Hornby Class 08

Look under the WOW factor a couple of days ago ....

Ah, A thread still pending to read following a holiday....

Thanks Dave!

"David Skipsey" wrote in message news:6B6me.1030$% snipped-for-privacy@newsfe3-win.ntli.net... Look under the WOW factor a couple of days ago ....

Forgive my ignorance but what is a "stall" test?

Michael

With the sound of it, the point the roads break or the motor burns out !

No, really, for DCC you have to apparently know the "stall" current of a loco. I.E the point when it stops due to load behind or little fingers pressing it too hard to the rail that the motor stops the gears turning.... I never bother myself !

Andy

Forgive my ignorance but what is a "stall" test?

Michael

"Michael" wrote

It's a test to see how much current a loco will draw before stalling. It's done usually to check out the current rating of DCC decoder needed.

John.

"Andy Sollis- Churnet Valley model Railway Dept." wrote

Oh dear, I must be doing things wrong. ;-)

I've around 40 OO & HO-scale locos with DCC chips fitted and never done a stall test yet. Never overloaded a chip either (in normal operation) so I think it's an 'urban myth' that stall tests are either desirable or necessary. It's a rare beast in the smaller scales to find a loco which needs more than a 1amp decoder, although I accept that there are some particularly heavy Continental outline locos which may need one with a higher rating.

I fib, actually I did a stall test once on an O-scale loco to see whether I could use a standard Lenz 1amp decoder in it.

John.

Dear Guys

Thanks for all of that, I haven't ventured into DCC so the info will be of value.

Phil: Its (historically?) related to the way the manufacturer of the decoder has specified its rating.

Eg - normal running current and a MAXIMUM (typically for up to 3 seconds)

- this in theory could be produced as the motor is starting (ie statioanry with no back emf to offset the applied voltage.. ie maximum current ie stall condition. (ZTC for example, but not uniquely, quote this) LENZ however, follow the idea of the maximum current (excluding initial surge which is hopefully only transient, or then catered for by on-chip overload protection) which is /likely/ to occur in practice - such as when the WHEELS slip (without additioanl weight from a hand).

The old 'stall current' value would be correct to work to if there was a 'fuse' onboard the decoder, which had no other internal protection. However, I believe most current designs, or worthwhile designs, all include onboard overload protection, and therefore this is no longer the critical value it may have been,

The operational current of the loco in use is what needs to be considered

- and if you have a long gradient, this would give time for the decoder to get warm due to any higher currents, and internal inefficiencies. (Hence Lenz suggest simulating this by holing back the model, so that the wheels slip ((not by forcing it down onto the track!)) )

IF you chose too low a decoder rating, you would experience 'blackout' periods when the decoder shuts itself down, due to excessive heating, and until its protection systems cooled down/ recovered. Obviously not desirable. And most likely in a HOT environment (like a roof layout in summer) or 'continuous running' (not likely at many exhibitions, including one I went to yesterday!)

Assuming that the decoders are not at their 'maximum speed/ full on setting' a decoder with back-emf compensation would be able to supply a higher current into the problem - eg of a stalled motor (stalled as in motor, not stalled as in lack of pickup from track!) - and this might easily exceed 3 seconds, and so a badly chosen decoder might enter its 'blackout' self protection mode.

In practice, I suspect, most 'decoder failures' (permanent type) are due to the motor-side-wires, or blue wire, or unused function outputs, touching a metal part of the body or chassis due to being inadequately insulated/ secured.

Phil

In message , Andy Sollis- Churnet Valley model Railway Dept. writes

You'd think that that was something the manufacturers ought to tell you. Then it only needs calculating once, rather than everybody doing it.

As has been pointed out trying to force a stall mechanically then measure current at full voltage risks mechanical damage to the loco as well as possible overheating the motor if you are not very quick. If you have a meter to take the current reading its much better to use the meter on its resistance setting, measure the DC resistance of the armature then calculate the stall current by Ohm's law. If you rotate the motor slowly while measuring you will find some variation depending on the position of the brushes on the comutator, use the lowest figure. Then if this value is for example 9 Ohms and assuming a 12V max supply, stalled current = 12/9 = 1.33 Amps.

Keith

Make friends in the hobby. Visit Garratt photos for the big steam lovers.

"John Turner" wrote

I ought to clarify this further I think, as my comments specifically related to current production British outline models. Models produced some years ago, such as Wrenn, Hornby Dublo and Hornby locos fitted with XO4 type motors might well have a maximum current draw in excess of 1 Amp.

John.

Personally, I have never done a stall test, didnt know what it was till now, have 30 odd locos chipped with Digitrax DS123** etc, and never had a problem on gradients or flats or continous running. And digitrax replace blown decoders anyway. Is it really neccersary? Rob

DS123** etc,

Sorry, read DZ

The current when it's stalled, not before.

Andrew

wrote

You're right - pedantic it is! ;-)

John.

Or maybe that the stall test should be to ensure that you can't stall the wheels of your loco when it hits an immovable object - i.e. that the wheels will still spin when the loco is prevented from moving. Measuring the current when that happens will give you a realistic maximum current figure.

I think Erik Olsen was the first to suggest that method on this group as a method of setting the maximum current on a loco motor by adjusting its weight to allow wheel slip at the optimum maximum current - a definite requirement for coreless motors if you want them to have a long life.

It also checks that the wheels will slip, in case you have been a bit over-exuberant with additional weight to increase a loco's tractive effort. If the wheels wont spin when it gets stopped with power on, then you run the risk of cooking the motor, and possibly a decoder chip as well.

JIm.

Standard magazine review test practice here in North America. Has been for donkey's years. Nothing new about it at all.

Maximum current draw is always tested and reported with the wheels spinning.

It's bad practice to have a loco weighted so that the wheels cannot spin.

-- Cheers Roger T.

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