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Running in locos as DC on DCC Wired Layout


sholidom
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If you want to run in a new loco - why not going to the local club? most of them have a test loop for various scales and probably even more than one for 16.5mm. This also allows you to show off with your new loco in front of your club mates.... :yes:

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1 hour ago, Pete the Elaner said:

I agree.

 

NMRA do not help matters though. You would think you can trust those who set the standards but I found this on their website:

"The DCC signal is an alternating DC waveform, which contains the digital information".

I wish they would take that rubbish down because the terms 'alternating' & 'DC' completely contradict each other. Current is either alternating or it is direct.

I read somewhere that some electrical engineers do not like a square wave described as AC, presumably because it confuses people into thinking it is a sine wave. I think they prefer to call it a bipolar signal.

A digital signal can be encoded within DC, but the DCC we all use does not work like this.

The use of the term AC , in electronic engineering is largely confined to sine wave classic alternating current  configurations , precisely because the term is very loosely defined  ( its also used rather erroneously  in audio and radio engineering to mean a waveform that has sinusoidal characteristics , even if the current is uni-directional) 

 

Firstly taking about voltage waveforms and AC is inherently contradictory , AC refers to current flow ONLY . If you are taking about the DCC voltage waveform ( which is what most  mean when they refer to "DCC")  this is NOT an AC waveform, This is a ( maybe ) a bipolar waveform  , 

 

DCC is also available as a unipolar waveform , this is what often used as the inputs to a pure Booster for example . 

 

 

It should be noted that the terms AC and DC can change as the point of reference changes , for example the current waveform exhibited at the DCC  power supply my be a pulsed DC current and not an AC one ,

 

The term alternating DC waveform is entirely appropriate , it means the DC current switches direction and is often used in non Sinusodial descriptions of digital waveforms 

 

I notice the NMRA specification never uses the term AC  in any of its specs.

 

Undoubtably DCC , when measured using the track as a reference exhibits a AC current characteristic , thats about as much as you can use the term AC in this context for.  but equally Bi-polar , unipolar , pulse train etc  are valid when used as appropriate 

 

IT should be noted that when pulse stretching is included the in DCC , the definition of AC is stretched even further 

 

So the real answer is " its complicated "

 

Dave 

Edited by Junctionmad
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I buy most of my stuff second hand because I find that cost effective and I  prove it is working absolutely perfectly on DC before I start ripping it apart to, mostly hardwire, a decoder into the model - impossible to return or dispute the performance once you have hacked it around and if it doesn’t work after I fit the chip I know who I kick

 

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5 minutes ago, Junctionmad said:

The use of the term AC , in electronic engineering is largely confined to sine wave classic alternating current  configurations , precisely because the term is very loosely defined 

 

Firstly taking about voltage waveforms and AC is inherently contradictory , AC refers to current flow ONLY . If you are taking about the DCC voltage waveform ( which is what most  mean when they refer to "DCC")  this is NOT an AC waveform, This is a ( maybe ) a bipolar waveform  , 

 

DCC is also available as a unipolar waveform , this is what often used as the inputs to a pure Booster for example . 

 

 

It should be noted that the terms AC and DC can change as the point of reference changes , for example the current waveform exhibited at the DCC  power supply my be a pulsed DC current and not an AC one ,

 

The term alternating DC waveform is entirely appropriate , it means the DC current switches direction and is often used in non Sinusodial descriptions of digital waveforms 

 

I notice the NMRA specification never uses the term AC  in any of its specs.

 

Undoubtably DCC , when measured using the track as a reference exhibits a AC current characteristic , thats about as much as you can use the term AC in this context for.  but equally Bi-polar , unipolar , pulse train etc  are valid when used as appropriate 

 

IT should be noted that when pulse stretching is included the in DCC , the definition of AC is stretched even further 

 

So the real answer is " its complicated "

 

Dave 

 

Thank you Dave - it is for these very reasons that I stated that DCC is not AC and I would maintain that position to avoid any confusion for people.

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7 minutes ago, WIMorrison said:

I buy most of my stuff second hand because I find that cost effective and I  prove it is working absolutely perfectly on DC before I start ripping it apart to, mostly hardwire, a decoder into the model - impossible to return or dispute the performance once you have hacked it around and if it doesn’t work after I fit the chip I know who I kick

 

This is entirely sensible and is what many of us do with locos that are not supplied DCC chip fitted. 

 

What I think I and others are trying to get across is that so called " running in " , simply requires the motor to turn and it matters not  if thats DC , pulse DC, half rectified DC, PWM DC ,  either from a DC controller  or a DCC decoder 

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1 hour ago, Crosland said:

 

Can we please stop this nonsense.

 

DCC IS AC.

 

It just isn't a 50 or 60 Hz, sine wave at 120 or 240V (i.e. AC mains), which seems to greatly confuse our American cousins, and some others too, it seems.

 

DCC is a low voltage square wave of low kHz frequency. Half the time the current flows one way, half the time the current flows the other way. The current alternates. It is alternating current. It is AC.

 

If it looks like a duck, quacks like a duck, ...

 

I'm sorry, but it isn't.

 

Dave got this right in this post where he says:

 

17 hours ago, Junctionmad said:

Not really , the voltage signal associated with dcc is correctly called a bipolar signal as from the perspective of the track the voltage swings about a “ nominal zero point “ ( which may or may not be gnd or 0 V ) 

 

An AC signal alternates the terminals (that would be rails in our case) both above and below the gnd or 0V level.

 

The DCC signal swaps the potential of the two rails but the polarity of the rails doesn't change.

 

To explain.  Take the left rail as an example.  The voltage may vary between zero and max negative voltage whilst the right rail varies between zero and max positive voltage with respect to gnd or 0V.  The left rail never becomes positve and the right rail never negative.

 

To quote this DCCWiki article > https://dccwiki.com/DCC_Power > "Since the signal is digital, the DCC waveform is a complex sine wave. The voltage on each rail alternates from positive voltage (High) to 0 volts. On an oscilloscope this appears to indicate a plus/minus signal, but it really isn't. It "sees" a negative voltage when the polarity flips on the rails, and the common point is energized. Since the signal changes at a very high frequency, a Digital MultiMeter (DMM) cannot read electrical values accurately. They do not sample at a high enough frequency to be able to calculate the value accurately, as they are typically designed to measure AC in the 50-60 Hertz range. Also, digital multimeters are designed to calculate and display the RMS value of a pure sine wave.

 

Using a DC function on a digital multimeter will not give accurate readings either, due to the changing nature of the signal. There are meters available that will accurately measure DCC signal, such as the RRampMeter, which was designed specifically for measuring DCC voltages and currents."

(my emphasis)

 

What is confusing about the DCC Wiki article is this >> the DCC waveform is a complex sine wave.  It should read a complex sum of sine waves, since by adding sine waves of different frequencies, amplitudes and phase we can synthesise any other waveform (this synthesis is known as a Fourier Series > wiki article).

 

Dave's post (five posts above this) is spot-on.

 

Art

 

Edited by Art Dent
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49 minutes ago, Junctionmad said:

 

The term alternating DC waveform is entirely appropriate.

 

That's complete rubbish because the two terms contradict each other. DC by definition is "a unidirectional flow of electric charge". If it alternates, it is not unidirectional.

 

I can understand not referring to a pulsed signal as AC, because it is usually (& wrongly) assumed to be sinusoidal AC.

Edited by Pete the Elaner
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49 minutes ago, Pete the Elaner said:

That's complete rubbish because the two terms contradict each other. DC by definition is "a unidirectional flow of electric charge". If it alternates, it is not unidirectional.

 

I can understand not referring to a pulsed signal as AC, because it is usually (& wrongly) assumed to be sinusoidal AC.

 

 

If you consider the typical method by which a DCC power signal is generated , ie a FULL bridge driver 

 

The feed to the bridge is DC , voltage is unipolar above system GND 

 

The drive to Half bridge A , is a unipolar DC pulse waveform 

 

The drive to Half bridge B is , a unipolar DC pulse waveform of alternative sign to drive A.  This is where the term alternating DC comes from , it means the voltage waveform  is  alternating between the two sides of the bridge   at no point in this circuit have we alternating current flowing 

 

Now the resulting DCC voltage waveform  when referenced across the output of the bridge driver , ie the track feed , is again NOT AC ( since we are measuring a voltage waveform ), and is correctly called a bipolar signal , ie the signal will be represented on an oscilloscope  ( using a differential probe only ) , is in effect , swinging from +- Vbridge supply/2  , ie its its a 20V Supply to the bridge  then the voltage is actually swinging around 10V, but to the and the loco the voltage is swinging around a nominal Zero point 

 

In none of these cases are we talking about current 

 

Now lets deal with Current 

 

Depending on how a DCC driven system rectifies the DCC waveform , the resulting current could be a form of AC, as in full rectification , yet many accessories and track powered  units , often only use half wave rectification , in this case the current comes from one or the other track , and hence is a DC current 

 

SO the correct answer is that the CURRENT waveform , may be AC or DC 

 

 

The point is that a DC derived signal , which exhibits symmetrical bipolar operation  can be described as an alternating DC waveform , its not a common nomenclature , this side of the pond , but any EE nows what being described.  The use of the term " alternating " describes the voltage waveform 

 

Like I said its complicated , simply stating that DCC is AC ( in all cases )  is NOT correct 

 

Dave 

Edited by Junctionmad
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10 minutes ago, Pete the Elaner said:

I agree with everything except  "an alternating DC waveform". This term is completely contradictory.

The term is not commonplace , but the alternating bit refers to the fact that  each side of the track voltage is an inverse of the other , ie it alternates.  You are mixing up AC and the word "alternating ".  perhaps a better term would be alternating bipolar waveform .  the term DC tends to get bandied around in voltage and current  descriptions with abandon  ( as does AC in lots of cases ) 

 

The term tends to be used to differentiate that a DC source was used to create the waveform 

 

 

to be a bit simplistic , lets say I operate a DC reversing switching randomly over the period of an hour,  I could describe the resulting signal as an AC signal , ( which it maybe ) but that would be somewhat misleading, I equally could say its a alternating DC waveform, which conveys  ( a) it changes polarity , ( b) its not a sinusoid and (c) I used DC to  create the signal 

Edited by Junctionmad
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1 hour ago, Art Dent said:

 

I'm sorry, but it isn't.

 

Dave got this right in this post where he says:

 

 

An AC signal alternates the terminals (that would be rails in our case) both above and below the gnd or 0V level.

 

The DCC signal swaps the potential of the two rails but the polarity of the rails doesn't change.

 

To explain.  Take the left rail as an example.  The voltage may vary between zero and max negative voltage whilst the right rail varies between zero and max positive voltage with respect to gnd or 0V.  The left rail never becomes positve and the right rail never negative.

 

To quote this DCCWiki article > https://dccwiki.com/DCC_Power > "Since the signal is digital, the DCC waveform is a complex sine wave. The voltage on each rail alternates from positive voltage (High) to 0 volts. On an oscilloscope this appears to indicate a plus/minus signal, but it really isn't. It "sees" a negative voltage when the polarity flips on the rails, and the common point is energized. Since the signal changes at a very high frequency, a Digital MultiMeter (DMM) cannot read electrical values accurately. They do not sample at a high enough frequency to be able to calculate the value accurately, as they are typically designed to measure AC in the 50-60 Hertz range. Also, digital multimeters are designed to calculate and display the RMS value of a pure sine wave.

 

Using a DC function on a digital multimeter will not give accurate readings either, due to the changing nature of the signal. There are meters available that will accurately measure DCC signal, such as the RRampMeter, which was designed specifically for measuring DCC voltages and currents."

(my emphasis)

 

<sigh>

 

It all depends where you take the reference.

 

An AC signal can have a DC component (an offset). The fact that neither rail goes negative (relative to where???) is completely irrelevant.

 

A loco on a layout has no fixed (e.g. 0V) reference. All it sees are the two rails. At one instant rail A is more positive that rail B. At another instant rail B is more positive that rail A. Current always flows from the more positive side to the other.

 

I will say it again, The current flows from A to B and from B to A at different times in the cycle. The current alternates. It is alternating current.

 

You can argue black is white all you like but I am sorry, you are wrong. There is no need to use terms such as alternating DC or bipoplar DC when AC is the accurate description of the DCC signal.

 

 

Edited by Crosland
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3 hours ago, Pete the Elaner said:

NMRA do not help matters though. You would think you can trust those who set the standards but I found this on their website:

"The DCC signal is an alternating DC waveform, which contains the digital information".

 

That's because a lot of Americans seem to refuse to accept that AC can be anything other than what they get from a mains socket.

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3 minutes ago, Junctionmad said:

  perhaps a better term would be alternating bipolar waveform .

No perhaps about it... that is infinitely better!

 

DC by definition describes current flowing in 1 direction, which DCC does not do. Alternating DC is therefore scientifically incorrect.

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Just now, Crosland said:

 

That's because a lot of Americans seem to refuse to accept that AC can be anything other than what they get from a mains socket.

Quite likely. It has unfortunately promoted poor terminology like 'modifying a point for DCC' which I find infuriating.

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This discussion is all very interesting but what type of voltage is fed to the motor?  The original question was whether a motor is best run-in, ie to bed the brushes in, on a DC controller or a DCC.  Surely it is the voltage to the motor brushes that is in question?  Pure DC or pulsed DC?  A battery is obviously the purest voltage.

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On 02/03/2019 at 17:32, grahame said:

. . . . or buy yourself an oval of setrack - easy and cheap.

 

G.

Yo Grahame

 

Oddly the most common sense answer.

 

If you need to run locos in on DC (why?) then a battery or a cheap train set controller and an oval of track. Whizz one way then the other. If happy with its running pop in the chip and off to the main layout. Job done.

 

I still don't know when  loco is run in, but I do know when it isn't working and how to fix it...or bin it.

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37 minutes ago, Jeff Smith said:

This discussion is all very interesting but what type of voltage is fed to the motor? 

It's a stream of dc pulses.  The number of pulses per period of time determines how fast the motor turns.  At maximum speed the pulses are virtually uninterrupted dc.

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2 hours ago, smokebox said:

It's a stream of dc pulses.  The number of pulses per period of time determines how fast the motor turns.  At maximum speed the pulses are virtually uninterrupted dc.

 

So that makes it a synchronous motor.....how then will it run on battery DC?

 

I think a DC motor speed/power is determined by voltage.

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3 hours ago, smokebox said:

It's a stream of dc pulses.  The number of pulses per period of time determines how fast the motor turns.  At maximum speed the pulses are virtually uninterrupted dc.

 

No, It's pulse width modulation (PWM). The number of pulses per second is constant. It is the ratio of on to off time (duty cycle) that determines the effective voltage.

 

Early decoders used a very low (100s of Hz) frequency, much like a DC pulse controller. This was very bad for coreless motors. Modern decoders (for at least 10 years, probaly 15+) have used much higher frequencies. At first they were still in the audible range but then increased to 40kHz or more to give a "silent" drive. It's this high frequency together with the motor inductance that means that the motor effectively sees a DC drive, except at the very, very lowest speeds.

 

1 hour ago, Jeff Smith said:

 

So that makes it a synchronous motor.....how then will it run on battery DC?

 

I think a DC motor speed/power is determined by voltage.

 

It's DC, see above.

 

EXcept for Marklin with their c-sin motor that requires special decoders.

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Ok, hopefully that clears that up.  Back to running in the brushes this is really a mechanical issue, what force turns the motor is probably irrelevant as long as the brushes bed in and track ok on the commutator.

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According to Wikipedia AC is

Alternating current is an electric current which periodically reverses direction, in contrast to direct current which flows only in one direction.

 

Just thought I'd throw that in

 

Or should we call it non-sinusoidal variable frequency alternating current (NSVFAC)

 

John

As for running in I generally don't

 

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10 hours ago, Jeff Smith said:

Ok, hopefully that clears that up.  Back to running in the brushes this is really a mechanical issue, what force turns the motor is probably irrelevant as long as the brushes bed in and track ok on the commutator.

 

And of course quite a few motors these days don’t have carbon brushes anyway, coreless etc as well as some iron rotor ones........precious metal fingers and so forth.

 

But I have never considered ‘running in’ a motor. Well made they should be optimum at the get go. It’s other parts, bearings, gears etc that can benefit from some polishing of the mating surfaces.

 

Izzy

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20 hours ago, Sir TophamHatt said:

...However reading through the first few posts of advice that DC was better, there were no reasons given as to why...

 

On ‎02‎/‎03‎/‎2019 at 19:10, 34theletterbetweenB&D said:

...Seeing the mechanism perform 'unassisted' by back EMF powered by a simple DC supply like a resistance controller enables the owner to see whether the drive train is mechanically sound, and any less then satisfactory running to be rectified. Having the mechanism exercised forward and reverse around the sharpest curves it will negotiate - left and right - on the layout typically allows any running issues to be spotted and rectified. If the mechanism degrades in performance once the decoder is fitted, it's an easy test to remove the decoder and determine whether the mechanism unassisted outperforms the decoder...

Those are my primary reasons. I first want to see how the mechanism performs without the overlay of DCC. And then repeat with decoder fitted to check that performance is further improved. Sometime it isn't, and no amount of alteration to what CV adjustments are available can amend matters.

 

The DCC decoder in respect of motor supply is just a DC controller, small enough to be onboard. If I am forking over cash above what the old style DC controller costs, I expect more. Sadly there are plenty of decoders on sale that don't deliver the full potential that DCC can offer in motor control. Easily characterised if you can compare the mechanism performance with and without DCC.

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