Yes it's important to understand all DC electric motors produce BEMF. You can measure the BEMF to determine what rate the motor is spinning at. Power plants use BEMF to keep all their big turbines rotating at the same RPMs.
Some model railroad decoders are designed to use the BEMF to regulate the engine's speed, much like a car's 'cruise control'. Some decoders don't have that feature. But BEMF is still there.
While it seems to be 'common knowledge' that consisting locos with BEM<F will make them fight, I have a pair of Proto 2000 Geeps that I run together all the time - so much so that I just gave them the same address, and reprogrammed the lights on each one so that only the lights on the forward and of the pair come on regardless of direction. They both have TCS decoders with BEMF. BEMF is NOT turned off. They do not buck and fight as most seem to think happens.
I also ran it with a QSI equipped Trainmaster stuffed in between the two Geeps, which was how they really ran according to an old railroad employee - crews would even make the extra moveds to get a Geep as the end loco if the consist wasn;t se tup that way, becuase they rode nicer than the Trainmasters. Still no issues witht he BEMF fighting each other, and all three have it enabled.
I have an AB set of FTs, the B is powered and has a Tsunami sound decoder. The A is powered and has another of the TCS decoders with BEMF - no fighting
Anyone using the Digitrax BEMF decoders in a system that defaults to using Advanced consisting with CV19, like NCE - unless you changed the BEMF CVs, when in consists, BEMF is OFF by default. Since Digitrax systems default to command station consisting, on there you WOULD have BEMF in consist.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
CSX Robert richhotrainBut, is torque compensation essentially the equivalent of BEMF? No. Torque compensation helps, but not as much as BEMF compensation. For example, in my the increase in torque will makes low speed stalls less likely but not prevent them, whereas BEMF compensation can prevent them(note that I am talking about stalls due to an increase in friction such as a bind in the drive train or something in the trackwork - NOT stalls due to a loss of electrical contact,which neither one will prevent). Torque compensation is a constant setting, but BEMF is a real time adjustment with feedback, so when an engine with BEMF compensation enabled encounters something that can cause it to stall it will actually increase the decoder's output yo prevent the stall.
richhotrainBut, is torque compensation essentially the equivalent of BEMF?
No. Torque compensation helps, but not as much as BEMF compensation. For example, in my the increase in torque will makes low speed stalls less likely but not prevent them, whereas BEMF compensation can prevent them(note that I am talking about stalls due to an increase in friction such as a bind in the drive train or something in the trackwork - NOT stalls due to a loss of electrical contact,which neither one will prevent). Torque compensation is a constant setting, but BEMF is a real time adjustment with feedback, so when an engine with BEMF compensation enabled encounters something that can cause it to stall it will actually increase the decoder's output yo prevent the stall.
I agree 100% with Robert. Sometimes torque compensation is also referred to as "kick start"
So here's my explination of BEMF and how it works.
V = IR (Voltage = Current x Resistence)
As a motor encounters mechanical resistence, it's electrical resistance actually drops because the arms slow down as it passes through the magnetic field. With a constant voltage, this creates an increase in current. dI = V/dR. This is why you are told to STALL a motor and measure the current then, because this is the maximum current that will pass through the motor.
What the BEMF does is "sample or measure" the current for a fixed voltage pulse setting. If it notices a change in current, then it knows there's an increase or decrease in load on the motor. So there's a super secret formula used by manufacturers to compensate for this to try to get the motor back to it's original speed. It involves calibrated table lookups as well as feedback functions (called convolution kernals in math) to get the motor back to it's original speed.
Every engine is different, every motor is different, so you have to adjust these settings individually.
Benefits of back EMF: Allows your train to run at a more consistent slow speed without stalling.
Drawbacks of back EMF:
1) Two trains with Back EMF on in a consist will "fight" each other. One motor will be trying to speed up, while another slow down. This can be problematic. I believe the solution implemented by QSI was to cut down on the amount of correction when the locomotive was set into a consist mode (CV19 not equal to 0) However digitrax uses a universal consisting scheme which circumvents CV19. Everyone who has a digitrax system should really switch it to use Advanced Consisting (check your manual)
2) People complain it's unrealistic for trains at speed on the main to maintain a constant speed when climbing and descending hills. To counteract this, you need to set the BEMF cutout variable. This is the engine speed in which the BEMF is turned off or reduced to achieve a more realistic acceleration/decelleration of the train on grades. This can also be used to help trains cooporate together when consisting.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
CSX Robert NCE decoders do not have Bemf compensation, their "torque compensation" is NOT BEMF compensation. The high frequency pwm drive of silent running decoders causes the motor to produce less torque at low speeds. Torque compensation applies additional longer pulses of power at low speeds to compensate for this loss of torque.
NCE decoders do not have Bemf compensation, their "torque compensation" is NOT BEMF compensation. The high frequency pwm drive of silent running decoders causes the motor to produce less torque at low speeds. Torque compensation applies additional longer pulses of power at low speeds to compensate for this loss of torque.
But, is torque compensation essentially the equivalent of BEMF?
I ask that because the majority of my decoders are NCE.
Rich
Alton Junction
As has been stated, BEMF is produced by the spinning motor, so it is not really correct to talk about a decoder having or not having BEMF, although that is the common usage. What we are really talking about is BEMF compensation.
I haven't seen much talk about why you would want a decoder with BEMF compensation. It is often likened to cruise control, and it does have that effect, but that is not why I like decoders that have it. I'll give three specific examples where BEMF compensation is useful.
1. The side rods on steam locomotives will often cause varying amounts of friction throughout one rotation of the wheels, causing uneven slow speed motion. BEMF compensation can greatly smooth this out.
2. When running at very slow speeds, something as simple as a bump at a rail joint can cause a locomotive to stop. BEMF compensation will power over the bump.
3. One loco I have in particular would pulse when running on DC(it sped up and slowed down at regular intervals - I'm sure it is something in the drive train that could be fixed, but so far I have not found it). After putting in a decoder with BEMF compensation, it runs as smooth as anything else I have.
tstage...NCE calls their's "torque compensation"...
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
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
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.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Geared Steam 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.
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.
GS, you nailed it. My layout is perfectly flat, no grades whatsoever. Thanks.
"The true sign of intelligence is not knowledge but imagination."-Albert Einstein
http://gearedsteam.blogspot.com/
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?
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
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.
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)
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.
betamaxBack 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.
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.
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.
greg - Philadelphia & Reading / Reading
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
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
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.
Thanks, Tom, I have to pull out my decoder owner manuals and review them all.
Not sure which do and which don't.
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.
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.
richhotrain 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
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?
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
Back EMF is a counter force that appears in a motor as it spins. It limits the motor from going faster as well.
Modelling HO Scale with a focus on the West and Midwest USA
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.