HI Reggie
On (02 Jul 97) Reggie Arford wrote to Gregory Procter...
But, some of your comments to him were aimed at my message so I will add to
the discussion:
AC> Then there is the beaut feature of
AC> positive creep whereby even more traction is obtained by
AC> deliberate, micro slippage possible thanks to microprocessors.
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GP> .................. The cycle repeats continuously, but the point
GP> of it is to keep the wheels immediately below the slipping point
GP>  for the highest percentage of the time. (same thing I know:-)
 RA> Umm..., no. I was told on a plant trip to EMD that they achieve
 RA> maximim tractive effort at 125% speed; that is, the wheels turn 1.25
 RA> times as fast as the rails go by. They call it "controlled creep".
Yes indeedy
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AC> However few locos are allowed to run steadily at optimum speed.
AC> In the real world it is stop, start, retard, accelerate. Steamers
AC> do not take so kindly to this and that is why for locos of
AC> comparable HP, the steamer achieves lower average speeds over
AC> a duty cycle, than do the diesels/ elecs.
 RA> You are misapplying the term "comparable horsepower".
I think I used that noxious term because it was in Gregory's message.
                                                                      
Remember,
 RA> steam locos do not have "a" rated horsepower as a Diesel does;
 RA> they cannot. As any motor turns faster, its power increases (up
 RA> to a point).    ^^^
Quite wrong! Most electric motors [both AC and DC] reduce in power
output as the speed increases. For loco installations, external controls
have to be installed to force a change in the supplied voltage as the speed
increases. This is done by raising the supplied voltage [ac, dc] or in the
case of AC by raising the supply frequency. Both these methods are common
in locos but rare indeed, in every other motor application.
To be precise, power output versus speed is an inverted Vee curve for the
defined supply voltage. The motor can be designed so the apex of the vee
occurs at a low speed or at a high speed. The vee is not usually symmetrical.
The maximum power output cannot as a rule be sustained [heating] and that is
why many traction motors were given a second "one hour" rating. *Very*
roughly, maximum drive power is available at half maximum speed. For AC
induction motors [the most common of all industrial motors] maximum drive
power is at near 85% max speed.
 RA> As an expansion engine (as opposed to a combustion engine), the
 RA> reciprocating steam "motor" increases its efficiency at lower
 RA> power output. (This is done not by throttling, but by adjusting
 RA> the valve timing to use less steam, and expand it more.)
 RA> Therefore, at the usual utilization rate of 70% of full power, a
 RA> steam engine /of proper size/ will be effective and efficient.
Correct
 RA> Why? A Beyer-Garatt (sp?) type engine, perhaps burning
 RA> pulverized coal or a coal/water slurry, would work quite nicely.
 RA> Note that on a steam loco, the axles are usually MUCH closer
 RA> together, so a 4-coupled steamer may have a similar rigid
 RA> wheelbase to a C-type (3 axle) traction truck.
These two versions of "rigid" wheelbase can hardly be compared because the
springing of the individual axles on the C- type track assures more constant
force on the wheels, than is possible with the steamer. Over rough track [is
there any other kind?? ]  the rigid steam drivers will often have two or
more wheels barely in contact with the rails, as the loco shimmies and bumps.
The C- truck is far from rigid, one axle may rise while its partner falls,
thus maintaining hefty contact with the [slippery?] rail. A bit like
independent front suspension on an auto, versus beam axle.
 RA> there were several types of articulated steam loco used on the
 RA> "trench" railroads. Some 0-6-0 locos had a ZERO rigid wheelbase!
 RA> These mechanisms could be adapted to modern use. A more powerful
 RA> steam locomotive could replace several MUed Diesel units.
Well, this is theory and dreamtime. The steam loco was killed by rising
standards of living, higher wages, necessity for high productivity. The 
roper
place for a steam loco today is in a museum. Take care!
Cheers......ALEC
... ....Horsepower was a wonderful thing when only horses had it
--- PPoint 1.92
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