>terry@cclru.unsw.edu.au (Terry Flynn) wrote:
>>dbromage@metz.une.edu.au (David Bromage) wrote:
>>>Terry Flynn (terry@cclru.unsw.edu.au) wrote:
>>>>Craig Haber <albatross@harnessnet.com.au> wrote:
>>>>
>>>>>
>>>>>Terry Flynn wrote:
>Actually, tvarious peoples' comments were nested about 4 deep here
>>>And just going off topic for a second, a steam locomotive can pull a
>>>heavier train than it could ever start. Opposite to a diesel, a steam
>>>locomotive has more power at speed than when starting.
>Bit of confusion here between power and tractive effort.
>It is true that the power expressed by a steam loco rises with the
>speed (but not indefinitely, of course). But "pulling" is dependent
>on tractive effort and, in common with diesel, this falls with speed
>(though not as fast as with diesel and, for the 1st 10-20 mph it
>doesn't fall much at all.)
>The formula for power (P), in terms of tractive force (F) and velocity
>(V) is
>P = F * V
>For the diesel, F falls off almost hyperbolically with V and so the
>product of F and V is a constant.... a consequence of the constant
>power development under these conditions.
>For steam, F falls off at a rate that is less than hypebolic (almost
>but not quite linearly over a wide range) and so power rises (if F IS
>linear, the the graph of P versus V is then a parabola)
>But, at any rate, F ALWAYS falls with both types as speed rises.
>On the other hand, static friction ("stiction") may be higher than
>kinetic friction and so it may be possible for any loco (steam or
>diesel) to haul a train it cannot start (without taking up slack,
>etc.). But this has nothing to do with the relationship between speed
>and power/force- and it applies to any tractive unit.
>Geoff Lambert
Its probably also worth pointing out here that the tractive effort that can be
developed by a traction motor at various speeds is in some cases in no way
related to the HP / Electrical output capacity of the Engine / Alternator
combination.
Traction motors are limited in their maximum tractive effort primarily by
their size and the amount of iron in the magnetic circuit, also the size and
thickness of the copper windings.
As the speed of a traction motor falls,the current thru it rises and the
voltage across it falls , so that the electrical input remains constant,but
the efficiency of the motor falls as it gets hotter.
The power loss in the motor is proportional to the square of the current it
draws and so as the speed of the motor falls so does its efficiency.
The maximum size of a traction motor is limited, by the distance between the
wheels, ie the guage and also by the maximum height of the locomotive,loading
guage,so that whilst it is easily possible to increase the HP of the prime
mover / alternator combination by increased cylinder size, turbocharging and
intercooling,its not always possible to correspondingly increase the power
rating of the traction motors to suit.
This leads to various differant HP ratings that can be delivered to the
traction motor sets at differant speeds and for differing times.
Ideally it would be nice to be able to increase the number of traction motors
from the usual 6 to 8 as engine HP increases,but loading and curve radii
prevent this.
This can mean that a 4000 HP loco cant start a heavier train than can a 3000
HP loco with the same number of traction motors.
cheers
Maurie Daly.