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The Electronics of Model Trains

Without a doubt one of my favorite toys when I was a kid was my electric train set. I can't recall what gauge it was, but it was about "Lionel" size, though not Lionel brand. For as much fun as it was though, there wasn't much that was realistic about it. This thing was all about raw horsepower, sparks, and the smell of ozone in the air. With a basic universal type (AC-DC) motor and simple rheostat controlled power supply you would have to use a lot of imagination to pretend you were driving the real thing. A slow, careful increase on the rheostat throttle would first make the motor hum and give off a faint smell of hot insulation. A little more and the wheels would start to jerk. "All aboard!" Finally the voltage would reach that magic threshold and she would shoot like a rocket, and if I was lucky it would even stay on the tracks! Unfortunately, today's model railroaders aren't able to experience this kind of excitement. Five-pole skew-wound motors, pulse-width modulating throttles, and DCC (digital command & control) systems have taken all the fun out of it. These electronic technologies have brought the hobby to such a level of realism that I fear what will become of our imaginations. :-)

OK, now that I have that sarcasm out of my system I'll try to share with you what I think are really cool technologies that have evolved in a great hobby.

Lets start with the improvements towards mechanical realism. Unlike the electrical beast from my childhood, real-life trains don't start and stop so quickly and erratically. If you've ever watched a real freight or passenger train leave the station you noticed that it started with an almost imperceptible movement of the wheels and then accelerated very, very gradually. That's because of the tremendous inertia of the train. A typical modern diesel-electric locomotive has about 4,000 to 6,000 horsepower - that's a lot of pulling force! Often you will see multiple units coupled together to provide the needed locomotion for the specific load. But then you factor in that the load of cars being pulled is hundreds or thousands of tons in weight and you can understand why they accelerate and decelerate so slowly. Two technologies designed to mimic this behavior are five-pole skew-wound DC motors and pulsed DC modulating throttles with "momentum" breaking.

Inside the model locomotive is a small DC (direct current) motor. Good quality units have a drive train that transfers the mechanical power to all of the wheels that contact the track. Early models, and

simple direct current motor with a 2-pole rotating armature

some inexpensive ones today, usually used a simple motor with a 3-pole armature. Combine this with an equally simple variable voltage power supply and you got something like what I described above. Though it was still fun to watch going around the track, it didn't behave like the real thing. One way to improve this was to manufacture motors with a 5-pole

N-scale "Dash-8" locomotive with 5-pole motor

armature. This gives the motors much smoother torque characteristics by reducing the angular increments between each pole. Manufacturers can improve the smoothness even more, especially at low speeds, by angling or "skewing" the armature poles relative to the shaft. This increases the manufacturing cost, but reduces the tendency of the motor to "cog" or have a preferred angular position with the field magnets.

Next, but no less important , is how you drive the motor. My train set used a simple AC

MRC model 260 Tech 4 controller w/ momentum

transformer with a wire-wound rheostat to vary the voltage to the track. Unless you combine this with a motor of exceptional quality you're not going to get very good results. With inexpensive DC power supplies, where the voltage is raised or lowered in a simple way, the effect is much the same. A major reason for the jerkiness when starting a locomotive is a mechanical characteristic called "stiction" (for static friction). This is the resistance to initial movement between contacting surfaces like bearings and gear teeth. This is why my early locomotive would hum and vibrate until the voltage to the motor was raised high enough to

A pulse-width modulated signal

overcome this opposing force - then it would lunge forward. To remedy this problem designers used circuits that would pulse the DC output to the motor. This has the effect of "nudging" the armature in order to break the stiction force in a more gentle way. One way to vary the speed of a DC motor using a pulsed output is with a method called "pulse-width modulation". An example is shown at left. Rather than vary the voltage level, or amplitude, of a steady DC current, you output a pulsed, full voltage signal, and vary the "on" time (or width) of each pulse using the throttle control. Though this offers smoother low-speed motor operation it also has the negative effect of causing the motor to generate more heat, which may shorten its lifespan. Quality system manufacturers, such as MRC, use more sophisticated technologies which can combine pulsing (to overcome stiction and inertia) with a steadier DC current to reduce the heating problem and extend motor life. Combine this technology with a five-pole skew-wound DC motor and you get unbelievably smooth action!

Taking realism a step further, additional technologies have been developed to simulate the inertia and momentum of long, heavy train "consists" (the specific locomotive and car makeup of a train). These systems operate by automatically ramping-up or ramping-down the output in a gradual way so that starting and stopping appears as it would with the real thing. Also, there is a feature available called "Positive Tracking Control". When a model locomotive pulls its cars up an incline it will normally slow down from the increased mechanical load. In the real world the train engineer would increase the throttle to compensate for this, but with model railroading you may not want to have to perform this duty each time an incline is approached. Control designers figured out a way to do this automatically by exploiting a DC motor characteristic called "back-EMF". If you pick up a small DC motor and manually rotate its armature it will act as a generator and output a voltage. With a moving model locomotive this output, or back-EMF, can be detected by the controller by momentarily interrupting its output and quickly taking a measurement to determine approximate motor speed. If it senses the motor slowing it can automatically increase its output to compensate - just as a real engineer would! Pretty cool?

Now we move on to something the computer science people can really sink their teeth into: DCC (digital command & control). Fortunately for me the good people at MRC provided an excellent primer on the subject, so I'll only need to outline the basic features and benefits here. You can click below to read the full primer.

MRC's Prodigy DCC system

Digital Command & Control was initially developed to solve the problem of operating multiple locomotives or trains on the same section of track. You can understand how several different locomotive motors driven off the same controller output simply wouldn't work. You couldn't drive one without driving all of them, and secondly, variations in motors and train sizes would cause some to go faster than others, ultimately resulting in a collision somewhere. With the evolution of digital electronic circuits a system was developed where the track was powered by a steady AC sine wave in which coded digital "packets" are imposed and then recieved by their respective decoding devices. This allows the use and operation of multiple trains - individually controlled - on the same section of track. A mobile "decoder" is a device with a specific digital address that controls aspects of a particular moving train, such as speed, direction, and maybe some lighting. An accessory decoder is typically used to control stationary items such as building lights or track turn-outs. These systems continue to get more sophisticated, and their uses are really only limited by your own imagination!   Click here to read more about DCC



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