Re: C Class Rumours

[mauried@commslab.gov.au (Maurie Daly)]

In article <7mtv2t$8qu$1@mirv.unsw.edu.au> G.Lambert@unsw.edu.au (Geoff Lambert) writes:
>From: G.Lambert@unsw.edu.au (Geoff Lambert)
>Subject: Re: C Class Rumours
>Date: Mon, 19 Jul 1999 00:40:07 GMT

>David Bromage <dbromage@omni.com.au> wrote:

>>TM wrote:
>>> The NSW WTT shows the following weights:
>>>                      Dead Weight                   Live Weight
>>>                           Tonnes                           Tonnes
>>> C class                 148                                   134

>>The dead weight isn't the actual weight, i.e. the force on the rails due
>>to gravity acting on the mass. The "dead weight" takes into account the
>>additional resistance of the dead traction motors.

>Gee, I've never noticed this before, what WTT was it in, and is there
>a list of similar statistics for other locos?

>Of course, this increased resistance masquerading as increased mass
>(or for the purposes of train loads being assumed so) would be
>speed-dependent, would it not?  Presumably the value given is some
>kind of average based on the likely speed range.  

>When a D-E loco is hauled dead and the traction motors are spinning,
>is there eny emf component to the resultant resistance?  If so, and if
>voltages appear at the terminals and currents come out of them, where
>and how are they dissipated?  Or do you need excitation current to be
>present for this sort of thing to happen?

>Geoff Lambert


This sort of stuff is normally not in WTTs , but instructions pages and 
various other forms of handouts.
Ive got a list of most NSW locos around somewhere.
The extra resistance is a mechanical one , not electrical.
DC motors when run backwards have a spinning armature upon which brushes
bear on the commutator and this introduces a small but not insignificant 
resistance , given that the armature of the motor is spinning faster than the 
axle speed, by the gearing ratio.
You are right also in that the motor will act as a very inefficient DC 
generator which will produce voltage across the armature windings.
As long as the windings are open cct ie disconnected , then no electrical load 
is generated.
If you did connect the motors to the dynamic braking grids then 
you would have a fairly inefficient electrical load which would increase the 
drag even more.
Obviously the more traction motors per loco,the more the extra 
rolling resistance.

MD