R30200 BR 0-4-0 steam loco

[kebang]

One of these arrived from Hattons a few weeks ago. I've run it in properly and I am amazed by its low speed performance. on its slowest speed it takes 90 seconds to complete a 14ft oval, which I think works out to a scale speed 7.5mph. I purchased it for shunting duties, but I doubted that it would run slowly enough. However I have been pleasantly surprised!
My controller is a very cheap PWM power supply from China.(Cost 2quid inc. postage) It accepts 6v to 24v input. I have it connected to a 12v 7ah lead acid battery (type used in computer UPS) which I recharge once a week, even though there is no significant voltage drop.
A question for the electricians:
The controller is very good but would I get even greater control if I connected it to a 9v rechargeable battery? The speed knob would still rotate the same amount, would the steps be smaller, giving greater control?
thanks
Kebang

[Dad-1]

I don't think you'll get it running any slower.
The reason being that it has a 12 v motor to run on DC
To reduce speed the controller is reducing the voltage.
There is a minimum voltage that the motor will turn at.
It doesn't matter what the maximum voltage is because
it's the slow running where all you're doing is reducing that
to the point of reaching a stalling input.

This is where DCC often helps as a decoder can send packages
of power, pulsing in above the minimum. Someone else can
elaborate on that if wanted but you run DC so honestly it's
irrelevant.

My slow running I look for 90 seconds to cover 305 mm, or 12"

Geoff T

[Bigmet]

This is a classic 'suck it and see'. Because the controller is a Pulse Width Modulation type, it is likely to produce slower running on 9V compared to the 12V supply, as what the motor sees from such a supply is an average voltage, thus: (voltage x time on + time off) / total time of on/off cycle. Since you have a timed lap at 12V, it will be simple to compare at 9V.

[kebang]

Thanks for that Geoff. I used to have DCC before the pandemic, but all that stuff is abandoned, at least in the short term, in our home in Tagbilaran. If we ever decide to return there I'll see what's salvageable. (I never really understood most of it, but it could run slow!)
So DC locos that can run really slow achieve that at the design stage, with gearing for example?

[kebang]

I'll suck it and see then Bigmet The experiment will cost me nothing!

[Bufferstop]

Gearing is only one aspect, it has to be offset against friction. Depending on the torque curve of the motor a 30:1 reduction can sometimes give a slower useable speed than a 40:1. You will sometimes see motors described as "skew wound" meaning that the pole windings are slightly helical which helps achieve a smoother transfer of attraction from one pole to another, less "notchy". There was a long discussion on this on one of the narrow guafe sites.

[Bigmet]

I'll suck it and see then Bigmet The experiment will cost me nothing!

I'll place my bet for the 9V outcome, 120 seconds for the 14ft oval.

... I used to have DCC before the pandemic... (I never really understood most of it, but it could run slow!)...

DCC uses the same pulse width modulation technique to supply the motor as your present DC controller, but there are two big differences.
First of these is that the track voltage is constant on DCC, it's always on full power, which aids pick up reliability. DC operation necessarily has the voltage really low for slow speed, and pick up is less reliable. This is often the limiting factor in low speed performance with DC, the motor could run slower, if only there was reliable power supply.
Secondly the frequency at which the pulse width modulation is applied. In mains operated DC this is typically double the mains frequency. The DCC decoder typically outputs at 20kHz or higher. For your DC controller operating off a battery supply, there will be some internal clocking to supply the modulation frequency, there may be manufacturer information.

...So DC locos that can run really slow achieve that at the design stage, with gearing for example?

Multiple factors in play, of some of which manufacturers appear to have limited comprehension. Motor characteristics already mentioned, use of the smallest diameter worm possible and a relatively small reduction ratio at this stage, achieving most of the reduction by spur gears, are the biggies. For my money this where a dedicated programmable microprocessor control of the mechanism (such as a DCC decoder) scores big time; reducing or even eliminating the perceived differences in performance between mediocre and well sorted mechanisms.

[kebang]

Experiment a bit of a failure it only took 1min 34sec on the 9v battery so no discernable reduction in speed....
Thanks for the DCC info, if we ever return home to Tagbilaran at least I will have a better understanding of it!
Meanwhile I am enjoying my return to DC model railways again (after the typhoon destroyed my first little layout at our rented home in the country, 15months ago )