Problems with LOAD feature

[CPRail]

Has anyone been experiencing issues with the LOAD feature?

I was at an op session on a NCE DCC layout over the weekend and I was running RailPro. One of the problems I was running into was the units would get "Motor Overload", or "Motor Overload +" warnings while using LOAD. As you know, these warnings shut everything down until you reset the units.

I only recall this happening while using the LOAD feature. Dial LOAD down to Zero and away you go with no problems. One move had 6 units that stalled on the Big Hill with the Motor Overload issue, which might've overloaded  the NCE system. Every unit in the consist did the overload thing. Drop the power down to 3 units and no issues, other than having to double the hill. Throughout the event, I'd get the weird overload on the flats and the hill while using LOAD.

All my units have Bridge Rectifiers and K-As, and there is a mix of LM-2S and LM-3S, mostly LM-3S modules.

Important Caveat on the layout in question: There are some mysterious electrical gremlins that affect DCC users, and trackwork is less than ideal in sections. The event in question had 8 DCC units with sound running, plus my 6 RailPro which were eventually reduced down to 3 after the Big Hill pull. Also the owner is a firm believer in heavily weighted cars, so the train I was attempting to pull on the Big Hill was 44 cars long and probably about 20 lbs in weight. The Big Hill is probably 20 to 30' in length with a sustained pull of about 2% but small sections that are higher. It's a real test of power.

Thoughts?

[KPack]

Ian, that's interesting...never heard of anything happening like that before.  I haven't had any issues with the load feature.  From the background on the situation it sounds like the strain of the locomotives may have tipped the scale, and add in the additional 8 DCC sound units and it could have overloaded the NCE system as well.  What is curious is why you noticed this with the LOAD function on and everything was fine with it off.  The LOAD function is really a mathematical algorythim that affects momentum and sound volume based on what you input.  I don't think it would put additional strain on the LM, besides putting a bit more power through the amp for sound.

If they motor overload tripped then it's my assumption that the locomotives were drawing too much power.  Does the NCE system scale up power based on usage?  Meaning, is it detecting a large drain and subsequently putting more volts to the rails?  That is a heavy train and that alone could trip the fault if the locomotives are really working hard. 

Here's what I would try next time....rather than running 6 units up front, how about put 4 up front and two in the back.  That should distribute the weight of the train a bit better I would think.  Railpro is perfectly situated to run DPU ops, I do it all the time on a friend's layout.  And with all your locos having KA's you won't have to worry about the DPUs dropping power.  My gut feeling is that distributing the power over the length of the train will solve whatever happened.

-Kevin

[Alan]

Were you also getting 'Low Voltage' warnings when the problem occurred?

My thought is the large loco current load on the NCE system may have caused it to drop in voltage especially if the electrical characteristics of the layout are less than ideal. The RP module would compensate for low voltage by lengthening the pulses. If the voltage were low enough and the locos under sufficiently heavy load then the module may have been at 100% pulse width (even though your throttle was not at 100%) which makes it essentially a conventional DC driven motor. 

In a DC motor, as load increases armature rotation slows which reduces the back emf and allows more current to flow to provide power to drive the load. If the voltage is low, the armature has to turn even slower so as to allow more current to flow so that the product of voltage times current equals the power needed to drive the load. The heat produced in the armature is proportional to the square of the current times the armature resistance. Thus, the heat quickly rises as the motor slows down because of the low voltage combined with heavy load.

[KPack]

^ I like Alan's explanation much better than mine.  I was mostly pulling stuff out of my hat.  His sounds like he knows what he's talking about.

-Kevin

[CPRail]

Hey guys,

Yes, I was seeing Low Voltage throughout the session, which was odd as he has the layout broken into different power sections. He did this as to not overload any one section, which multiple sound units have done in the past. The odd part was I was the only one running in my section for most of the night and I would see 14 volts, then 4 volts than 14...

I did initially have it set-up as a DPU as I knew it was going to be a monster, and saw Kevin's video with DPU, so I know it works. It backed out of the spur all happy, but when I attempted the run on the hill I couldn't get it to move - of course, it was hung over two sections with the lead locos in one and the DPUs on another section which had other DCC units in play. Sigh.

I think Alan has it right. Lucky for me, I took another iteration of the same train back down that session. After all the switching, there's another 30-40 cars to come back up. I'll try the DPU idea again, but with only 4 units. I hate doubling the hill!!

Why anyone wants to play with DCC, I have no idea!!

[Alan]

A point of clarification...

When RP reports motor overload it isn't actually measuring the temperature of the motor itself. There is no temperature sensor attached to the motor. What is actually being over-driven (overheating) are the output transistors in the LM PWM drive circuit.

Functionally, it is all the same. Just wanted to clarify in case someone was wondering how the LM knows the motor temperature. It doesn't. But it does know how much current is flowing through the output transistors and their temperature.