AC vs. DC

[dcushing]

I'v seen a couple of threads on this and other forums discussing ac traction motors vs. dc traction motors and I got to wondering if real railroads mix both ac and dc in the same lash-up. In other words, would it be normal to see SD70m's and Sd70ACe's on the same train? What would be the advantage or disadvantage of such a mix?

Duncan

[harryadkins]

The prototype railroads mix anything and evreything to get the job done. All modern diesels have MU (Multiple Unit) controls and work together. I saw a CSX consist the other day that had a Dash9, an SD40-2, and a GP-38. In fact, it's rare to see matched locomotive sets.

Harry

[up_8677]

And some locos (ES44AC comes to mind) can have their tractive effort modified in the computer so they can play nicer with other types of units.

[NS9030]

I think its more common on NS, all their ES40's and SD70's were DC till 2008. So its more likely to see an ES40DC matched with an ES44AC. But AC still has the advantage and thats why NS has stopped buying DC motors...

[GaryG]

When working hard at low speeds, AC traction motors don't overheat the way DC ones do. Additionally, when dynamic braking, AC traction applies retarding force at much lower speeds. DC traction needs higher rotation speed to apply retardation - 1MPH (or lower) for AC, >5MPH for DC. At least that's what I've interpreted from what I've read.

The biggest AC advantage ... when it gets down and dirty (hard pulls at low speeds), AC is much better. One shortcoming, AC is more expensive when purchasing a loco.

GaryG

[Kali]

The disadvantage of mixing them - this is a reach but the only thing I can think of - would be uneven low speed pull/retardation. Very easily overcome in software though.

[TrainMaster1]

Roads and crews for that matter would always prefer pure sets of one power type and even manufacturer if possible (due to transition variations). But in many cases you have a train to move and those are the locos you have right now. Get them up front and get out of town. It is not optimal all the time but you are here to make money ultimately and move the freight.

DC motors do tend to overheat at lower speeds. Each on comes with an LCS rating (Lowest Continuous Speed). Below that number you run the risk of frying the motors and damaging them. And yes DC motors generally need to be above LCS for dynamics to work. AC has many advantages for traction motors and moving heavy trains over grades (up and down).

Nick

[_o_OOOO_oo-Kanawha]

I don't think this has been mentioned before, but there is a distinct difference in the workings and therefor construction of the traction motors: AC motors are brushless. Their rotors are of the self inducing cage type, perhaps some electical engineer can explain their workings better than me.

So no risk of flash overs or arcing at the collector brushes, a major maintenance advantage, especially since DC traction motors were usually axle hung and hence unsuspended.
Perhaps today not so much as when high speed passenger running over reasonably well maintained track was commong in the 40's and 50's.

I presume in modern AC units on high adhesion trucks, the traction motors are fully suspended.

It has been asked in another thread on the AC vs DC subject: 4 axle vs 6 axle. Freight only US railroads still seem to favor 6 axle Diesel locomotives, while most European/Russian/Chinese electrics are 4 axle or double 4 axle loccomotives. 6 axles in two trucks are considered not suitable for high speed/passenger service. 6 axles in three trucks have been tried but the tracking/adhesion of the middle truck is often a problem.

[Kali]

For a squirrel cage motor ( most common current type ), it's pretty basic - which is good, because I hate studying electrics and I suspect I still might not have got this quite right:P; if you run three-phase AC current through electromagnets in a circle, you get a rotating magnetic field. If you stick a bunch of rods in the middle of that and attach them to an axle, then the rotating magnetic field induces current in the rods - that then interacts with the magentic field to produce a mechanical force, like any other motor, so it tries to spin the axle as fast as the magnetic field is rotating, although it never quite manages it ( that's slip ).

Where it becomes usefully different to DC motors is here; the reaction force depends on how strong the magnetic field is - current - like a DC motor, but the motor speed is controlled by the frequency of the AC supply which has nothing to do with how much of it there is. Therefore you can precisely control how fast the wheel turns - very handy for a rail application & explains the much better adhesion of AC units - totally independently of effort, at low speeds at least - and also control the effort independently of speed too.

Alstholm have built some motors with permanent magnets which are even simpler, but I think they're heavier and a bit bigger.