Model Forum / General / Railroads / May 2005

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.

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

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

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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.

You're right - pedantic it is!  ;-)

John.

DS123** etc,

Sorry, read DZ