Dennis,
Right. Fingers are fast but brain is slow.
Russ
----- Original Message -----
From: "Dennis C. Henry" <dchenry@g...>
To: <BRHSlist@yahoogroups.com>
Sent: Sunday, 14 April, 2002 11:36
Subject: Re: [BRHSlist] short-term current rating discussion [was F's]
> Russ, Ken, and others who may want to look at this again,
>
> If Ohm's Law were the only thing working here, a rising voltage would lead
> to a rising current. There is another effect, called the "back emf" of the
> traction motor, which increases with speed. This back emf opposes the
> applied voltage of the DC generator. As speed increases, there is more of
> this cancellation, and the traction motor current (amperage)
> falls. Indeed, during dynamic braking, it is the back emf that generates
> the current that flows into the resistor grid, and this current produces a
> magnetic field in the motor that opposes the field winding, and keeps the
> speed under some control.
>
> Dennis
>
>
> At 09:20 AM 4/14/02 -0500, you wrote:
> >Ken,
> >
> >The traction motors on a diesel-electric motor can only get rid or
> >a certain amount of heat. Beyond that point they will fail by either
> >shorting or the literal melting of the windings.
> >
> >Presuming full throttle operation the slower the locomotive is moving
> >the higher the amperage. This directly relates to Ohm's Law. As
> >speed increases the voltage rises and the amperage drops.
> >
> >Locomotives have "Short Time Ratings" and "Minimum Continuous
> >Speed". They are equipped with ammeters so the engineer knows
> >what is going on. (Most early switch engines were not.)
> >
> >There is a overload factor built into this so a unit can run at, say
> >950 amps, but only for, say 20". At that point either the speed
> >has to increase or the load has to be reduced.
> >
> >The "Minimum Continuous Speed" is the speed at which the unit
> >can be operated in throttle position 8 for as long as you want
> >without worrying about traction motors burning up.
> >
> >On most SD40-2's with 62/15 gearing that is 11.5 mph.
> >On the gone and forgotten LMX's with 83/20 gearing it was 18.0 mph.
> >
> >Russ
> >----- Original Message -----
> >From: "Ken Martin" <kmartin@c...>
> >To: <BRHSlist@yahoogroups.com>
> >Sent: Sunday, 14 April, 2002 02:14
> >Subject: Re: [BRHSlist] Last years of the F's
> >
> >
> > > Russell Strodtz wrote:
> > > >
> > > > Gordon,
> > > >
> > > > I think Railfans probably worry
> > > > about short time ratings more than the Railroads do.
> > > >
> > > > Russ
> > >
> > > Can someone explain what "short time ratings" are ?
> > > I don't know the term.
> > >
> > > Ken Martin
> > >
> > >
> > >
> > >
> > > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
> > >
> > >
> > >
> >
> >
> >
> >
> >
> >Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
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