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/
>
>
>
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