I was on reddit, asking about a problem with my engine, and somebody said I might have to play around with Back Emf and speed tables. I have definitely heard of both of those, but I have no idea what they are. Anyone got a short descriptor for each?
Thanks, ron
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Very short and basic:
Back EMF is a sort of electrical feedback from the motor to the decoder. “Some” decoders can detect this feedback and use it to adjust speed, for example, when going up or down an incline.
Speed tables are a set of CV’s that tell the decoder how much power (actually PWM precentage, ie, what speed to move at) for a given speed step. It’s usually used for speed-matching locos, limiting top speed, etc.
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Ohhkay, i understand, thanks. So you probably woudnt need BackEMF on something like a yard switcher (what i am working on right now) that works at slow speeds and things?
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What is your problem with your engine? The Reddit reply may not be the applicable solution.
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Sure, you could use back EMF on a yard switcher, for example to keep the speed at a given speed step constant whether you’re pulling a long cut of cars or none.
Oh, and it just dawned on me: Another use for back EMF is with the sound decoders that notch down the sound of the prime mover as the loco gets up to speed. It’s a more pronounced change the heavier the train.
Two questions…
- What decoder is in your locomotive?
- What locomotive do you have?
Back EMF is more important at slow speed than higher speeds. You want your switcher to crawl and to do it smoothly.
Tom
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Ohhh ok, understood. My locomotive is an older Athearn RTR SW1500 in BNSF paint. It was originally DC when I got it, I made it DCC a while ago, and recently, I did my first DCC/Sound installation on it. I used a Digitrax SDXH167D decoder in it. Heres my reddit Post. This describes more of my issue. In the comments, you can see somebody recomended Back EMF.
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back electromotive force is the voltage generated by a motor proportional to it’s speed. It is measured by the decoder by temporarily (msec) not applying a voltage to the motor.
it can be used to regulate the motor speed by adjusting the PWM value to obtain a desired BEMF value. May be most noticiable at low speeds.
speed tables can be populated with a PWM value (motor voltage) for each throttle value so that the motor runs at the speed corresponding to the throttle setting
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Back EMF is a force generated in a rotating machine, which limits its speed. This voltage can be monitored and used to control the speed when the load changes.
Speed tables are used to customize the slow, fast and middle speeds of the locomotive. It can limit the maximum speed, set the minimum, etc. The decoder can use these settings for finer control of the motor as per your wishes.
Hi there. I read your question on Reddit. Here are my two-cents worth:
-I agree that BEMF can cause erratic performance, but mostly for steam engines. Looking at your setup, it should not be an issue. The Digitrax default value should be OK.
-Speed table: This allows you to adjust the speed of your motor. Once you will get your mechanical/electrical bugs fixed, you may want to adjust your motor speed using the 28 speed steps option. This is best done using a computer interface (JMRI). Otherwise, you can set your CVs according to the Digitrax instruction sheet for switching speed (not using the 28 speed step table).
Simon
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Ohh kay, so I could probably not change anything BEMF related? Thanks for all your guys help. Also, I feel like I read somewhere you might want to tweak your speed tables if you run your locomotive at slower speeds (my case with my yard switcher), is this true?
Ron
Looks like your desired speeds are programmable by using the computer interface. Likely you will have to play with the values, while physically observing the train speed until you get it the way you like it. Experiment, observe, correct…play!
Ok, thank you!
Ron
Keep in mind there are 28 speed steps to adjust. Unless you need to super fine tune the engine to run with another engine, you can just adjust CV5 (top speed). Lowering this CV lowers the engine’s top speed. It lowers all the other speed steps down automagically. Adding an amount to 3 and 4 (start and stop momentum) can make the engine ramp up to a start and coast to a stop smoothly. For a switcher, maybe try CV3-4 at 15, and CV5 at 125 and see how it works.
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Ok, ill try that, thanks so much. Thanks for helping, never played around with CVs before.
Ron
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Looking at the video, it looks like dirty wheels/track, or a loose wire on the motor.
Simon
Just looked at the video, I think Simon may be correct. I would try cleaning the track and the wheels. Stopping and starting aren’t going to be fixed by a CV.
Maybe try running the engine on DC to see if it works with that. If it runs OK on DC power, it’s a decoder issue, not a wiring or dirty track issue.
p.s. How much power is your DCC system sending to the track? I have one decoder I bought that runs fine on 12V DCC, but anything over that and it overheats and shuts down, runs a little ways then stops.
Another word for Electro-Motive Force (EMF) is voltage. The EMF is generated by moving conductors through a magnetic field - moving a magnetic with respect to conductors also works. Voltages in motors generally increase with increasing speed, hence a limit on voltage will create a limit on speed.
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Reading through all of this, I humbly suggest that everyone bounced around the practicality of this subject, but never really hit the target.
Put extremely simply, every potentially magnetic object that moves through another magnetic flux field - and is wired - will produce electric current in those wires as it moves through the magnetic field. The exact reverse is also true: if electricity is applied in the proper configuration, it will cause movement through that magnetic flux field. Motors and relays all work with this principle.
This means that electricity causes the armature inside a motor to turn. However, if that armature is turned mechanically, it will actually create electricity backwards through its inputs. IOW, it becomes a generator instead of a motor.
This action is referred to as Back Electro-Motive Force, or Back EMF. It’s used to advantage everywhere electricity is used, mainly to build and utilize generators.
However, it can also be used to control motor speed over varying loads. If a model train is going downhill, the motor is very lightly loaded and gravity can force the train downhill with great speed. When it does this, the motor is being “pushed” by gravity and will generate more Back EMF - electricity going back the other way.
As others have mentioned, DCC decoders can measure this Back EMF and use it to limit the current to the motor, thus slowing it down while gravity is trying to make it speed up. In normal operation, this includes very minute motor speed and Back EMF differences, as opposed to the worst-case example of going downhill on a grade. At very low speeds, the Back EMF is compensating for the loads created by the simple friction (or lack of friction) of the mechanical components in the model loco’s drive train. It does this in thousands of a second, adjusting current up and down to compensate for friction/no-friction as movement continues.
In real trains, this is similar to Dynamic Braking, but on a much larger scale and to save fuel usage. A giant “load bank” of resistance is placed in the locomotive. When going downhill the motors are lightly loaded and attempting to produce current going backwards. The Dynamic Braking’s resistance load bank is switched into the motor circuit, creating an artificial load on the motors when going downhill to use the backwards current the motors are producing and to lower their speed. The greater the resistance load, the more it will slow the motors down, even though gravity is trying to force the motors to go faster. This resistance load is rejected in the form of heat, which is why you see Dynamic Braking blisters on top of locomotives with giant fans similar to the radiator fans. It’s also why you often see NO Dynamic Brakes on locos from a railroad that is mostly flat throughout its range (Illinois Central comes to mind). Dynamic Brakes are generally a waste of money with those railroads.
Currents are much smaller in model train locomotives, so no fan-cooled load banks are required. Nevertheless, it factors into the calculations for motor current relative to decoder current limits.