MOTOR FULL LOAD CURRENT

[Tom]

I have come to learn that there are at least several electrically knowledgeable posters on this site.

For some time now I have experienced that when linking two or more locomotives together after having set the Motor Full Load Current automatically they will not stay the same distance away from each other when not physically connected.  Sometimes it is not even close.

Having determined by trial and error that two locomotives will maintain an equal distance from each other at all speeds when the mA are about 200mA apart I manually set one locomotive 100mA above its automatic mA and the other 100mA below its automatic mA.  Which leads to some questions an electrically challenged RailPro user has:

•   Why does setting the Motor Full Load Current automatically not work as it is supposed to work, at least with regard to when it comes to linking two locomotives (both Athearn Genesis GP9) to work with each other?

•   Is there a much simpler methodology that RailPro users may have developed to get locomotives to run at the same speed when linked?

•   The most important question is what possible damage is likely to occur by setting the Motor Full Load Current manually 100 mA or more from the automatically developed one?

Tom

[Alan]

If your GP9s are new I would break them in well and see if the problem doesn't go away by itself.

It has been my observation stall current readings are different each time I test. Not by a huge amount, maybe +-50mA. Why, I don't know. I confirmed with my ammeter that a loco does return different readings at wheel slip under what otherwise seems identical test setup. So, it is the locos not the HC. I attribute it to the fact they are toys, not precision machinery. I do the auto stall test several times noting the reading. Then I average the readings and use that.

Note the stall reading during repeated tests of each GP9. Do the same loco results vary by a large amount?

[Tom]

Alan,

I agree with your stall limit test variations observations and have wondered if the motor temperature could have anything to do with it; among other potential variables.  Variations are within the 50 mA +/- range you mentioned, but that does not explain or address the speed variation between locomotive using the automatic setting of the load current.  It seems that if the automatically set load current is as reflective of locomotive performance as Ring says it is, it should work – within reason.  I am a little disappointed that the automatic load setting does not work entirely well for whatever reason.  GP9s are not new and I have no idea how used they might be and Ring is definitely oriented towards diesels as well as newer versions, so maybe older HO motor electrics are a problem for Ring in this regard?

Do you have any comments on how to easily get all locomotives running at the same speed (top speed changes have little or no effect) and whether or not they could be damaged by setting the load current manually too far from what the automatic comes up with?

On another related BTW issue, when locomotives are linked the overall speed at a speed % is less than for each locomotive individually not linked.


Tom

[Alan]

To be honest, I have never ran two locomotives uncoupled to see if they maintain spacing although Ring does just that on the promo video. I have not had unusual gear wear, coupler breakage, or anything of the sort that would indicate the locos aren't playing well together. My layout cannot be powered up at the moment (in the midst of fascia panel installation, lots of bare dangling wires) so I cannot test maintaining spacing of locos for you.

I don't believe you will damage anything regardless of stall current setting. Even if full track voltage with unlimited amperage supply was applied to the motor (DC mode) you would not damage it. It would just take off at 90 MPH! The LM has internal over current protection so you won't damage the module. It will simply shut down if overloaded. If greatly different stall current settings makes the locos run nicely together then I would do it that way.
 
Yes, I too have noticed the throttle position difference between single loco and multi-loco consists although it is a small difference. I have no explanation for why that is.

Curious, what stall current do your GP9s measure? 100mA is a big difference on a 300mA loco but a small difference on a 900mA loco.

[KPack]

Keep in mind that load sharing is not speed matching.  It is very likely that two locomotives will run at different speeds when uncoupled.  Think about it....the principle of load sharing is based on the locomotive modules detecting a load, and then distributing it across the consist.  If a lead locomotive isn't coupled to anything, then it detects no load, and therefore tells the following locomotives to contribute less to the overall pull.  So yes, the following locomotive will usually run much slower than the lead if they are uncoupled.  How much slower depends on the individual locos and their settings.

You'll notice that the effects of load sharing are much easier to see when pulling a longer/heavier train.  Try it and you'll see.

That being said, there have been a couple of locomotives I've built and installed Railpro in that I had to dial in the MFLC manually in order for them to play well.  These were frankenstein locomotives though....motors were not original and much work was done to the drivetrains. 

Also, going along with Alan's comments, I will typically run my MFLC settings three times on a new locomotive to get the most accurate one.  Does it really matter?  Probably not, but I do it anyways.  If you think about it, the MFLC will drop slightly after first time due to the wheels/rails getting mildly polished from the spinning.  Running the locomotive for a bit beforehand to warm up the motor and loosen the gears a bit may also make a difference.  It will likely be a small difference, but a difference nonetheless. 

Out of curiosity I have re-run the MFLC on locomotives that had it set years ago, and remarkably the values have largely been the same, maybe within 10 mAh of the original or right on.

-Kevin

[Dean]

With new locomotives, I set the full load current using the automatic mode. Then after running my break-in routine, I measure the full load current again. Usually, there is a drop in the full load current.
Running a four or five engine consist pulling 30 heavy cars, ( 8 oz+ each ) I watch the couplers. The couplers on the locomotives stay tight, going up or downhill.

My break in procedure:

 The main thing you are trying to accomplish during a break in of a DC motor is to seat the brushes and polish the commutator.

The armature ( the entire rotating part of the motor including the commutator) moves back and forth in the housing as the speed and load changes.

Getting the brushes to seat is pretty straightforward. Run the engine at varying speeds in both directions for about 20 minutes each direction.

Getting the proper polish takes current passing from the brushes to the commutator and running unloaded doesn't supply enough current. After running light as above, I attach a coal car to the engine that is half full of lead shot. This gets the same 20-minute treatment as the engine running light.

I checked the current draw of some Kato powered engines by placing my finger in front of the engine to prevent it from moving and ran it at full speed for a few seconds. The wheels were allowed to spin on the track. New, out of the box, most engines drew 270ma to 310ma. After break-in, they would draw 220ma to 260ma. Of course, the gearboxes and other running gear also got some break-in time too. But with Delrin plastic in the gearboxes and running gear, I don't think they made much difference.

[ I was an electrician in a rolling mill for 35 years. Over 90% of the motors in the plant were DC. My break in procedure is based on my experience working on these DC motors. ]

[G8B4Life]

Going to do a bit of a thread hijack here, but it is relevant to the discussion.

After reading this topic I decided to look at what the Patent had to say about the load sharing function, which is very little with only a little bit of information on one part of the logic.  You can easily look it up if you want, I won't put it here as it's not the point of my post.

What I did read which I found strange was the way the modules control the motor. We all know the DCC waveform is square wave right? I've never put an oscilloscope on anything let alone a DCC decoders motor output but I've always assumed that that would also be square wave as well, all the PWM motor control that I've read about is all square wave. Well it appears that Ring has done it differently. From the Patent:

"wherein at least one control module varies a voltage on a motor of a respective motor powered rail vehicle in accordance with a periodic wave shape of a subsonic type between about 2 and 20 Hz with an approximate triangular shape"

What would be the benefit of driving a motor with a triangular waveform (disregard the word approximate for this question)? From what I can gather from reading the above it looks like it's easier to "set" the average voltage that the motor sees without having to drive the output transistors at supersonic speed like modern DCC decoders have to but that's the only thing I can think of.

- Tim

[Alan]

... What would be the benefit of driving a motor with a triangular waveform (disregard the word approximate for this question)? From what I can gather from reading the above it looks like it's easier to "set" the average voltage that the motor sees without having to drive the output transistors at supersonic speed like modern DCC decoders have to but that's the only thing I can think of.
- Tim

Fewer parts count in the module. The triangular (dual log slope) waveform is the natural output from an oscillator due to timing network charge and decay of C. Wave shaping to a neat square wave requires several more components i.e. triggers. The additional wave shaping provides little benefit to motor control because of the motor's rotating mass relative to the signal frequency. The motor couldn't respond accurately to the sharp rise time of a square wave anyway so a slow rise/fall wave works just as well. There is a power loss using a triangular waveform versus a square wave however that is easily compensated with higher frequency.

Voltage across C over time:


To further hijack the thread, this subject will come into focus when I post my DCC-simulated power supply circuit for Jacob.

[KPack]

I'm so confused, lol.

[Alan]

I'm so confused, lol.

That's why the magic is always kept inside the box out of view! 

[Dean]

Would this explain why a DC only locomotives buzz when they are put on a DCC track?

[Alan]

Yep. That and a DCC signal is AC so the poor DC loco is trying to reverse directions 7,000 times a second. That's why they get hot.

[TwinStar]

Voltage across C over time:

The last time I saw this there were sharks and tornadoes coming out of it. That's my level of electrical knowledge.

[G8B4Life]

Thanks Alan, I was hoping you'd be able to provide an answer.

It makes more sense now. When I read the words "approximate triangular" I envisaged the typical straight sided sawtooth pattern that one sees all the time in images, perhaps "flattened" on top by the word approximate, not a log slope.

With the dual log slope waveform, how much harder is it to work out what the average voltage the motor sees is compared to a square waveform (which is easy)? Given the "1000 speed steps" we have the 2 to 20Hz duty cycle mentioned in the Patent doesn't sound like it'd give that.

- Tim

[nodcc4me]

Kevin, Jacob, you are not alone. If Alan and Tim ever got together and started talking electronics there would be a lightning storm, the likes of which the world has never seen.