A speed limiter for a Universal Motor in an old Lionel Train

  • Electrical
  • Thread starter NTL2009
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  • #1
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Summary:

I'm looking for feedback on this before I get too far. I want to set a max speed limit on this model train to avoid derailing.

Main Question or Discussion Point

I have a "262 Lionel Line" O gauge train, from the early 1930's. It has sentimental value, my Father-In-Law (RIP) got it as a gift when he was about age 10. I've set it up each Christmas, a short track around the tree. But it is difficult to control, the gearing is high (it goes fast), so reducing power to it to reduce speed results in frequent stalling at some points. It's a very fine line between too fast and stalling (like keeping the 0-100 knob from maybe 28-32, it's that 'tight'). I'd like to be able to set it and let it run, but I often have to modulate the throttle to keep it in limits. Typically, I need to advance the knob to ~ 50~60 to start from a stop. Especially now that my two grandsons were thrilled with it this year, I'd like to be able to let them run it next year.

FYI, here's a coupe videos of this model:
Some details:
The controller I'm using is a fairly modern one, it uses an SCR/TRIAC to chop the 60 Hz 20V peak AC wave, so under normal operation it does see a 20V peak, which I think is good for providing torque under partial power. The train draws ~ 1.0~1.3 Amp running, and draws ~ 2 Amp to start. I tried a motor controller that puts out a 25KHz, 0~100% duty cycle wave, connected to a 15V 7A DC supply. No real difference in operation. I had thought about a much lower frequency chopped wave, but I doubt that would be much different than a 60 Hz chop (which I guess is really 120 Hz for a Universal motor). I also tried a constant current supply, and that seemed about the same as well.

My plan:
I don't think I can do much from the supply side alone. I think I need feedback from the train speed. I thought about feedback from the train back to the controller, but that got complex. Now I'm thinking about a local electronic switch in the locomotive. Something that could easily be un-done to restore the train to original condition, if needed. My idea is to add an optical detector on the locomotive, a convenient place is underside where a 'push rod' comes through. I could obtain a pulse for each wheel revolution. One rev of the drive wheel takes the train ~ 4.25" (~ 11cm). I measured my slowest/fastest practical speeds for one lap to be from ~ 15 inches/sec to 25 inches/second. So that would give me ~ 3 ~ 6 intervals detected per second, which should be enough for good response. The 3 pole motor turns about 5:1 with the drive wheels.

I really just want to limit the top speed. I think I can use two 555 timers such that I keep power switched ON to the motor when the interval between pulses from the detector is > X mSec, and I'd cut power (internally) if the interval is < X mSec. I could have a small pot on the locomotive to fine tune the top speed. I think this should be fairly simple, though I haven't detailed out the exact configuration yet. And I can full wave rectify the input so I'm just working with DC.

The advantages I see is that should be pretty easy to undo if desired, and it doesn't require a special/modified power supply. I can use that just as I do now, and adjust it to control acceleration - it just wouldn't have much effect once the train has exceeded a set speed (the train would start cutting power internally at that point).
 

Answers and Replies

  • #2
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Do you enjoy digital projects? This sounds like a fun Arduino type project.
 
  • #3
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I could do it with an Arduino, I've coded up a few projects with those, but those are bigger than the one or two 555 timers that I think I can do it with (need to actually design and test that, but I'm pretty sure that's easily done). I'd still need the optical detector and motor switch circuit, so I don't think that is my best option.
 
  • #4
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I bet you could get wireless comm so that the speed controller is not on board the train.

There are Arduino forums where you could post that idea and get lots of specific suggestions.
 
  • #5
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Why does the train stall at certain parts of the track? Are they uphill? It may be more important to improve the conductivity of the track
 
  • #6
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I bet you could get wireless comm so that the speed controller is not on board the train.

There are Arduino forums where you could post that idea and get lots of specific suggestions.
Yes, that was my original plan. I figured I'd use that optical detector to send IR 'beeps", or ultrasonic "beeps" at each interval, and detect those with the Arduino. Then all the 'smarts' would be on the controller side.

But I kind of like the idea of having this as a "limit switch" on-board. I can have a disable switch to bypass it, and it works as normal. That way, it's not dependent on two separate systems working together.

Why does the train stall at certain parts of the track? Are they uphill? It may be more important to improve the conductivity of the track
It's short track with tight curves ( 8 - 9" straghts, 8 - 10" 45° curves), so it is tougher pulling cars through those curves. And as I mentioned, they gear these things to go really fast. Which I don't understand - it's a kid's toy, it should be rugged, geared to overcome friction, curves, etc. If I had the mechanical ability, I'd really like to gear this thing down from the 5:1 ratio to maybe 20:1. If I run the controller above ~ 40%, it starts running away and will throw itself off the track. I'm no motor expert, but Universal Motors have series field windings, and I think that series windings tend to 'run away' RPM wise? That doesn't help.

A few years ago, I ran some 16 GA leads to the 'far end' of the track (which isn't very far!), to avoid IR loss on the steel/nickel track. That didn't seem to help much. This year, I soldered jumpers across the joints on each of the four sections I have mounted to boards. That did help, it is more consistent in speed across the lap, but it's still 'tweaky' to get the speed just right.

ETA: Not only that gearing, but in the box from my Father-In-Law was a rheostat controller. I can only imagine that putting resistance in series with a supply would make this issue much worse. These motors are going to draw more current under load, which would then drop the voltage, limiting the current further. I think the 1950's controller I had for the 1950's Lionel trains had a sliding contact that tapped progressively from the windings of the secondary, so that would have been a low impedance source, and I think would have worked far better than a resistor.
 
  • #7
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I think the 1950's controller I had for the 1950's Lionel trains had a sliding contact that tapped progressively from the windings of the secondary, so that would have been a low impedance source, and I think would have worked far better than a resistor
As a kid my American Flyer set had sliding pickup and my neighbor's O GAUGE Lionel set had a beautiful double slider toroidal wound transformer. The rheostat is not ideal but all the old singer sewing machines used them quite nicely and I think they were series wound motors.
Both train sets certainly went fast enough to fly off the track but you had to be nearly full throttle...... Maybe 90% throttle.....so I think the PWM set-up may somehow not be ideal. The American flyer put out 15 V rms max.
Yes the stators on the AC-DC motors are series wound with the commutators but I am unaware of runaway effect (but I'm no expert here).
I tried a motor controller that puts out a 25KHz, 0~100% duty cycle wave, connected to a 15V 7A DC supply.
Did you look at the voltage at the motor as you tried to vary the motor speed? I don't understand why this doesn't work unless the controller is not working as you surmise.
 
  • #8
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NTL2009 said:
I tried a motor controller that puts out a 25KHz, 0~100% duty cycle wave, connected to a 15V 7A DC supply.
Did you look at the voltage at the motor as you tried to vary the motor speed? I don't understand why this doesn't work unless the controller is not working as you surmise.
Oh it worked, the duty cycle varied just as expected on a 'scope. It's just that it didn't work noticeably different than straight DC (constant V or constant I), or the chopped AC from the TECH II O-gauge rated train controller I had.
 
  • #9
Tom.G
Science Advisor
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Brainstorming here. How's this for a control approach?

  • Startup: Current control, maybe Constant current up to some (predetermined) speed (as sensed by average voltage)
  • Running: constant power; value set by the operator; with a hard voltage upper limit to avoid derailment
Cheers,
Tom

p.s. Hmm... sounds like a bench power supply with current limit and voltage limit
 
  • #10
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Hi !

Been a while, but I built a 12 Volt PWM controller for some Hornby OO/HO. It was much, much better than the supplied rheostat, especially at low speeds, but still exasperatingly non-linear.

( Given magazine's circuit used germanium transistors, efficiencies were low and heat-sink remarkably large... )

Also, IIRC, the pulse frequency had to be tailored to the motor type. There was a broad 'sweet spot' based on the motor's induction, pole-count and maximum RPM. Too low, 'cogged'. Too high, eddy-current losses...

And, yes, a hulking great diode to catch back-EMF spikes, without which precaution, the circuit promptly died...

IIRC, there was a later up-grade using a 'make before break' multi-position wafer switch with resistance ladder determined by iteration to give more intuitive response. Think of it as a 'hardware look-up table'...

IIRC, analogy was made with eg trams or subway step-controllers which had 'experimentally determined' values of resistances or turns between the taps to give a 'more friendly' response.

FWIW, if you used a two-wafer switch, half the second's poles (minus one) could be wired to energise DPDT reversing relay...
==
Sorry, tad terse due two sub-editing cats. I think they want second-breakfast...
 
  • #11
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I could do it with an Arduino, I've coded up a few projects with those, but those are bigger than the one or two 555 timers...
I would try to look up similar (frequency-to-PWM) applications of some PIC microcontrollers. With some of those, it'll be smaller than an 555 :wink:
 
  • #12
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In some situations, 'brush noise' may be used as motor speed feedback.
 
  • #13
Tom.G
Science Advisor
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In some situations, 'brush noise' may be used as motor speed feedback.
Much easier to just measure the back-EMF when the drive is disconnected, as between pulses if using a PWM drive. You have to wait for the switching transient to die down first.
 
  • #14
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Much easier to just measure the back-EMF when the drive is disconnected, as between pulses if using a PWM drive. You have to wait for the switching transient to die down first.
I'm generally familiar with this technique, but haven't found specifics on how to make or buy something that does this. The references I saw actually has some sort of sensor on the motor, I'm not sure how to use the back EMF. This is also a very noisy environment - the two pick-ups on the center rail, and wheels on the outer rails create a lot of intermittent contacts.

I took the first step on the logic with two 555 timers on the bench (w/o the power drive components or motor connected). Maybe I'm just getting old, but I scratched my head over coming up with something that would just cut power when the sensor sent pulses less than (for example) 300 mSec apart, and would pass full power if the pulses were grater than that time. And it has to default to the ON condition. A micro-controller is starting to look more attractive!

Tonight I came up with something that might work, hard to say w/o trying it. I wired a single 555 timer as a re-triggerable one-shot. So the output stays high (which I use to turn power OFF) if the pulse interval is less than one-shot timing (train is moving fast). But it also turns OFF for that amount of time after each pulse. The end result would be 0% Duty Cycle when train is running just fast enough to keep the 555 re-triggered, and if the speed drops to 2/3rd that rate, the DC would be 33% (the 555 times out during that 1/3rd extra time between pulses), and DC is 50% at 1/2 speed.

I'd need to check again, but I think it runs at a good speed at ~ 30% Duty cycle with a pulse power supply. So that might be a good sensitivity to keep it running at that speed that is ~ 2/3rd of my one-shot time - faster will start reducing the duty cycle. I guess I can't really tell if this is too far under damped, over damped, or Goldilocks.
 
  • #15
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Best of luck !!

I've tried using back-EMF sensing to regulate speed of big windscreen-wiper motors re-purposed to drive a wheeled robot's base, wasted several months. An optical feedback disk per motor would have been more productive. Happens that by using the motors' existing speed taps, I got enough control as-is...
 

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