Most motors will perform quite satisfactorily with a standard decoder configuration, the only configuration necessary will be to set the address. However, many engines will perform better with some tweaking and with some, it is essential. This is particularly the case with the older Hornby Dublo engines. Older engines often run at unrealistically high speeds as demanded by the “toy railway” expectations of the day and need taming.
The first CV to consider is the one to set the top speed or, more accurately, the voltage delivered to the motor at the highest speed setting. That is governed by CV5 and, if that is changed, then CV6 which governs the voltage delivered at mid-speed should also be defined. CV2 governs the start volts, the engine should just start moving when the throttle is set to step 1. Two other useful CVs are those governing momentum (acceleration and deceleration), CV3 and CV4.
Thus the basic configuration is Table 6, as follows:
| Start volts CV2 | Mid speed CV6 | Top speed CV5 | Accel rate CV3 | Brake rate CV4 | |
|---|---|---|---|---|---|
| Suggested start | 0 | 127 | 255 | 25 | 12 |
| To tame high speed | 0 | 100 | 200 | 25 | 12 |
| Shunter | 0 | 64 | 127 | 15 | 8 |
The values in this table correspond to fractions of the total voltage available. In other words, with a nominal maximum 12 volts available to the motor, a value of 1 for start volts means ¹/₂₅₆ × 12V = 0.047V. This is not to be confused with the number of speed steps which will be 128 for most recent decoders.
To be pedantic, it should be appreciated that the decoder is not reducing the voltage to the motor in a conventional sense. It works by sending pulses of full voltage with varying spaces between them so that the effective average voltage produced by these pulses changes. It is the same system as used by the more sophisticated DC pulse width modulation (PWM) controllers but in miniature.
CV2 is of most use for older decoders in older engines, where the initial stiction needs a higher starting voltage to get things moving. The value in CV2 is added to the speed step issued by the controller. I expected my Hornby Dublo engines to need help in this way but I had underestimated how effective a good back-emf system can be. The TCS decoders I have adopted have their own advanced system of back-emf and I found that a newly fitted engine would creep along beautifully on a throttle setting of just 1, straight out of the box.
The system works by measuring the back-emf produced by the motor between the power pulses and compares it against the speed setting called for. If the motor doesn’t start turning at a speed setting of 1, then it will generate zero back-emf. The decoder detects this and delivers a longer pulse of power the next time, then checks the back-emf again during the next off period and adjusts the next pulse length as necessary. It is quite amazing to think that this all takes place 19 thousand times or more every second!
More precise control can be achieved by defining another set of variables, grouped in Table 7. This is a set of CVs that define the momentum by setting values for the acceleration and deceleration over three defined sectors of the total speed range. Setting these CVs can give a useful improvement in response but the values are affected by the values set in Table 6.
Some suggested initial values for Table 7:
| Acceleration | Deceleration | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Base accel CV3 | Lower change CV125 | Mid accel CV126 | Upper change CV127 | Top accel CV128 | Top decel CV132 | Upper change CV131 | Mid decel CV130 | Lower change CV129 | Base decel CV4 | |
| Straight line | 25 | 85 (¹/₃) | 25 | 170 (²/₃) | 25 | 12 | 170 (²/₃) | 12 | 85 (¹/₃) | 12 |
| S-curve | 30 | 32 (¹/₈) | 20 | 192 (¾) | 30 | 18 | 192 (¾) | 12 | 25 (¹/₁₀) | 15 |
I have shown CV3 and CV4 from Table 6 to complete the picture, CV3 sets the acceleration from rest to the lower intermediate point and CV4 sets the deceleration from the lower intermediate point to a halt. It is not necessary to set the same intermediate points for deceleration as for acceleration. Bear in mind that if the value set in either CV127 or CV131 is higher than that set in CV5, then the top acceleration or deceleration rate will not be used.
However, setting up the values in Table 7 is all very well and good when you have a fixed-formation train such as my APT or Midland Pullman. A locomotive will have a totally different momentum when it is running light or shunting than it will when hauling a train. When the engine is attached to several hundred tons of a load, the total momentum will be significantly greater than when unattached. The PowerCab has a Momentum button which allows the values in CV3 and CV4 to be reset on the fly, which is very handy when shunting and picking up a train. A light engine might need values in CV3 and CV4 of, perhaps, 15 and 8 as suggested above for shunting, whilst needing much higher values when attached to a heavy train. This suggests that, for a locomotive, setting the values in Table 7 might be counter-productive if CV3 and CV4 are to be changed on the fly.
There is another table for even more precise control involving the definition of voltage at each of 28 speed steps. This overrides the settings of CV2, CV5, and CV6, but it seems to be a lot of trouble to go to for little additional benefit. I think it is worth experimenting thoroughly with Tables 6 and 7 before venturing into this area.