Keep DCCWiki free of advertisements, contribute today.

From DCCWiki, a community DCC encyclopedia. Please contribute today to keep it free.

Back EMF (BEMF), is like having cruise control for your loco's. This helps prevent trains from slowing down when going up a grade and from speeding up when going down grade. Sometimes model trains stop when trying to roll slowly across a turnout or other rough trackage. Back-EMF control provides a solution for this.

Back ElectroMotive Force is the voltage generated by the spinning motor. BEMF (Back-EMF) is proportional to motor speed and independent of motor load. BEMF may also be called 'Generator Coefficient,' 'Generator Constant,' 'Voltage Constant' or 'Counter ElectroMotive Force.' BEMF is specified in more than one way, although the end result is the same. One definition of BEMF is "volts per thousand RPM," or "Volts/KRPM." A second definition of BEMF is "Volts/(rad/sec)" where (rad/sec) is Radians per Second.

Using the above, the decoder can measure the electric motor's speed with something called Back-EMF, and apply power accordingly to keep the motor's speed constant. The result can be a train that will maintain the same speed regardless of the grade, number of cars being pulled, or condition of the track.

When an electric motor's armature turns, a certain amount of electrical kick-back is generated. This kick-back is called Electro Motive Force (EMF) or Back-EMF. The faster the armature turns, the more EMF is produced. This voltage can be measured to determine how fast the motor is going and can be used to control the speed of the motor - like cruise control. Some manufacturers simply call it Back-EMF control and some people call it Cruise Control.

One of the things this can be used for is cruise control. That is, trains go up and down grades while maintaining the same speed. Some people like this idea while others think it's a bad idea - it is NOT prototypical. Some people just want to see trains run and don't want to be bothered by continuously throttling up and down when going up and down grades. For layouts that are primarily used for display, this could be good - set the train's speed and let it run unattended. But for people who enjoy running their trains, prototypically or otherwise, this feature can take away from that enjoyment - for those, Back-EMF can be disabled while programing the decoder.

If you want to use Back-EMF for this purpose, there is something you need to be aware of. It is all but impossible to make two locos run at the exact same speed. If you couple two locos together, both having 100% back-EMF control, they will fight each other. The one that is fastest will want to pull the slower one, and the slower one will want to hold back on the faster one. The more they pull against each other, the harder they will try - until you hear wheels scrubbing. This is called the push-me-pull-you syndrome. Even if they are off by only 1/4 inch in 10 feet, it can be enough to get the syndrome started. And once started, it only intensifies.

To counter this effect for consists, many manufacturers have made various provisions to allow coupling Back-EMF controlled locos together. Some do not allow Back-EMF control at speeds above switching (more about that later), and others provide for turning Back-EMF off when locos are MUed together.

Another use for Back-EMF control is to have smooth running over turnouts and other rough trackage at slow speeds. This is especially important for switchers running at very slow speeds over points and frogs. Some manufacturers specialize their Back-EMF only for this purpose - cutting out when the loco reaches a certain speed. Digitrax decoders can be set up to run at 100% Back-EMF for switching purposes, but does not automatically cut out at higher speeds.

[edit] Digitrax Variation

Digitrax provides several options for this. First, it can be turned on and off at will. Second, a certain amount of tolerance can be programmed in - in two different ways (making it scalable); how much Back-EMF to use and how fast to use it. And lastly, they provide for two different amounts of Back-EMF to be used based upon whether the loco is consisted to another loco or not. This last feature allows you to program full Back-EMF control when not consisted, and have some tolerance for the other loco when consisted via Decoder-Assisted Consisting.

Like all things with versatility, Digitrax's Back-EMF is a little more complicated to program than some others. Think about it. If all you can do is turn it on and off, that's all you need to learn how to do. Actually, you can operate Digitrax's Back-EMF decoders this way if you choose. But, if you want to run two locos together under Back-EMF control, you will need to learn how to program that feature into the decoders as well.

To do all of this, Digitrax uses three CVs (55, 56, & 57). To explain how each of these work, you have to understand that the "target" speed is the speed with which the loco is supposed to be going according to the throttle setting you have given it. CV#55 is used to tell the decoder how fast to compensate for speed differences. CV#56 uses historical information to act as a shock absorber - to keep from over reacting. CV#57 tells how much tolerance from the target speed is allowed. CV#57 is divided in half. The first hex digit is for how much Back-EMF when not consisted, the second digit is for how much when consisted via Decoder-Assisted Consisting.