As DCC continues to advance and the motor control becomes more refined it becomes more realistic to consider speed matching locomotives that never could have been speed matched before. 40 years ago speed matching was unheard of, because all you could do was set your locomotive on the track and increase the voltage. Now, however, you can control the locomotive as much or as little as you want to. This has created its own problem, however, because while in the past there were not enough options for controlling a motor, now it may seem that there are so many options that one cannot even begin to understand where to start. The purpose of this guide is to start out at the basics so that anyone can start to speed match locomotives, and then get more complicated so that you can do as much as you want.
For the basic user there are two things to consider as you start your speed matching. One is momentum and the other is actual speeds. Momentum is the rate at which the locomotive speeds up and slows down and actual speed is how fast the locomotive is going once it has overcome momentum. We will start with the most basic settings and describe them, and then move to more advanced settings, but at each point we are trying to control the momentum and actual speeds at different points through the speed curve. There is another term that should be defined, which is speed curve. When a locomotive speeds up and slows down it is not a linear rate of speed, there are points where the locomotive is speeding up faster or slower than other points, and therefore the speed of the locomotive is better depicted as a curve than a line. It should be noted that this guide to speed matching can be used, not only for matching two model locomotives, but it can also be used for matching a locomotive to its prototypical running conditions.
For the basic user there are only 5 CV's to consider. (This may seem like a lot, but when we get to the advanced user having around 40 CV's to consider the basic settings will seem even more basic.) There are two CV's dealing with momentum and 3 dealing with the speed of the locomotive. First there will be the short explanation, and then there will be a more descriptive explanation if you need it.
Short explanation CV3 is acceleration (how fast the loco speeds up). The higher the value the longer it takes. CV4 is deceleration (how fast the loco slows down). The higher the value the longer it takes. CV2 is start volts. Set the voltage for Speed Step one. CV6 is mid volts. Set the voltage to the motor in the middle of the speed curve. CV5 is top volts. Set the voltage to the motor for the highest Speed Step. A value of 18 in CV2, 6, and 5 corresponds to approximately 1 volt.
Long explanation (grab onto your seat, because I'm long-winded.....) Let us first consider momentum. There is a CV to set the acceleration rate of the locomotive (CV3). If CV3 has a 0 in it, this means that there is no acceleration rate, which means that when you turn your DCC throttle up the locomotive will be at whatever speed you set it to as soon as you set it there. However, in real life, locomotives take a long time to get moving, so you want your model to also take a long time in getting up to speed. The larger the value that you set in CV3 the longer time it will take the locomotive to get up to whatever speed you set it at. Typically we suggest trying a value around 20 or 30 to start with, and then you just have to play with it to see what you like. Once you get into the range of 80-100 it will take the locomotive such a ridiculously long time to speed up that its not worth it. The highest value possible for the CV is 255, but if you put this value in you might as well go get a beer because by the time you get back the locomotive will still be accelerating. The other momentum CV is CV4 and it controls the rate at which the locomotive decelerates. It works the same as CV3.
Now let us consider the actual speeds of the locomotive in the speed curve. The most basic settings split the speed curve into three points: the starting point (CV2), the middle point (CV6), and the highest point (CV5). So, in the end, the basic settings make it look more like a speed angle than a speed curve, but it is still more realistic than just using the decoder as it comes in the package. The highest value that may be entered into any CV is 255. CV2, 6, and 5 take the speed range and divide it by that value of 255, meaning that a value of 1 in any of these CVs will be the slowest your motor can possibly run and a value of 255 will be the fastest your locomotive can possibly run. So in order to duplicate the original speed range while using these CVs you would enter a value of 1 into CV2, a value of 127 into CV6, and a value of 255 into CV5. If you then want to slow down the speed of the locomotive when it is in the lowest points of the speed range but maintain its speed at the top of the speed range you would lower the value in CV6 to 100 but leave CV5 at 255 and CV2 at 1. If you wanted to increase the starting speed of the locomotive you would increase the value in CV2 while leaving the other values. You can use any of these CVs that you want while leaving the others at 0 and the ones left at 0 will continue to operate at their defaults. So in the first example you could change CV6 to 100 while leaving CV5 at 0 and CV2 at 0. And in the second example you could change CV2 to 20 while leaving CV5 and 6 at 0. Now, this applies to speed matching because if you have one locomotive that is geared in such a way that it starts out at a high rate of speed and another one that is geared to start very slowly, then you would increase the value of CV2 in the locomotive that starts slowly. Also, if one locomotive is very fast at the highest speed setting but another locomotive runs at a medium speed when it is turned all the way up, the one that is running faster can be made to slow down by adjusting CV5. These CVs can also be used to achieve prototypical running. For example, if you have a switcher that runs very fast, this is unrealistic. Switchers only operate in the yards and should therefore run rather slowly, but sometimes locomotive manufacturers gear them in such a way that they are running at the same speed, if not faster, than all the other locomotives. If this is the case then you can simply lower the values in CV5 and 6 and this will decrease the whole speed curve.
Now it is time to consider the next most advanced set of features. Just keep in mind that these adjustments are achieving the same thing as CV2-6 just with more variables in order to further define the desired speed curve.
Short explanation Momentum (Table 7 in Advanced Programming Guide) Acceleration CV3 – First Acceleration rate CV125 – starting point for second acceleration rate CV126 – Second Acceleration rate CV127 – starting point for third acceleration rate CV128 – Third acceleration rate Deceleration CV132 – First Deceleration rate CV131 – ending point for first deceleration rate CV130 – Second Deceleration rate CV129 – ending point for second deceleration rate CV4 – Third Deceleration rate
Actual Speed (Table 9 in Advanced Programming Guide) A total of 28 CVs that define the voltage going to the motor at 28 points through the speed curve.
Long explanation Let us once again start with momentum. The problem with setting the acceleration and deceleration rates in CV3 and 4 is that most locomotives in real life do not have the same acceleration and deceleration at all speeds. In other words, it take a lot to get a locomotive moving, so the acceleration rate is very slow, but once its moving it is easier to speed up so the acceleration rate gets faster. The same idea pertains to deceleration. In order to achieve this you can divide the speed curve up into 3 different segments and give each their own acceleration/deceleration rates. Not only can you define the rates themselves, though, you can also define at what point through the curve that rate will become active. The points where the rates change work similarly to CV5 and 6 in that if you set CV125 to a value of 85 (255 divided by 3) then the first acceleration rate will be active for the first 1/3 of the speed curve, at which point the second acceleration rate will become active. Then, if you set CV127 to a value of 127 (255 divided by 2), then the second acceleration rate will be active from 1/3 of the speed range through 1/2 of the speed range. Also, because of this value of 127 in CV127 the third acceleration rate will be active for the second half of the speed range. For control of the actual speed, beyond what is provided in CV2, 6, and 5, there are a total of 28 CVs that can be used to define the speed curve in whatever configuration you desire. This table must be activated using CV29 (add a value of 16 to whatever value already exists in CV29). The CVs used are CV67-CV94, with CV67 defining the beginning speed of the speed curve, CV68-93 define all points between the starting and ending point, and CV94 defines the highest possible speed of the locomotive. The values in these CVs work the same as those in CV2, 6, and 5, so 127 is half speed and 255 is top speed. You can do the math to determine what values you want to use in your speed table, but it really comes down to just playing with it til you get it to do what you want. There is one last function of TCS decoders that must be considered when speed matching locomotives, especially if the other engine is not a TCS equipped locomotive. TCS decoders come equipped with BEMF which provides very good slow speed control. The BEMF is auto-adjusting, however, so that it also provides excellent control in the top end of the speed curve. The only problem is if you are using a non-TCS decoder that is not equipped which such excellent speed control because the locomotives may end up fighting each other. Because of this we have provided options for setting BEMF up so that it cuts out at certain points. To read more about this, go to our BEMF page.