how can you pull so much on such a small amount of rail? is it not the fact of power vs traction and steel is not very easy to get traction on. another few questions are what is the voltage that traction motor?
Does the voltage stay the same and the amperage change?
How does ac give you better traction because power is still getting to the rail with dc?
how do you determine tractive effort? is it the torque that goes to the rail or the weight. or its ability to slip…
It seems like the contact area should matter. In fact, while it’s not the whole story, contact area can essentially be ignored when calculating friction between rail and wheel – it all depends on the force pressing on the rail. And steel is not necessarily hard to get “traction” on. Have you ever tried to pull out a tightly press-fit pin and had it refuse to budge? That’s friction.
You can calculate this friction by multiplying the weight of the loco x the static friction coefficient of steel on steel, which varies, but is roughly 0.2-0.6.
Again: heavy drag-freight locos don’t have lots of wheels to increase the area of contact for friction purposes. They have lots of wheels to carry their greater weight on driving wheels, which increases friction and therefore traction.
If you put a locomotive in any notch and let it accelerate, as it speeds up, the voltage to the traction motors will go up and the amperage will go down. The volts X amps will be constant because for each notch a locomotive makes a constant amount of horsepower.
AC traction allows a higher coefficient of friction (normally known as adhesion), so the locomotive can generate a higher maximum pulling force. The nature of AC propulsion and it’s ability to react almost instantaneously allow this. The DC traction systems can’t react as fast because the big, inductive windings in the generator and traction motor are involved and inductors don’t like to have their current changed (think of it like inertia…)
Which governs? Adhesion or torque? The simple answer is whichever is least. If you are pulling a train at 12 mph and the locomotive is capable of pulling 100,000 lbs but your adhesion for the current conditions only allow 80,000 lbs, then adhesion governs. If you try to pull harder, you slip. If you speed up to 20 mph and the locomotive can only make 40,000 lbs of pulling force, then the locomotive’s engine and propulsion system will govern.
On most DC traction motor locomotives not only does the voltage change because of the changing speed of the generator/alternator, but the motors go through a series of wiring changes. This is call transition. The stators and rotors go through different series and parallel connections to keep the voltage and amperage within limits. There are some locomotives, many switchers and lots of Baldwins, that do not transition.
With AC locomotives the power going to the rails is not DC but is variable frequency AC. The AC locomotives are able to get higher adhesion because the traction motors are limited to a maximum RPM by the frequency generated by the inverters. That reduces the probability of slippage and limits the amout of slippage possible. In fact by allowing a certain amount of wheel slip (the builders call it creep) the adhesion can exceed the theoretical 25% of steel on steel.
0.2 Coeff of friction means 20% of the weight on the wheel can be sustained as traction.
For DC locomotives of the Dash 2 vintage, the typical, dependable adhesion is 18-21%.
For 60 series, Dash 8s and newer DC locos, it’s roughly 27%
For AC units, its 35%
P.S. Transition is a “red-herring”. It exists because of pracital upper limits on the voltage. It has no measurable effect on wheelslip control and adhesion. (In fact Dash 2 and newer four axles don’t have transition, and 50 series and Dash 8 and newer have “generator” transition)
This varies depending on what locomotive but a general ball park figure is around 600 -900 volts from the generator to the traction motor. I know that a U.K class 50 locomotive (1968 vintage so im talking old) the generator output is 2500 amps, 940 volts, DC at maximum power. Todays modern locomotives with their AC alternators can typicly double this figure, so its alot of power. ie, 4-5000 amps.
As speed increases, amps decrease but voltage increases. Tractive effort also falls away as speed increases. This is all accomplished by contactors inside the locomotive. As generator volts rise the amps also decrease. Its known as generator field weakening, look apon it as a sort of electrical gear changing. Another word for it maybe transitioning. The change in pitch you hear from an SD40 for instance when its in run 8.
The AC traction motor is brushless. You can input more amps into a brushless motor hence more traction. They also run cooler and are more reliable than the old DC motor. Theres more to it than that but those are a few fundamentals!
I just read the article at alkrug.vcn and it is indeed excellent. Thank you for sharing this link, it has been saved in my archives and will be referenced often.
Its certainly very imformative and answered alot of unanswered questions as far as i was concerned. I got that link origonaly from one of the guys on here. Its one of the best around. Glad to have helped Wayne.
That was a neat little page there. This is always an interesting subject as there is so much more to moving a train than just pulling power. And of course it’s just as important to stop.