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johnarcher

Old controller with coreless motors.

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Posted (edited)

I believe that coreless motors don't like feedback control?

 

I have, and still find good, an old (30 years?) ECM Compspeed. It is the version with switchable feedback, can be feedback off or on 1 or 2. 

With feedback on it is still very good with 'normal' motors, would it be OK with coreless in the feedback off position?

 

(I apologise for the depths of electronics ignorance that lie behind this question).

Edited by johnarcher

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Posted (edited)

Coreless motors provide less back EMF (feedback) than Iron cored motors due to much less inductance, so if the controller has the option (as you state) I would suggest turning it off especially as it wont have the desired effect. (unless it is a high frequency PWM)

Also coreless motors will overheat more readily than iron cored motors if overloaded so tend to be more fragile but if kept within their operating spec should be fine.

 

EDIT I hope you can understand the little bit of tech speak creeping into the answer.

Edited by melmerby
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19 minutes ago, melmerby said:

Coreless motors provide less back EMF (feedback) than Iron cored motors due to much less inductance, so if the controller has the option (as you state) I would suggest turning it off especially as it wont have the desired effect. (unless it is a high frequency PWM)

Also coreless motors will overheat more readily than iron cored motors if overloaded so tend to be more fragile but if kept within their operating spec should be fine.

 

EDIT I hope you can understand the little bit of tech speak creeping into the answer.

Thank you, back EMF is OK, inductance got past me a bit.

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Posted (edited)
19 minutes ago, johnarcher said:

Thank you, back EMF is OK, inductance got past me a bit.

Simply, Inductance is the result of winding wire into a coil, if you include an iron core it increases it considerably, more inductance means more BEMF, as a result the iron core in a motor is the major contributor of Back EMF.  As core less motors do not have an iron core BEMF is much lower. This lack of iron core which in an iron cored motor acts as a heat sink is the reason coreless motors can overheat (and fail) more quickly under overload conditions.

Edited by melmerby
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1 minute ago, melmerby said:

Simply, Inductance is the result of winding wire into a coil, if you include an iron core it increases it considerably, more inductance means more BEMF, as a result the iron core in a motor is the major contributor of Back EMF.  As core less motors do not have an iron

core BEMF is much lower. This lack of iron core which in an iron cored motor acts as a heat sink is the reason coreless motors can overheat (and fail) more quickly under overload conditions.

Thank you, lucidly explained.

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By my standards it is the middle aged feedback controllers like the ECM  Compspeed which do the damage.   The oldies tend to be variable transformers or variable resistors which are pretty much core less friendly.   If you want to mix and match the motors powering your locos why not get a Morley?  Our Vector runs coreless and 1950s Hornby Dublo powered locos double headed no problems, except you need a delicate touch on the 160 degree off to max travel on the control knob.

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1 hour ago, DavidCBroad said:

By my standards it is the middle aged feedback controllers like the ECM  Compspeed which do the damage.   The oldies tend to be variable transformers or variable resistors which are pretty much core less friendly.   If you want to mix and match the motors powering your locos why not get a Morley?  Our Vector runs coreless and 1950s Hornby Dublo powered locos double headed no problems, except you need a delicate touch on the 160 degree off to max travel on the control knob.

Thanks for the suggestion, if I need to replace the controller that is one option I have looked at.

I would not have considered using any other Compspeed with a coreless motor, but in this case I wondered, as the feedback can be turned off.

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If I remember correctly, the switchable feedback version of the Compspeed was introduced precisely for the purpose of controlling coreless motors, with the rise of the Portescap RG4.

Cheers, Dave.

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Posted (edited)
33 minutes ago, DLT said:

If I remember correctly, the switchable feedback version of the Compspeed was introduced precisely for the purpose of controlling coreless motors, with the rise of the Portescap RG4.

Cheers, Dave.

Thank you, I hope you remember correctly.

I'm sure your memory is better than mine, I can't remember at all how long ago I got the Compspeed, or whther RG4's were around at that point.

Incidentally, in case it jogs anyone else's memory, this is the beast in question (Compspeed F IIRC)

 

compspeed.JPG

Edited by johnarcher

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Posted (edited)
7 hours ago, johnarcher said:

Thank you, I hope you remember correctly.

I'm sure your memory is better than mine, I can't remember at all how long ago I got the Compspeed, or whther RG4's were around at that point.

Incidentally, in case it jogs anyone else's memory, this is the beast in question (Compspeed F IIRC)

 

 

Without knowing what the output from your Compspeed actually looks like (anyone?) it's impossible to say if it's OK for coreless motors or not. You could try a simple test. Try it and if there is any indication of overheating stop immediately.

 

There are (at least) two reasons why coreless motors are a bit different from iron core motors. They don't have a lot of momentum (like a flywheel) and they don't have a lot of stored magnetic energy (like a solenoid). As soon as the power to them is interrupted they decelerate quite quickly. When the power is restored they immediately try to accelerate to match their generated EMF to the applied voltage (minus the voltage dropped through their internal resistance).

 

If the power is being pulsed it's this continual fluctuation of the EMF that produces the undesirable heat. When power is restored the difference between the actual speed (represented by the EMF) and the demanded speed required by the applied voltage produces a substantial current pulse and that current pulse generates a lot of waste heat in the unavoidable internal resistance in the motor's windings.

 

 

Edited by AndyID
Typhoo

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3 hours ago, AndyID said:

 

Without knowing what the output from your Compspeed actually looks like (anyone?) it's impossible to say if it's OK for coreless motors or not. You could try a simple test. Try it and if there is any indication of overheating stop immediately.

 

A problem with that is that by the time any heat generated in the winding, which doesn't have any sort of former, has reached the rest of the motor e.g. case, it may have done irreparable damage.

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9 hours ago, maico said:

The PICtroller is a feedback design that is claimed to detect coreless. I haven't read any reviews on it though.

 

http://www.malcolmsminiatures.co.uk/PICHH+-+hand+held+PICtroller

 

I've tried one.  I wasn't impressed; the "auto detection" circuitry causes a loco to shudder as it is turned on (every time you change direction!), and the low speed control wasn't particularly good on the coreless models in question (various tiny motors and up to some models with Portescap RG4's).   Conclusion, nothing like a real Pentroller - I have one, and access to several more.    The problem was solved by converting the locos to DCC with quality decoders.   

 

I've had better devices for DC, but they seem to come and go as commercial supply. 

 

 

- Nigel

 

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Thanks for all replies.

I suppose I have no idea what the output is like with the feedback off - is it pure DC then?

Of course if DLT is right (and obviously he usually is) and that setting was added for Portescaps it should be OK.

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It's quite possible the Compspeed outputs DC even when the feedback is on. I suspect it does but I cannot confirm it. The name and the two feedback settings suggest that it might be using  positive feedback - sometimes referred to as feedforward.

 

DC motors slow down under load because their windings and some types of brushes have quite a bit of resistance. As the load increases the current increases and more voltage is dropped across the motor's internal resistance causing the motor to run slower.

 

Feedforward senses the current and increases the applied voltage to compensate for the voltage lost by the internal resistance. That can make the motor run at close to constant speed regardless of the load. In effect the feedforward is cancelling out some of the motor's internal resistance.

 

To make this work the controller has to have some idea about the motor's internal resistance and that might explain the two feedback settings (or maybe not!)

 

BTW If you have an analogue meter it might have an input jack marked "output". That puts a capacitor in the circuit which blocks DC and allows you to measure the AC component of a signal. If the output is pure DC you should not be able to detect any AC voltage. You might be able to do the same thing using an external blocking capacitor with any meter set to measure AC volts.

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7 hours ago, AndyID said:

It's quite possible the Compspeed outputs DC even when the feedback is on. I suspect it does but I cannot confirm it. The name and the two feedback settings suggest that it might be using  positive feedback - sometimes referred to as feedforward.

 

DC motors slow down under load because their windings and some types of brushes have quite a bit of resistance. As the load increases the current increases and more voltage is dropped across the motor's internal resistance causing the motor to run slower.

 

Feedforward senses the current and increases the applied voltage to compensate for the voltage lost by the internal resistance. That can make the motor run at close to constant speed regardless of the load. In effect the feedforward is cancelling out some of the motor's internal resistance.

 

To make this work the controller has to have some idea about the motor's internal resistance and that might explain the two feedback settings (or maybe not!)

 

BTW If you have an analogue meter it might have an input jack marked "output". That puts a capacitor in the circuit which blocks DC and allows you to measure the AC component of a signal. If the output is pure DC you should not be able to detect any AC voltage. You might be able to do the same thing using an external blocking capacitor with any meter set to measure AC volts.

Thank you, I understood most of it. I think my comment re DC was silly, DC with feedback  would still be a problem with coreless, for the reasons people have said above.

Anyway I have found a mention in another place (S4 forum?) by someone who has run Portescaps with a Compspeed F with feedback turned off for years, so I hope all should be well.

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Posted (edited)
On 08/10/2019 at 10:03, johnarcher said:

Of course if DLT is right (and obviously he usually is)

 

Oo-err, I would never claim that!

But thanks very much anyway.

Cheers, Dave.

Edited by DLT

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5 hours ago, johnarcher said:

, DC with feedback  would still be a problem with coreless, for the reasons people have said above.

 

 

Actually it would not because the output from the controller is pure DC ;)

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Posted (edited)

The problem with coreless motors is that the lack of BEMF confuses the controller, so it cannot follow the power requirements of a coreless motor under differing load conditions in the same way it would with an iron cored motor.
 

With PWM (Pulse width modulated) what happens is a short duration pulse of power at the full output voltage is sent to the motor, which will generate a BEMF which is dependent on speed etc.

This BEMF is evaluated and the controller calculates whether the next pulse should be wider or narrower to control the motor's speed.

Coreless motors do not generate much BEMF so the controller over-widens the pulse width.

Sending a wide pulse (at full output voltage) at low motor speeds to a coreless motor will likely destroy the winding.

This is only a problem with low frequency PWM systems where the pulse repetition rate is in the 10s or 100s of Hertz (Cycles per second)

High frequency PWM using 10K Hertz or more repetition rate (as used on most DCC decoders) is fine.

 

If the output is unmodulated (pure) DC and feedback is derived to control this output, it should not cause a problem but might not control very well.

 

 

Edited by melmerby

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1 hour ago, melmerby said:

The problem with coreless motors is that the lack of BEMF confuses the controller, so it cannot follow the power requirements of a coreless motor under differing load conditions in the same way it would with an iron cored motor.
 

With PWM (Pulse width modulated) what happens is a short duration pulse of power at the full output voltage is sent to the motor, which will generate a BEMF which is dependent on speed etc.

This BEMF is evaluated and the controller calculates whether the next pulse should be wider or narrower to control the motor's speed.

Coreless motors do not generate much BEMF so the controller over-widens the pulse width.

Sending a wide pulse (at full output voltage) at low motor speeds to a coreless motor will likely destroy the winding.

This is only a problem with low frequency PWM systems where the pulse repetition rate is in the 10s or 100s of Hertz (Cycles per second)

High frequency PWM using 10K Hertz or more repetition rate (as used on most DCC decoders) is fine.

 

If the output is unmodulated (pure) DC and feedback is derived to control this output, it should not cause a problem but might not control very well.

 

 

 

Hi Memberly,

 

All DC motors, including coreless motors, produce a BEMF that corresponds to the speed of the motor. If they didn't they would run faster and faster until they self-destroyed. The BEMF is exactly the same as the EMF that would be produced if the motor was run as a dynamo. As I tried to explain previously (probably not very well) the problem with coreless motors is they don't have any electrical energy storage or much mechanical "flywheel" action, so they slow down immediately when the current is interrupted. The reason the BEMF reads low on a pulsed feedback controller is because the motor actually has slowed down when it's sampled.

 

Pure DC feedback controllers do control all types of DC motors very well. I know that to be true because I build them.

 

Cheers!

Andy

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Posted (edited)
31 minutes ago, AndyID said:

 

Hi Memberly,

 

All DC motors, including coreless motors, produce a BEMF that corresponds to the speed of the motor. If they didn't they would run faster and faster until they self-destroyed. The BEMF is exactly the same as the EMF that would be produced if the motor was run as a dynamo. As I tried to explain previously (probably not very well) the problem with coreless motors is they don't have any electrical energy storage or much mechanical "flywheel" action, so they slow down immediately when the current is interrupted. The reason the BEMF reads low on a pulsed feedback controller is because the motor actually has slowed down when it's sampled.

 

Pure DC feedback controllers do control all types of DC motors very well. I know that to be true because I build them.

 

Cheers!

Andy

 

Why are coreless motors chosen for some model trains, is it smaller size  ?

The Swiss company Maxon make numerous brushed micro-motors including powerful ironless rotor ones

 

https://www.maxongroup.com/maxon/view/content/Overview-brushed-DC-motors

 

They also have brushless ranges

 

https://www.maxongroup.com/maxon/view/content/Overview-brushless-DC-motors

 

Edited by maico

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Posted (edited)

Thanks again to all contributors, I do have the impression that my Compspeed with feedback turned off should be OK?

 

In fact I have never used a coreless motor before (could not afford RG4's in the old days, and most of them don't like worm gears I believe), so may I ask those who have used both, and built more locomotives than I have (four, and that some years ago), which combination would likely give the best results (in terms of smooth, slow running rather than power) in a small 0-6-0 that I have planned. (4mm).

I could use a good, maybe large reduction gearbox like High Level, even maybe the 80:1 or 108:1 Loadhauler, but that would mean a smaller motor - N-drive 1015 perhaps, or the little eBay Chinese 10mm square one. (A  Road Runner would mean the same motor choice).

Or - a more compact gearbox (Branchlines or London Road single-stage 38:1, maybe 50:1) with a better motor, I am tempted by the 1219 coreless one High Level are now selling (which has bearings suitable for worm gears), hence the subject matter of this thread.

Edited by johnarcher

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1 hour ago, AndyID said:

 

Hi Memberly,

As I tried to explain previously (probably not very well) the problem with coreless motors is they don't have any electrical energy storage or much mechanical "flywheel" action, so they slow down immediately when the current is interrupted. The reason the BEMF reads low on a pulsed feedback controller is because the motor actually has slowed down when it's sampled.

 

Pure DC feedback controllers do control all types of DC motors very well. I know that to be true because I build them.

 

Cheers!

Andy

This is what I was taught about BEMF:

All coils, (even open wire types as on a coreless motor) take a finite time for the field to collapse and the collapsing of the field produces BEMF.

The more magnetic field the more BEMF there is. You can show this on an oscilloscope using a large choke, with and without an iron core.

 

Most motors used in model railways have a flywheel and other added mechanical mass so the motor does not slow down significantly between pulses, the BEMF is lower becuse there was less magnetic field  to collapse in the first place.

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1 hour ago, melmerby said:

This is what I was taught about BEMF:

All coils, (even open wire types as on a coreless motor) take a finite time for the field to collapse and the collapsing of the field produces BEMF.

The more magnetic field the more BEMF there is. You can show this on an oscilloscope using a large choke, with and without an iron core.

 

 

 

Hi Memberly,

 

You are absolutely correct about BEMF induced by a collapsing field, but that's not the EMF we are interested in. We are trying to determine the EMF induced in the windings as they move through the motor's magnetic field, just like the EMF produced by a dynamo, and that EMF is directly proportional to the motor's speed.

 

More here: https://studyelectrical.com/2015/02/back-emf-and-significance-in-dc-motor.html

 

Cheers!

Andy

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4 hours ago, maico said:

 

Why are coreless motors chosen for some model trains, is it smaller size  ?

The Swiss company Maxon make numerous brushed micro-motors including powerful ironless rotor ones

 

https://www.maxongroup.com/maxon/view/content/Overview-brushed-DC-motors

 

They also have brushless ranges

 

https://www.maxongroup.com/maxon/view/content/Overview-brushless-DC-motors

 

 

 

Good question Terry.

 

I don't know of any manufacturers who put coreless motors in their products but some might. Modellers started using them some time ago because they were a lot quieter and "smoother" (less cogging)  than the usual open-frame motors available at that time. If you look at the Maxon blurb you'll see that they are really good at handling rapid speed changes, but that's not exactly something we need. Unfortunately they are a lot less robust than iron-cored motors, and they can be a lot more expensive.

 

Things changed when the likes of Sagami started producing really good "cored" can-motors (sadly Sagami is no longer in business and if you see any of their motors for sale at a reasonable price I'd encourage you to buy them.) They are just as smooth and quiet and they are also quite robust.

 

Coreless motors are a bit more efficient. That means they generate less waste heat, but that's not something we typically have to worry about either. In terms of size, there isn't a lot of difference.

 

Cheers!

Andy

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