[SOLVED] Poor low speed running - Hornby Star - Lokpilot(?)

[DK123GWR]

I have purchased a Hornby Star class on ebay, which was DCC fitted. CV8 reads 151 and CV7 reads 89, I believe indicating that this is some sort of LokPilot. At very low speed steps it is a jerky runner. At around speed step 8 it can suddenly jump up to a very high speed. Often it is just brief wheelslip, but sometimes it races off and has to be brought to a stop. CV29 is set to 34, so analogue running should be disabled. I haven't yet had the chance to test on DC, but will report back when I can. Does anyone have an idea what might be causing this sort of behaviour?

 

EDIT: The jerkiness at low speeds appears to be the same issue, but with more frequent and shorter duration periods of 'running away' - several per rotation - on a loco which is (apart from that major issue) running correctly.

 

EDIT: I turned off BEMF by setting CV49 = 0. This has solved the running away issue, although the loco probably needs a few further tweaks it is now usable.

Edited November 13, 2023 by DK123GWR

[ISW]

On 13/11/2023 at 17:21, DK123GWR said:

EDIT: I turned off BEMF by setting CV49 = 0. This has solved the running away issue, although the loco probably needs a few further tweaks it is now usable.

Has this reduced the 'pulling' power of the loco?

 

I've found that with BEMF 'on', if you put your finger infront of the loco you can feel it increase the power. With BEMF 'off', a finger just stops the loco.

 

Ian

[DK123GWR]

2 hours ago, ISW said:

Has this reduced the 'pulling' power of the loco?

 

I've found that with BEMF 'on', if you put your finger infront of the loco you can feel it increase the power. With BEMF 'off', a finger just stops the loco.

 

Ian

I haven't performed that specific test, but yes, I think so. Previously the loco was moving off at speed step 1 (albeit eratically) whereas afterwards (and before I altered the start voltage) it was only doing so at about step 5. I think that's what you'd expect to happen. As I understand it, the BEMF is a proxy for the motor's speed, and the chip is meant to use the reading to determine whether the motor is running faster or slower than expected at a given speed step, then change the voltage to compensate. In this case, the motor was running slower than expected, so the voltage was increased to speed it up. However, for reasons that I don't understand (and, since it's working OK without BEMF, don't need to) it must have been severely overcompensating at times, leading to the sudden acceleration.

Edited November 15, 2023 by DK123GWR

[34theletterbetweenB&D]

If this is an early lokpilot (such as Bachmann badged and sold a decade past) this was a recognised problem to do with the BEMF set up, because the factory settings were optimised for a large motor with flywheel. As a result they worked very well on centre motor both bogies driven mechanisms, but were distinctly rough on smaller motored mechanisms with no flywheel, typical in steam models. Essentially the BEMF sampling was not frequent enough, and the feedback applied too coarse for a small motor.

 

This could be rectified in CV's 54 and 55. With BEMF enabled in CV 49, try CV 54 at 15, and CV 55 at 40, that should produce a readily seen change; after which apply small changes to optimise performance. The motor control is very good, but I found with smaller motors it was very 'tweaky' to set up, single digit changes in these two CV's would make a difference. HTH.