What are traction motors?

Can someone explain to me exactly what traction motors are and what they do and how weather effects them? I have an idea but I'd like to know the exacts.

#7470 wrote:Can someone explain to me exactly what traction motors are and what they do and how weather effects them? I have an idea but I'd like to know the exacts.

Alright, Traction Motors are the actual things that move the train down the track. They are mounted in the trucks, the prime mover has a generator on the end of it, this makes power for the traction motors. Weather affects the rails, by making them wet, the moteos spine, because they slip on the wet surface. Hope this helps.

yes. The traction motors are connected to the wheel. The actual "engine" in the engine; prime mover is just a large generator, and provides the traction motors with electricity to move the train using the crank shaft's to spin the wheels. That is why an electric engine can be so much more powerful; doesn't need a generator, it directly takes electricity from the cat.'s

The wheels slip for the same reason your car wheels slip on ice, but there is a lot less friction with metal on metal, than rubber on asphalt.

Wet leaves on rail in the autumn is a severe problem for railroads not only in N.A. but also in the U.K.. In fact when I went to the web to find some info to post here, I was overwhelmed by how much there is. There is even a whole entry in Wiki devoted to the issue, entitled, appropriately enough, "Slippery Rail". It affects both freight and passenger, and is perhaps most problematic for passenger/commuter operations due to the higher speeds, crucial stopping distances involved, and signal faults (due to the unwanted "insulation" the leaves cause between wheels and rails) not to mention an observant, impatient public. One engineer on the web wrote he saves his vacation time and extra days off for this time of the year just so he doesn't have to deal with the problem! In addition to the operating difficulties, it also causes severe and expensive mechanical damage, mostly flat spots from sliding wheels. Also, it's not just the first train over the leaves that has to deal with it. Apparently after the first wheels crush the wet leaves, they are not gone. Instead it leaves a hard slippery residue that is very hard to remove, one source equating it to Teflon! I was surprised that traditional sanding is not a cure-all either. Some rail companies utilize special equipment meant to "blast" the residue away. Thus, though nobody except the grouch mentioned on the Saturday trip has implied as such, CSRR's problems are not caused by faulty equipment or operators. Just a seasonal problem that makes trains even more difficult to operate than most people realize.
-Alan S.

Oh wow thanks. I had no idea then. From what I was told or heard or maybe thought was the electric power came from the rails? But I could never believe that because rails are not electrified like subways. So all my 18 years of life to finally realize that the engine is actually a big generator for the electrical generator for the traction mtors! Wow thats a lot to swollow but thanks for the advice. Im still confused but at least I have a much much better and accurate idea.

#7470 wrote:the engine is actually a big generator for the electrical generator for the traction mtors!

Exactly right.

http://en.wikipedia.org/wiki/Diesel_locomotive

-otto-

Here is Picture of a DC traction motor

The axle of wheelset mounts in open bearings and the pinion gear will mesh with the axle gear.

Now, having been instructed on how traction motors MOVE a train, if the locomotive is equipped with DYNAMIC BRAKES, the traction motors can be used to help STOP or retart train movement. They can be turned into generators to help dissipate electric power, thus retarding the movement of the locomotive. This effect is used in addition to the air brakes to help control the train braking. It is "similar" to the effect of a large truck down shifting gears to help it retard momentum when going down hills. Dynamic braking can substantially reduce wear on the brake shoes and control run out or run in on the train.

http://en.wikipedia.org/wiki/Dynamic_braking

Instead of using the air,which sometimes gives you a kicker in the train esp w/trailer train flats,you can just go after the dynamics to slow your train,until the floor of the cab starts to get warm. That's when you need to back off of them a little.

Ok, to expand on the whole thing:

Diesels locomotives in the US are diesel-electric. What this means is that the diesel engine literally just spins a big generator. This generator makes electricity, which then gets routed to traction motors, which are mounted on the trucks. It's those motors that actually move the train.

Diesel engines have narrow powerbands, and let's face it, a 3,000 HP transmission's gonna be a big thing, so, it's just easier to turn it into electricity.

There are some types of locomotives in Europe that are diesel hydraulic - much different. But they're a minority.

Diesels are often called AC or DC. What this means is the type of electricity that's fed into the traction motors, DC is the older technology, but the TMs require brushes and commutators, which wear down and are vulnerable to dirt. AC's newer - it's brushless, so the motor has only the rotor as a single moving part. But you need electronics to flip the power around to create the rotating field that spins the rotor. Both use century old motor technology, but it took until recently to get the electronics that make AC work for locomotives. Since AC uses a simpler motor, they require less motor maintenance, but they cost more to buy - that gap's narrowing fast though.

There's a few ways to mount the traction motors. The most popular in the US is have them rest partly on the axles and partly on the truck frame. It's good for low speeds and heavy duty applications, but it's rough on the track at high speeds. Much of Europe, and the electric units in the Northeastern US, the traction motors are supported entirely by the trucks, and couple to the wheels via a flexible coupling. This is easier on the track, but costs more and has a maintenance premium. For high speed applications (and some other special applications), the traction motors are mounted to the car/locomotive body, and drive the gearbox and wheels via a driveshaft. The Pendolino uses a setup that looks kind of like a rear wheel drive car (or an RDC), the TGV has the motor in the 'normal' location, but mounted to the body, and has a CV joint to the gearbox. This gives them a very light truck, which gives high speed stability and safety.

For a number of years, the French used a 'monomotor' design, where each truck had only one motor, geared to all the axles. This gave them somewhat better adhesion, but restricted axle movement, so it demanded very smooth track - that's why the French unit tested in the 70's on the NEC was such a dud. The AEM-7 has a motor per axle, and a very flexible truck that doesn't mind our track.

Traction motors typically have blowers mounted in the locomotive to keep air going through them. High power levels generate heat, and it's a limiting factor, especially a low speeds.

Dynamic braking means the motors are turned into generators to make electricity. This power's routed to resistors, or on some new electric locomotives, back out through the overhead wires. Contrary to what some believe, the motors are NOT 'put into reverse'. The actual mechanisms for how they are made to generate electricity are different between AC and DC - AC's done via some electronic slight of hands, DC's done by changing the connections around with automatic switches. AC has the advantage here - a good AC locomotive can provide dynamic to very low speeds, and in fact, some multiple unit trains (like subways) can actually come to a near stop with just motors alone. Diesel locomotives tend to have blowers to help cool their resistors, and those can be loud, since it takes a lot of air to cool things off. Dynamic used to be an option, but I'm pretty sure it's standard on everything now.

As surprising as it may sound, traction motors aren't actually that powerful in the world of electric motors - pumps at large water or sewage plants can easily have motors rated over 2,000 HP, and nuclear plants have pumps with motors rated upwards of 10,000 HP per pump (!). Those things are big - the size of a UPS truck, or bigger.

Great posts, everybody. Interesting stuff !

I'm just wondering- with all the added machinery required for a diesel-electric application as opposed to a straight electric one that goes directly to the traction motors, why has there traditionally been such a prominence of diesel-electrics in the US ? It sounds like greater power, less machinery (ie. less maintenance), longer life-span, less break-downs, etc. are the hallmark of the electric.

As far as I can tell, the only drawback to electric is the additional infrastructure.

Passenger wrote:As far as I can tell, the only drawback to electric is the additional infrastructure.

Exactly ... the US is big. Very big. And if you can take your electrical generator with you, for several thousand miles, and not have to maintain catenary for that length of track ... it's a huge cost savings.

Perhaps as diesel gets more and more expensive per gallon, the offset will begin to swing towards more electrification, but for now, the cost analysis is squarely in the big brute's court.

As for leaves ... UGH. They are a royal PITA. We routinely have slippage problems on the BRSR on the hill coming back to Blue Ridge (it's a pretty steep, tight, windy uphill passage), when it's wet during fall leaf season excursions. We solve it with sand, and a GP38 instead of a GP18 (pretty much a wash on horsepower, but that extra 100k lbs helps a lot) ... and the occasional 2nd or 3rd attempt.

- litz

I understand your point in theory, litz, but in actuality, at the time of the heyday of railroads, our national system was made up of many smaller roads. Just randomly looking through my guide, I see the following mileage for many roads around 1929. Just a sampling includes: Bamburger -36, Clinchfield - 309, Cumberland & Pennsylvania - 50, Lehigh & New England - 217, Louisiana Midland - 77, Michigan Northern - 250, Rutland - 413, Uintah - 70. It would seem that maintenance costs would make electrification pay for itself very quickly. My instinct tells me that diesels just inherited steam locomotive set ups, but were the steamers that much more economic over electric ? Did the industry dislike, distrust, or not be familiar with electric, or be concerned about safety concerns ? It's a testament to the quality of this post that I now have a better understanding of the way that diesel-electrics are propelled. Honestly, I knew that they were generator driven but I believed that there was an on-board traction motor which delivered power - not merely a prime mover and generator. It seems to me that to put all that power to convert for the purpose of driving small motors is redundant and seems like overkill. I can't help but wonder why a more direct approach (ie. electrification) wasn't built as soon as it was perfected.

My personal thought is the cost of putting up catenary is prohibitive to most other railroads. I have UNP in my IRA (you now know my usual disclaimer) and I trust them to have made a properly economic decision on this.