Coreless Motors - Back EMF - Feedback Controllers - DCC

[DLT]

9 hours ago, AndyID said:

Typically that's done by cutting power to the motor periodically and sampling the voltage produced while the motor acts as a generator (the EMF). Depending on the period (the time between samples) the motor may or may not tend to overheat.

 

As I understand it, that is how the original ECM Compspeed worked.  As time went on, various different versions appeared, some with switched levels of feedback for precisely the reason under discussion!

The original purpose of the Compspeed was to improve the performance of the "conventional"  motors of the time, ie open-frame type.

[AndyID]

5 hours ago, DLT said:

 

As I understand it, that is how the original ECM Compspeed worked.  As time went on, various different versions appeared, some with switched levels of feedback for precisely the reason under discussion!

The original purpose of the Compspeed was to improve the performance of the "conventional"  motors of the time, ie open-frame type.

 

If anyone has an oscilloscope it would be interesting to see a pic of the waveform.

[Andy Reichert]

26 minutes ago, AndyID said:

 

If anyone has an oscilloscope it would be interesting to see a pic of the waveform.

 

Make that a co-located oscilloscope

 

Andy

[melmerby]

I have an oscilloscope but not an ECM controller.

I do have a LF BEMF PWM controller built from a Wireless World circuit about 50 years ago!

It is very low frequency, you can hear the motor pulsing, especially at low speeds, however it works very well with an XO4 motor. I wouldn't like to subject a coreless motor to it.

[stewartingram]

28 minutes ago, melmerby said:

I have an oscilloscope but not an ECM controller.

I do have a LF BEMF PWM controller built from a Wireless World circuit about 50 years ago!

It is very low frequency, you can hear the motor pulsing, especially at low speeds, however it works very well with an XO4 motor. I wouldn't like to subject a coreless motor to it.

Sounds like the one I use. I must have built about 15 over the years. I have a (very) large collection of locos, but not much layout to operate yet, so they get little more than a run the length of the garage, back & forth. I was wary about coreless, and almost didn't but the Model Rail J70 because of that. However, it has had its turn on the "shuttle, and seems quite happy. I may have other coreless, I'm not sure without checking. Reading the article in WW only yesterday, I get the impression that the controller should be happy with coreless, but I'm not sure yet.

Only loco I've experienced failing was a Mintrains 009 Baldwin, belonging to a mate of mine. It had been in constant use during a local exhibition and the chimney melted, and drooped forward! We put that down to the heat from the bulb on the front though.

 

Stewart

[AndyID]

47 minutes ago, melmerby said:

I have an oscilloscope but not an ECM controller.

 

Likewise. I suppose a meter measuring AC in series with a capacitor to block the DC might provide a clue.

[melmerby]

2 hours ago, stewartingram said:

Sounds like the one I use. I must have built about 15 over the years. I have a (very) large collection of locos, but not much layout to operate yet, so they get little more than a run the length of the garage, back & forth. I was wary about coreless, and almost didn't but the Model Rail J70 because of that. However, it has had its turn on the "shuttle, and seems quite happy. I may have other coreless, I'm not sure without checking. Reading the article in WW only yesterday, I get the impression that the controller should be happy with coreless, but I'm not sure yet.

Only loco I've experienced failing was a Mintrains 009 Baldwin, belonging to a mate of mine. It had been in constant use during a local exhibition and the chimney melted, and drooped forward! We put that down to the heat from the bulb on the front though.

 

Stewart

Hi Stewart

Which issue is it?

I think the article was something like:

"Towards better control of small DC motors" or similar.

I got rid of mine years ago but I've found access to PDF copies online.

 

EDIT Found it.

Page 333 July 72 edition, I built 4 originally on Veroboard using lockfit transistors, then built a couple more on etched PCBs.

Good issue, I also built the Phase Locked Loop stereo decoder on P 315!

Edited April 20, 2020 by melmerby

[Michael Edge]

18 hours ago, melmerby said:

Which is unlikely to be connected to coreless motors as on their own they have little inertia and stop dead (almost) when the power is off)

A decent gearbox without a worm drive married to any motor with a flywheel will do that.

It is the coreless motor causing this, no matter how efficient the gearbox is. The loco isn't just running on a bit, it will creep on as far and as long as it can go. It's only a minor nuisance, the section switch is normally turned off when the loco has stopped anyway - otherwise turn the controller direction switch off.

[stewartingram]

11 hours ago, melmerby said:

Hi Stewart

Which issue is it?

I think the article was something like:

"Towards better control of small DC motors" or similar.

I got rid of mine years ago but I've found access to PDF copies online.

 

EDIT Found it.

Page 333 July 72 edition, I built 4 originally on Veroboard using lockfit transistors, then built a couple more on etched PCBs.

Good issue, I also built the Phase Locked Loop stereo decoder on P 315!

Available on line here:

 

https://www.americanradiohistory.com/UK/Wireless-World/70s/Wireless-World-1972-07.pdf

[melmerby]

16 hours ago, melmerby said:

Hi Stewart

Which issue is it?

I think the article was something like:

"Towards better control of small DC motors" or similar.

I got rid of mine years ago but I've found access to PDF copies online.

 

EDIT Found it.

Page 333 July 72 edition, I built 4 originally on Veroboard using lockfit transistors, then built a couple more on etched PCBs.

Good issue, I also built the Phase Locked Loop stereo decoder on P 315!

 

5 hours ago, stewartingram said:

Available on line here:

 

https://www.americanradiohistory.com/UK/Wireless-World/70s/Wireless-World-1972-07.pdf

isn't that what i had said?

[maico]

On 20/04/2020 at 19:25, AndyID said:

 

If anyone has an oscilloscope it would be interesting to see a pic of the waveform.

 

Ask Mike, he has an oscilloscope and has played around with various controllers

 

 

[Chuffer Davies]

12 hours ago, maico said:

 

Ask Mike, he has an oscilloscope and has played around with various controllers

 

 

I found this very interesting in that I hadn’t appreciated that an unsmoothed source could actually result in better low speed control of a motor than a smoothed DC source, even when using coreless motors.  Fortunately I already use Gaugemaster controllers as standard and so we have had the benefit without realising it.

Unfortunately it doesn’t get us any nearer to answering questions about feedback controllers though.  

Frank

[melmerby]

On 22/04/2020 at 12:11, Chuffer Davies said:

I found this very interesting in that I hadn’t appreciated that an unsmoothed source could actually result in better low speed control of a motor than a smoothed DC source, even when using coreless motors.  Fortunately I already use Gaugemaster controllers as standard and so we have had the benefit without realising it.

Unfortunately it doesn’t get us any nearer to answering questions about feedback controllers though.  

Frank

 

A little bit of AC injected into the DC output helps to keep the motor running at low speeds, some controllers had an adjustment so you could vary it

[maico]

What sort of output is the Modelx? Their website gives zero info...

[NIK]

On 24/04/2020 at 10:29, melmerby said:

 

A little bit of AC injected into the DC output helps to keep the motor running at low speeds, some controllers had an adjustment so you could vary it

Hi,

 

Do you mean AC (where the current goes positive and then negative) or DC pulses?.

Just out of interest which controllers had an adjustment for AC injection?.

 

Take care.

 

Nick

[DCB]

On 21/04/2020 at 23:52, maico said:

 

Ask Mike, he has an oscilloscope and has played around with various controllers

 

 

Who actually did the tests.  Did he compare the two for powering dis similar locos when double heading?  I use Morleys etc mainly because they do allow double heading and banking with locos from different makers and different eras,  Coreless Oxford and Hattons with Bachmann and iron age Hornby Dublo and Triang most combinations work, whereas on a resistance control Bachmann slip furiously while the Hornby Dublo barely even bothers to emit a buzzing noise.

[d00m]

It's a really long time ago, and slightly off topic but....  'AC injection'.... What would be the effect on a coreless motor if you were to use a standard DC controller with a Relco track cleaner? 

[melmerby]

8 hours ago, NIK said:

Hi,

 

Do you mean AC (where the current goes positive and then negative) or DC pulses?.

 

 

Take care.

 

Nick

A small amount of AC is added to the clean DC so you end up with DC with a ripple.

Other variations were such as using unsmoothed DC from a bridge rectifier and a resistance controller.

This was normal before the days of electronic controllers.

 

E.g. my first mains controller (1950s) was an H&M (?) transformer to reduce the mains to 15v, then an H&M selenium bridge rectifier to produce DC then a Tri-ang RT42 controller. It worked well with the Triang locos of the day.

(I had started with the Triang P40 battery controller with the RT42 but the batteries didn't last long and it were expensive to buy.)

 

 

[t-b-g]

I do recall, many years ago, when Portescap motors first appeared, fitting one into a loco that ran on several different layouts at exhibitions. There was one layout which had a controller that may have been an ECM Rambler (with the push buttons instead of a knob) but it was a long time ago so I cannot be 100% sure, where it ran like a pig. It stuttered and lurched forward and would not run smoothly. A friend who is an electrical engineer, explained that the controller was trying to "read" the back EMF but because the motor didn't  give the same sort of back EMF as a conventional motor, it was struggling to work out what to do. He reckoned that the low current consumption of the motor may have been the problem as the controller was one that adjusted its output to compensate for load, gradients etc. to maintain a constant speed. He said that fitting a resistor in series with the motor would cure it but in the end I think we just swapped the controller for another one.

 

I haven't  experienced any problems since. So these problems with coreless motors on feedback controllers are not a total myth but I think a problem with one type of controller with one type of motor has maybe become a bit of an urban legend.

[Crosland]

3 minutes ago, t-b-g said:

I haven't  experienced any problems since. So these problems with coreless motors on feedback controllers are not a total myth but I think a problem with one type of controller with one type of motor has maybe become a bit of an urban legend.

 

No, certainly not a myth but, as ever, the devil is in the detail.

[t-b-g]

The problem is that if one person has a problem with one motor and one controller and puts two and two together and makes 5, it can spread like wildfire. "My mate says you can't  use feedback controllers with coreless motors". Soon everybody knows!

Edited April 28, 2020 by t-b-g
Spelling

[Chuffer Davies]

1 hour ago, d00m said:

It's a really long time ago, and slightly off topic but....  'AC injection'.... What would be the effect on a coreless motor if you were to use a standard DC controller with a Relco track cleaner? 

Here again, if my memory serves me correctly, when they were first made available customers were advised against using Relco with Portescap motors.  
Regards,

Frank

[maico]

8 hours ago, DavidCBroad said:

Who actually did the tests. 

 

Click on the video!

 

Mike then made a second video showing how to modify the Morley with smaller capacitors to improve crawl performance

 

 

Edited April 28, 2020 by maico

[RBAGE]

Having spoken to a lot of people with direct hands-on experience, and realised that there are a lot of unproven myths flying around, would anyone disagree with the following? Coreless motors are compatible with pretty well all analogue control equipment made in the last ten years – Gaugemaster, Modelex, Morley. They're not happy with older, non-switchable feedback controllers such as KPC, AMR and the Gaugemaster HH (black front panel) but seem absolutely fine with ECM panel-mounted models (if not the push-button ECM Rambler). Pentrollers and Pictrollers are fine also but using coreless motors with ancient rheostat controllers by the likes of Hammant & Morgan (H&M) may not be a wise choice if fine control is your goal. There are no problems with using coreless motors with up-to-date DCC equipment but issues a'plenty (as there always are and always will be) with bargain-price decoders. You may be able to adjust the CVs but it's probably easier to stick with Zimo, Lenz, Digitrax and other reputable makes.

[NIK]

2 hours ago, melmerby said:

A small amount of AC is added to the clean DC so you end up with DC with a ripple.

Other variations were such as using unsmoothed DC from a bridge rectifier and a resistance controller.

This was normal before the days of electronic controllers.

 

E.g. my first mains controller (1950s) was an H&M (?) transformer to reduce the mains to 15v, then an H&M selenium bridge rectifier to produce DC then a Tri-ang RT42 controller. It worked well with the Triang locos of the day.

(I had started with the Triang P40 battery controller with the RT42 but the batteries didn't last long and it were expensive to buy.)

 

 

Hi,

 

I think we are at cross purposes.

That's not AC. AC is a current that flows in one direction and then back the other way and does this repeatedly.

 

I had a selenium rectifier that got overloaded. It took decades for me to forget the smell.

 

Take care.

 

Nick