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  • RMweb Gold

Just a general query about the old AMR slow-speed controllers (talking DC only here).

 

I've been using one for several years and I really like it, it gives excellent slow speed control for a range of non-coreless motors, such as Mashima or those found in many RTR locos.

 

I've also used it with Portescaps, with no apparent ill effects.

 

I know virtually nil about electronics and have no idea if the AMR slow-speed controller features 'feedback' or not.

 

My query really concerns whether it will be OK to use on the current generation of coreless motors found in RTR locos like the new Bachmann 94XX etc. (even though for other reasons, I'm not likely to use it on my 94XX now).

 

Are these AMR slow speed controllers actually feed-back controllers?

 

Many thanks.

 

Edited by Captain Kernow
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18 minutes ago, Captain Kernow said:

Are these AMR slow speed controllers actually feed-back controllers?

 

Yes, in so far as they sense the back emf of the motor. The debate concerns the degree to which the control alters in response to the 'feedback'. I think the degree chosen for the AMR was a good one, and I consider them to be suitable for coreless. Others might disagree about that. Before he retired, Len Rich (Mr AMR) did a version of the controller with a 'high/low' feedback switch on it, which was even better for coreless motors.

 

One of the best handhelds around IMO.

 

 

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  • RMweb Gold
2 hours ago, Miss Prism said:

 

Yes, in so far as they sense the back emf of the motor. The debate concerns the degree to which the control alters in response to the 'feedback'. I think the degree chosen for the AMR was a good one, and I consider them to be suitable for coreless. Others might disagree about that. Before he retired, Len Rich (Mr AMR) did a version of the controller with a 'high/low' feedback switch on it, which was even better for coreless motors.

 

One of the best handhelds around IMO.

 

 

Thanks, Miss P, that's really helpful to know.

 

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  • RMweb Gold

Interestingly, I think I'm seeing differences in the performance of individual locos, depending which of two AMR hand-held controllers I'm using (the 'main' one and the 'spare').

 

There is no obvious difference between them, to look at.

 

Is it possible that one is giving more 'feedback' than the other?

 

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  • RMweb Gold

Hello Captain

 

I have some All Components handheld controllers (which used to be made by Kent Panel Controls).

 

I had one repaired, and one of my normally good locos was 'jumping like a kangaroo'. The company somehow 'took off' (or vastly reduced) the feedback for me and the problem was solved.

 

Brian

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9 hours ago, BMacdermott said:

Hello Captain

 

I have some All Components handheld controllers (which used to be made by Kent Panel Controls).

 

I had one repaired, and one of my normally good locos was 'jumping like a kangaroo'. The company somehow 'took off' (or vastly reduced) the feedback for me and the problem was solved.

 

Brian

 

Hi Brian,

 

Yes, feedback can be either negative which acts to stabilize a system or positive which acts to reduce the difference between the input signal and the a actual position or speed. The positive type is sometimes referred to as "feedforward".

 

If the feedforward gets too ambitious (too much gain) the system goes unstable and starts to oscillate wildly, hence your kangaroo observation. That's bad enough in a model loco but it can be a disaster if the system is controlling the rudder on a supertanker :o

 

Cheers,

Andy

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7 hours ago, AndyID said:

 

Hi Brian,

 

Yes, feedback can be either negative which acts to stabilize a system or positive which acts to reduce the difference between the input signal and the a actual position or speed. The positive type is sometimes referred to as "feedforward".

 

If the feedforward gets too ambitious (too much gain) the system goes unstable and starts to oscillate wildly, hence your kangaroo observation. That's bad enough in a model loco but it can be a disaster if the system is controlling the rudder on a supertanker :o

 

Cheers,

Andy

Alternatively, you can always 'reverse the phase polarity relays' or similar nonsense, which always gets Star Trek out of trouble!

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  • RMweb Gold

The more I think about this, the more I realise how little I understand 'feedback'.

 

Given that I am a complete electrical ignoramus, would someone please be able to explain, in the most basic of layman's terms, exactly what 'feedback' is and what it does?

 

I've also heard of the term 'back emf' in connection with 'feedback'. Again, without getting in any way technical or complex, what exactly is 'back emf', please?

 

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  • RMweb Gold

Hi CK I am more a biologist than a physicist but here’s my understanding of back EMF...
 

An electric motor, once the armature starts to rotate, develops a current which opposes that which is driving it. This will be low at slow speeds but as the armature accelerates there comes a point at which it balances out the driving current at which the motor speed will stabilise. On a fixed magnet motor this is the maximum speed of the motor if the driving current is constant but on a motor with field coils the back EMF can be reduced by weakening the current in the field coils which allows the motor to accelerate further... hence field weakening on traction motors fitted to diesel electrics.

 

My understanding of feedback controllers is that they monitor the back EMF and increase the driving current to speed up the armature if a drop is detected .... or vice versa if it increases.

 

Probably an oversimplification!

 

 

 

 

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33 minutes ago, Captain Kernow said:

The more I think about this, the more I realise how little I understand 'feedback'.

 

Given that I am a complete electrical ignoramus, would someone please be able to explain, in the most basic of layman's terms, exactly what 'feedback' is and what it does?

 

I've also heard of the term 'back emf' in connection with 'feedback'. Again, without getting in any way technical or complex, what exactly is 'back emf', please?

 

 

I'll give it a shot :)

 

Any control system has an input (the demand if you prefer) and an output (the actual output). The difference between the two is called the error.

 

In our case the input is a speed demand (the controller position) and the output is a voltage that makes the motor rotate. The difference between the speed demand and the actual speed is the error. The controller continuously cranks the voltage up or down to try to minimize the error. That's the feedback bit - positive feedback in this case.

 

Obviously the controller needs to be able to determine the motor's actual speed for this to work. On big systems this might be done with an  electronic tachometer but in a model loco it's done by sensing the voltage produced by the motor when it's running as a generator rather than a motor. That happens every time the controller cuts the current supplied to the motor at the end of every power pulse. (That's the back-EMF bit.)

 

So for this to work the power supplied to the motor has to be pulsed rather than steady DC. The continuous pulsing generates some heat in the motor. That depends to some extent on the pulse frequency and the motor's momentum.

 

There is another way to do it without any pulses at all but I don't think any commercially available controllers work that way.

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