Coreless and feedback

[steve howe]

As a non-techie, and waiting to be shot down in flames, may I ask what's probably a very simple query? I have been using feedback controllers for many years starting with the ECM Compspeed and now various brands of panel and hand-held jobs including the Gaugemaster. Most of my locos have been standardised on Mashima motors with (in most cases) a High Level gearbox which has been a very satisfactory combo on the slow-speed shunting planks I favour. Recently I have acquired a couple of locos from a late friend which are fitted with RG4s. Now, I am dubious about running them with the feedback controllers in case they are not compatible and it damages the RG4s. Unlike the Pentroller, none of my feedback controllers seem to have the facility for switching off the feedback. Anyone any advice on the matter?

Steve
(in darkest Iron Age Cornwall)

[Guy Rixon]

Steve, a motor is damaged by a pulsed power-supply if it has precious metal brushes and if it is light enough to spin down and stop in between the pulses; the working lifetime of the brushes is limited by stops and starts rather than continuous running. Small, coreless motors often meet both of these criteria and can motors generally do not. But it also depends on the details of the controller, specifically the length of the gaps between the power pulses.

The chance for the motor to spin down in the inter-pulse gaps depends on the duration of those gaps. Simple feedback controllers control the output by varying the length of the pulse, so they have long gaps at low output and slow speed - exactly when a light motor might be most likely to spin down. Clever controllers, notably the Pentroller, keep the gaps small for all outputs and vary the peak voltage of the pulse instead. So if your controllers of choice are the variable-pulse-height kind your motors are safe.

If you do have variable-pulse-width controllers, it ought to be possible to fit a filter to the loco to smooth out the pulses and avoid the spin-down. Basically, the filter would be a keep-alive capacitor, but with a very small capacity since it only has to keep the motor turning for fractions of a second; thus it probably be a small assembly. It might be possible to repurpose a commercial keep-alive unit, or you might need to get a circuit design from someone with practical electronic skills (not me!) and build it yourself. A home-build circuit would (I guess, without ever having built one) be only a handful of discreet components, so not at all difficult to solder up. As a bonus, the filter would also keep the motor from spinning down if the pick-up was slightly intermittent.

HTH,
Guy

[Will L]

I think the problem with feedback controllers and core less motors was largely restricted to the original ECM designs which cycled relativity slowly. The lesson was learned and later controllers cycle a lot faster. Many DCC chips work the same way don't they? and I don't believe there is any problem with core less motors.

[nigelcliffe]

The problem is with low frequency feedback designs which had large impacts on the motor. ECM are one such design, some Gaugemasters are another (the black panel designs in the current range).
So, I would find an alternative controller for the existing layout. There are some Gaugemaster designs which are fine (yellow panel in current range) or the 30Euro MedVend handheld unit from Hungary would be another candidate.


High frequency PWM is fine, as shown with numerous DCC chip designs, the better European DCC chip makers are include documentation for Faulhaber and Maxon coreless instrument motors (ie. same types as used in RG4 motor gearbox combinations).

- Nigel

[Martin Wynne]

The RG4 gearboxes are reversible, meaning that the motor can be driven by the rolling momentum of the locomotive. It seems very unlikely that the motor AND locomotive would be able to come to a halt in the short interval between pulses. The problem is more likely to be overheating of the windings, in the absence of the heat-sink properties of a core. Most pulse controllers have a peak pulse voltage way above the nominal 12volts of the motor. Try supplying the controller from a car battery* instead of the usual 16V RMS AC, which is usually nearer 20V RMS on open circuit, peaking to over 30 volts.

*In practice a sealed lead-acid battery -- http://uk.rs-online.com/web/p/lead-acid ... s/5375472/

Other forms of smoothed regulated 12volt supply are available.

Martin.

[dal-t]

MedVend handheld unit from Hungary

Quite interested in those, Nigel, but their website doesn't seem to give any indication of where they can be purchased (unless the Ungarish version is fuller than the English - I could only ever follow a few words of the language, even when living nearby). Don't suppose you know any sources of supply?

[nigelcliffe]

MedVend handheld unit from Hungary

Quite interested in those, Nigel, but their website doesn't seem to give any indication of where they can be purchased (unless the Ungarish version is fuller than the English - I could only ever follow a few words of the language, even when living nearby). Don't suppose you know any sources of supply?

Email the contact on the English page, and the maker will reply in decent English. Ordering is email order to Hungary, PayPal payment. All quite painless and rapid.

[dal-t]

Many thanks, Nigel, I'll give it a try.