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ECM Rambler Minor -is it a feedback controller?


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Hi,

 

I have "found" an ECM Compspeed Rambler Minor in my pile of "useful for later stuff". I use Compspeed F's on the main layout which feature a "No feedback" switch for my Portescaps. The ECM Rambler Minor (its the one with a panel mount and a plug in rambler with buttons for speed ,direction, overload etc), looks a nice controller but has no technical data with it. The question is -Is it a feedback output controller so will harm the brushes on my Portescaps?

 

Thanks in advance for your help - I never fail to be amazed about the level of knowledge out there!

 

Tony

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Hi,

 

I have "found" an ECM Compspeed Rambler Minor in my pile of "useful for later stuff". I use Compspeed F's on the main layout which feature a "No feedback" switch for my Portescaps. The ECM Rambler Minor (its the one with a panel mount and a plug in rambler with buttons for speed ,direction, overload etc), looks a nice controller but has no technical data with it. The question is -Is it a feedback output controller so will harm the brushes on my Portescaps?

 

Thanks in advance for your help - I never fail to be amazed about the level of knowledge out there!

 

Tony

 

It's not so much damage to the coreless brushes, as the high impedance of the coreless motors coils that generates a lot of back EMF when the motor runs. (High pulse power is different and might cause noise and wear)

This high back EMF does not operate the feed back circuit properly, and most feedback type circuits then apply a higher voltage than required and the controller tends to lurch the motor into life.

 

Conventional motors draw more current at the start, and do not generate the same amount of back EMF till much higher revolutions, which is where the circuit draws it's feedback signal.

 

 

A simple test.........

 

 

You will not harm the Portescap motor by trying it carefully on the 0 to 12volt controller, as long as it smoothly controls the speed from zero upwards, then it should be fine, if however it causes the motor to speed suddenly, as the controller goes from zero, then it will not suit a coreless motor. It will have done no harm to the coreless motor to test it carefully like this.

 

As with all motor tests, do not increase the voltage to 12volts to race the motor with no load, not a good idea!!!

 

You can trick a feedback controller to work with coreless motors by adding resistors in series, or diodes in series, but it also reduces the potential max speed, however this may not worry you if say, a shunter, the difference is not dramatic and adjustable by picking the right values.

 

However again a problem is the resistors can get hot and in say an N gauge loco or small 00 type there might not be space for the extra resistors, a large loco, no problem.

 

Hope this helps,

 

Stephen.

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Just in case it's queried by other members, the coreless motors are "safe" on most controllers, they work well, but where the power is applied in a stream of pulses like a variable mark space ratio DC , the change from zero to the max voltage and back induces a big back emf spike from the relatively large coils in the motor.

 

It makes the motor "jump" on each pulse, and can cause arcing on the delicate commutators that a lot of coreless use. It makes them hum as well, so not a good idea with an expensive type of motor.

 

You could smooth the power pulses with capacitors in the loco, (with diode protection),, but it loses the extra control it gives, and it is simply kinder on the coreless motor to use plain DC, or a feedback controller adjusted to work with them.

 

Stephen.

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Hi Stephen,

 

Thanks for your detailed explaination of how to test for feedback, I will give it a go (another thing to add to the "to do" list!), presumably a motor alone (with an axle & wheel)would be an acceptable option to see the effect of te feedback cutting in. As a long term solution if it does give feedback adding resistance to the fleet of portescap locos (over 60) is not really a preferred option.

 

Cheers Tony

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If there are 60 locos it is worth modifying the ECM, not the locomotive, capacitors across the output of the ECM will do it, with a couple of diodes in series to lower the output to suit the more sensitive coreless motors.

 

 

If the reversing switch is not accessible, and some ECM were centre off, then it needs a dual circuit in the output lead, two large capacitors, I would estimate without testing the circuit about 200uf, each with a protection diode to prevent damage in reverse, (you can used non polarised capacitors as an alternative but they are more expensive than adding a cheap diode.

 

The capacitor will smooth the signal, losing some low speed control, but coreless are good at slow anyway.

Getting the right value will need experiment, it could be as high as 1500uf if the waveform is square wave pulse.

 

The test is to run at low speed one of the motors and add the cap and diode across the leads, any hum should vanish, if it does not increase the value.

 

Equally it would really do no harm to use the 1500uf anyway, at worse it will smooth it to DC. Any higher and the sensitive motors will run on the stored power!

 

A proper test would need an oscilloscope to evaluate the wave form, (any access to one?) and a value could be chosen to suppress the square wave.

 

All of this beggars the question of the feed back, and the output of the whole controller could be lowered by adding silicon diodes in series, (double string, one for each polarity). Diodes have a voltage drop in forward conduction and easily lower the voltage without heat unlike just adding resistors. At a guess three (six in all) would lower the voltage applied, and the feedback should work, again more could be added, but it will reduce max speed available.

 

If your are electrically experienced these notes should make sense, if not I can post the circuit to add to the lead from the ECM. The modified ECM would still work with normal motors but have less control of very low speeds than standard, but you could make the whole thing switchable, just like the bigger unit.

 

Hope this helps,

 

Stephen.

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Just had a search through some old modeling mags to find an advert for ECM controllers ,

found one in a 1984 mag , it would look like all Compspeed controllers were a feedback type ,

the ' F ' is marked as switched but none of t he others .

 

I also found a letter in an old MRJ where the writer had found a simple solution to overcome any

problems using feedback controllers and Portescap motors , he simply connected a seperate motor

to the controller output to run in parallel with the track supply .

 

I assume this then dispenses with the need for capicaitors and diodes and takes some of the feedback

away from the Portescap .

 

I hope this may be of help .

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Hi Mike & Stephen,

 

Yes I have come to the conclusion that it is a feedback type. I could do the conversion/adaption that Stephen suggests but I think the best solution would be to flog it on flea-bay and buy a Gaugemaster handheld type W which has no feedback and a lifetime gaurantee! At least that way I can spend my modelling time building stock and locos towards the new layout.......

 

Thanks anyway

 

Cheers Tony

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