I don’t know what BEMF (Back Electro Motive Force) is in DCC and what it does exactly. I’ve read my DCC system manufacturer’s manual and the explanation of BEMF is still extremely vague. Sort of like those “reverse draft angle” couplers, lol. I’m looking for examples of where BEMF is advantageous.
Basically a decoder with BEMF monitors the decoders usage of power and applies extra power when it is needed to maintain the same speed. Works really well for super low speeds.
Back EMF is a counter force that appears in a motor as it spins. It limits the motor from going faster as well.
In DCC, the decoder can sense that reverse voltage and use it as a form of “cruise control”, as well as changing sound effects.
So your locomotive will maintain a set speed regardless of the load change when going up or down a grade. It can also change the sound to indicate the engine is working more or less as a result.
I’m with the OP. I have no idea what BEMF is either.
But, from the first two replies, it sounds like BEMF is somewhat analogous to cruise control in an automobile.
Is that a fair analogy?
Rich
Rich
YES it is
When I first set up a new engine for on my layout I usually test to see if the BEMF is turned on by placing my finger on the front of the engine and then speeding it up.
The engine will begin pushing gains my finger as I am holding it back and I can then feel the engine begin pushing harder and harder against my finger until the engine wheels begin to slip.
If it passes this test then I feel the BEMF is set about right and usually leave it alone!
If not then I will see how it runs in a train and make any adjustments from there - MOST of the time I do nothing!
BOB H - Clarion, PA
Thanks, Bob.
Now my true ignorance of BEMF will show.
Do all decoders have a BEMF feature?
I have simply never paid attention to this feature.
Rich
Rich,
Some do, some don’t. Early on very few decoders had it. NCE calls their’s “torque compensation” while Lenz calls their’s “load compensation”.
Here’s a good read from Tony’s Train Exchange on the topic:
DCC Motor Control with Back-EMF and P-I-D
Granted, the article is 6-1/2 years old now but it should still be fairly applicable.
Tom
Thanks, Tom, I have to pull out my decoder owner manuals and review them all.
Not sure which do and which don’t.
Rich
I know that BEMF is buggy in many sound decoders. Many people recommend you shut off BEMF in a sound decoder as it will cause your loco to randomly “reset” even if your track has consistent power or no dirty spots.
I’ve never found that to be true in any of my Loksound, QSI, or Paragon2 decoders, Metro. I did have the reset issue with a Atlas HH660 switcher. However, it was attributed to dirty track. Once I cleaned the track the problem went away. In fact, I ran that very switcher around my layout for a 1/2 hour + last night with no incident whatsoever. Some decoders are more sensitive to dirty track than others.
Tom
a motor is a lot like a generator. If a mechanical input causes it to spin, it can generate voltage/power.
when the motor is turning, the BEMF opposes the current through it. For a given input voltage, the less BEMF the more current it draws. The greater the load, the slower it turns, the BEMF is lower and it draws more current. This helps maintain a more constant speed.
I don’t know how a decoder can measure BEMF while applying a voltage to a motor. I believe either the decoder attempts to measure the BEMF in between the PWM pulses, or it suspends PWM to make the measurement.
By measuring BEMF, a decoder can regulate speed by adjusting power to the motor (the duty cycle of the PWM pulse) to maintain a desired BEMF. One way to look at this is that the input to the controller isn’t the voltage/power to the motor, but the desired BEMF.
The only time BEMF can become a problem is when running two or more engines in a consist with them all having BEMF turned on. If one of the engines runs slightly (and I mean SLIGHTLY !) slower or faster than the other, the BEMF will be constantly fighting trying to even things out.
If the second engines senses things are starting to slow (because it pushes the lead engine for a split second) it will try to increase the speed to mantain it’s preset level. The lead engine feels this slight push and thinks it’s starting to go too fast, and the BEMF tries to slow it down. The multiple units fighting against each other can create some not so smooth operation. That’s why most guys turn BEMF off when using engines in consists.
Mark.
Going all the way back to the basic meaning of Back ElectroMotive Force…
If you spin a permanent-magnet motor, it acts as a generator, producing current. This happens no matter what is causing the motor to spin (fingers on the shaft, connection to an internal combustion engine, analog DC or DC processed through a DCC decoder) A DCC decoder with a BEMF detection circuit can sense this current, which shows up as resistance to the motor current and rises predictably with motor RPM. All else stems from this basic characteristic of the motors we use.
Chuck (Modeling Central Japan in September, 1964)
To fine tune Chuck’s response, BEMF uses voltage reference, not current.
For example - to get a motor to spin 3000 rpm, you need to apply 12 volts. If you manually spin that motor at 3000 rpm, it will produce 12 volts. The amount of current produced is based on the windings size - like a transformer. Both the motor in your train engine and the alternator in your car produce 12 volts, but the current ratings are MUCH different.
When you are running your engine along at 6 volts, there are small gaps (in milli-seconds) where power is not applied, allowing the decoder to read the voltage produced by the motor at that speed … which should be the same 6 volts that is being applied. As you approach a grade, the motor will slow down under the extra load, even though you are still applying the same 6 volts. With the motor turning slower, it’s now producing less voltage than the same 6 volts as the input. The circuitry automatically increases the speed of the motor so the output voltage again matches the input voltage - just like your car automatically applies more throttle to maintain speed up and down hills … different principal, but same idea.
Mark.
The best decoders have a high rate of sampling, meaning they check things out every tiny fraction of a second, and not every half-second or longer interval. If you had the very best BEMF software in a decoder, but it only sampled every half-second (begging the question), you would probably find your consisted locomotives bucking against each other and leaving you distinctly displeased with your conisting experience. Triple or quadruple the sampling rate and you get a decoder that does a much finer job of responding to imperections in pickup, gear mesh, drive-train variances, trailing rolling stock rolling resistance, and other problems that plague our DCC models.
In that respect, all modern decoders, from the now-older Tsunami to the modern LokSound 4.0 and QSI Titan, have user-adjustable ways to control locomotive motion. Those with the inclination will succeed, and those who won’t delve into the manuals will carp here and on other forums about how they don’t control their locomotives well. I got a distinctly hesitant and jerky BLI DC J Class 4-8-4 to run like it has a fluid drive with a Tsunami installed. I posted a video of it leaving the roundhouse here about two years ago.
Crandell
Assuming that one of my decoders has the BEMF feature, how can I activate it and how can I test it?
In other words, what can I do to see that it is working properly?
And what will I observe if it is working properly?
Rich
Rich
It your layout is relatively flat, you probably wouldn’t notice, if you have some grades, your train will maintain a constant speed on the flat, uphill and downhill. If you see it slowing down and speeding up, chances are it is turned off, needs to be adjusted, or it doesn’t have that feature. I have 5-6% grades so it pretty easy for me to tell. [swg]
GS, you nailed it. My layout is perfectly flat, no grades whatsoever. Thanks.
Rich
Actually BEMF is even more effective when crawling along at slow speeds. A motor turning at 1/4 or 1/2 volt input is more sensitive to even slight binds or pressures in the drive train and will easily fluctuate above or below the input voltage.
Try turning off the BEMF, you’ll find your slow speed control is no where near the same. A lot of decoders will either automatically, or by user setting, have the BEMF cut out somewhere between 1/4 and 1/3 throttle. Real trains don’t have cruise control - when they approach a hill or decend a hill, they have to adjust the throttle … most operators prefer to emulate this as well.
Mark.
LION knew nothing of BEMF so him looked it up (about five minutes on Google–you get what you pay for) and understands that all DC motors create a Back Electromotive Force. So the BEMF is always present in your locomotive, and usually it does nothing, because nobody asked it to do anything. Apparently some decoders can make use of these voltages to control other things, like the voltage to the motor to account for fluctuations of some sort.
Sounds like a good Idea. The LION uses no DCC and him uses a regulated 10.5 volts on his tracks. There is more amperage available than I will ever need: so there is a constant power on the tracks, and the trains speed up going down hill, and slow down going up hill. I guess that is realistic enough. I think I’ll leave things the way they are.
ROAR