Hi Z-nauts -
I'm not really sure why current demand is seen to be a factor - as has been
written earlier in the thread, Marklin Z locomotives hold records for
length of continuous running, though I must admit we don't know what
electrical set-up was used for those test runs. You could e-mail Marklin,
but my suspicion would be that standard control equipment was employed, set
at a very modest output.
I do admit though that Z locos get quite hot when run over longish periods.
Having said that, this I assume is catered for in their design as I've
never had one stop or burn out due to overheating.
As far as friction / effort is concerned, what about employing a sensitive
spring balance as used in laboratories? Used horizontally, it should show
effort in grams before slipping occurs.
Hope this helps
Steve
"To be free, to be who you are"
----------
From: Jacob Munkhammar <jacob@...>
To: z_scale@...
Subject: Re: [z_scale] Locomotive current demands
Date: 04 July 1999 04:17
From: Jacob Munkhammar <jacob@...>
From: BJKRONEN@...
Since motor current demands are affected by load (number of cars/wagons)
there is no clear definition of what a "standard" load might be. We
"think"
wheel slip represent the maximum load condition on a loco. We "think"
it
represents that point where any more cars/wagons cannot be pulled by
that
particular loco. In any case, it would be the same point for a given
loco
every time.
Comments please? A better way? Are we on a fool's path?
Actually, maximum load isn't when the wheels spin. To explain this I'll
have to give a crash course about friction.
There are two types of friction. I do not know their English names, but
I'll try to explain anyway.
One friction is the "gliding friction". It is the resistance an object
gives while *moved* over a surface.
The other friction is the "threshold friction". This is the resistance an
object gives when it is stationary and you *try* to move it.
The "threshold friction" is always higher that the "gliding friction",
often much higher.
This is easily illustrated with car tyres. Once the wheels lock, we have
much less braking power. This is the princible behing anti-lock breaks;
they keep the wheels at just about locking, that is where friction is
greatest.
Another way to easily test this is to put an un-powered loco on a long
piece of rail, then lift one end of the rail. You can lift it quite high
before it slips, but once it is moving you don't need much angle at all
to
keep it moving.
Friction is actually measured as this angle - or rather, these two
angles.
The other angle, for "glide friction", is the angle where the loco stops
again.
So, when you meassure a spinning loco, it is only encountering "gliding
friction". Before a moving, loaded train starts spinning, it must be
subjected to a load that makes it pass the "threshold friction".
And after this lenghly lecture, do I have a way to measure current demand
at maximum load? No. None other than to put weights on a freight car till
just before the loco slips.
I hope this helped more than confused.
/Jacob
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