Blown Back EMF?

[ISW]

Is it possible to 'blow' the BEMF function of a DCC chip? I ask as I have a Heljan Class 47 that I had fitted with a Hattons 4-function DCC chip. I did check that the BEMF was enabled. I noticed it wasn't pulling properly on my gradients so, as I had a spare Hattons 4-function chip, I replaced it.

 

The Class 47 worked very well after that, pulling 8-coaches up a 1in45 gradient without issue. I did notice (from the NCE PowerCab display) that the Class 47 was pulling ~0.45amps even on light engine, and ~0.75amps with the 8 coaches on the level. That's quite a bit more than my other locos. On the gradient this went up to ~0.95amps (it didn't exceed 1amp was far as I could tell). I left it running round the layout for ~30 minutes with each ~5 minute loop including ~1 minute each time on the uphill gradient.

 

Then, without warning or any obvious changes, it suddenly wouldn't pull up the gradient. The 'finger test' (holding the loco back at the front) showed the BEMF was not working again. So, has the BEMF function blown?

 

Ian

[Crosland]

High current will not "blow it" even if that's possible. BEMF is a voltage and is highest when the motor is spinning freely under no load.

 

Does the decoder have CVs to tune the BEMF? It should, otherwise all you get is a compromise "one size fits all". Have they been corrupted? 

 

[ISW]

58 minutes ago, Crosland said:

Does the decoder have CVs to tune the BEMF? It should, otherwise all you get is a compromise "one size fits all". Have they been corrupted? 

Andrew,

 

The Hattons 4-Function DCC does have the CV(s) to adjust the BEMF. What I do know is that the old chip (1) had the BEMF set, but it didn't seem to work. Then the new chip (2) had the BEMF set to work, and was working, until I used it on ~0.95amps for a while.

 

I use DecoderPro to check /amend the BEMF, so I don't know what CV I'm adjusting. What would be the CV settings for BEMF 'on' and 'off' so I can check them directly?

 

1 hour ago, Crosland said:

High current will not "blow it" even if that's possible. 

I'm pleased to hear that, but I thought the Hattons DCC chip was rated at 1amp, and I was getting pretty close to that with the Class 47 on the gradient.

 

Ian

[Nigelcliffe]

11 hours ago, ISW said:

Andrew,

 

The Hattons 4-Function DCC does have the CV(s) to adjust the BEMF. What I do know is that the old chip (1) had the BEMF set, but it didn't seem to work. Then the new chip (2) had the BEMF set to work, and was working, until I used it on ~0.95amps for a while.

 

I use DecoderPro to check /amend the BEMF, so I don't know what CV I'm adjusting. What would be the CV settings for BEMF 'on' and 'off' so I can check them directly?

 

I'm pleased to hear that, but I thought the Hattons DCC chip was rated at 1amp, and I was getting pretty close to that with the Class 47 on the gradient.

 

Ian

 

I don't know which CV's do what in a Hattons decoder.  Presumably they offer a manual ?     There is a preference in DecoderPro to turn on CV numbers in tool-tips when hovering over a field.   

 

I suspect you're at the margins, and overloaded things, and it did some sort of internal reset changing the values.   Heljan locos have a reputation of drawing a lot of current.   Lenz decoders are generally conservatively rated for power output, so may work better.  Or some of the Zimo MX630-639 range have much higher power ratings (1.8A continuous, 2.5A peak).   

[RAF96]

When you are adjusting the BEMF CV you are defining the cut off point. The default is usually value 128 i.e. always on.

Edit - from there one can venture into PID motor control.

Each decoder should provide guidance for setting these parameters in the decoder manual.

Edited March 20, 2021 by RAF96
Add PID

[ISW]

2 hours ago, Nigelcliffe said:

I suspect you're at the margins, and overloaded things, and it did some sort of internal reset changing the values.

That does make sense. I'll check to see if any of the 'settings' have changed.

 

2 hours ago, Nigelcliffe said:

Heljan locos have a reputation of drawing a lot of current.

I can now believe that. Most of my locos take ~0.3amp in normal use, but the Heljan takes ~0.7amp under the same duty.

 

2 hours ago, Nigelcliffe said:

Lenz decoders are generally conservatively rated for power output, so may work better.  Or some of the Zimo MX630-639 range have much higher power ratings (1.8A continuous, 2.5A peak).

Many thanks for that. I'll have to see what is suitable (8-pin) and reasonably priced as an alternative.

 

Ian

[ISW]

34 minutes ago, RAF96 said:

When you are adjusting the BEMF CV you are defining the cut off point. The default is usually value 128 i.e. always on.

Thanks, that's useful information for me to use while checking the settings.

 

34 minutes ago, RAF96 said:

Edit - from there one can venture into PID motor control.

Now you are moving into 'advanced' level that's a little beyond my current expertise ...

 

34 minutes ago, RAF96 said:

Each decoder should provide guidance for setting these parameters in the decoder manual.

I don't think the manual goes into such detail unfortunately.

 

Ian

[ISW]

I've had a browse of the manual (leaflet) and it says:

"Back EMF:
Back-EMF is self-adjusting and can be either on or off. Set CV61 to 1 (default) for on, 0 for off.
If you want Back-EMF to turn off after the locomotive is underway (for example, if consisted locos fight
each other, this will make consisting smoother) set CV10 to the speed step at which you want it to turn off.
(e.g. CV10 = 15 will make Back-EMF turn off at speed step 15."

 

It was set for 'enabled' in JMRI (CV61=1) and the back EMF cut-off set to 0, by which I presume means 'self-adjusting'. I retried the Class 47 and it behaved normally (ie, with BEMF) and pulls up the ramp no problem. I'm therefore reduced to 'assuming' that the chip has some auto overheating mechanism that reset themselves once they 'cool down'.

 

If (when?) it happens again, I'll try setting the BEMF cut-off to 128 (always on) to see if that over-rides the auto cut-out. However, that'll only be for 'investigation' purposes, as I'm sure if the chip does have an auto cut-out and I over-ride it for any length of time I'll probably fry the chip.

 

If this does occur on a regular basis, then I'll be looking at the Zimo MX630-639 range suggested above.

 

Ian