than a weak original.
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Re: Bad performance at higher speed with DCC decoder

Postby Bufferstop » Mon Nov 01, 2021 10:08 am

Unless it was previously replaced even a two rail H-D couldn't have come out of Binns Road with a neo magnet. Mid 80s they appeared, were scarce and dammed expensive. Is it the HD early ringfield that takes up much of the cab?
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Bigmet
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Re: Bad performance at higher speed with DCC decoder

Postby Bigmet » Mon Nov 01, 2021 3:53 pm

Bufferstop wrote:... Is it the HD early ringfield that takes up much of the cab?

Certainly does in the 8F. I have accidentally acquired a pair of these, both very good, with ample traction, and neither of them can have seen much use as the tyres show no wear, motor windings clean and bright, brush carbons full length, they look almost ex-factory. Current draw running at full speed with a 60 wagon (express) freight 700 -750 mA , take the train off and they still draw 670-700 mA. Stalled current on nominal 12V DC, 1.4A. As near to nothing, identical results from both examples.

Don't know what the cheapest good 3A+ continuous rated decoder would be: at least there's plenty of room in the tender to house it, with lots of metal available for extra heat sinking, should anyone want to run one on DCC...


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