Hi Folks!
In the Locomotive section of the Feb. 2016 issue of trains I noted the following, "With locomotive propulsion systems now standardized with A.C. traction, an option to have only four powered axles versus six is available" (p. 18). Why have three--and in some cases, four--axle trucks been preferred in the past, but according to this article there seems to be a desire for only two axles of a three axle truck being powered?
Thanks & God bless!
AC traction motors are more efficient. Having 4 traction motors is mechanically easier and less expensive to maintain than 6. However, the same number of axles are still needed to distribute the weight. Have you ever seen an FL-9? It was an F-9 that could run as a straight electric as well as diesel electric. That capability added extra parts and weight to the standard locomotive. Thus, the FL-9 had the standard two axle truck in the front, but a 3 axle truck in the back. Only 2 of those three axles were powered. The third one simply helped to support the extra weight.
RRR RailRetardedRev Hi Folks! In the Locomotive section of the Feb. 2016 issue of trains I noted the following, "With locomotive propulsion systems now standardized with A.C. traction, an option to have only four powered axles versus six is available" (p. 18). Why have three--and in some cases, four--axle trucks been preferred in the past, but according to this article there seems to be a desire for only two axles of a three axle truck being powered? Thanks & God bless! RRR RailRetardedRev
Only two railroads have ordered AC drive locomotives with that six axle/4 motor arrangement;BNSF and Florida East Coast.
The other North American Class 1 railroads continue to order CC units;six axles and six traction motors....
BNSF wanted an AC traction locomotive with the performance and cost to purchase of a DC motored 6 axle, eliminating two traction motors met their price point/performance requirements.
FEC is a basically flat railroad so they feel 4 motors are good enough.
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
Thanks so much PAJR; I get it. You also answered a question I hadn't considered asking, namely about the FL-9 wheel arrangement. When I saw a picture of one it looked a little odd to me, but there was nothing in the caption that would have lent a clue regarding the arrangement.
Thanks so much CARNEJ1; you provided a good insight into the mind of BNSF & FEC buyers at least in this matter.
My wishes to you & PAJRR (sorry I misspelled your handle in my reply to you) for blessed, safe railroad endeavors this year.
I'm looking forward to learning more from all you RR savvy folks in the future!
The BNSF locomotives with four powered axles also have a mechanism to lift some of the weight off the dead axle and put more on the powered axles at low speed when more traction is needed. At speed all three axles on the truck are loaded the same to prevent track damage.
BNSF probably has the right idea about leaving the option of equiping the middle axles with traction motors. Inverter and AC motor technology has changed so much in the past few years that we may expect 6 axle DC to cost more than AC.
A local HVAC man informs that newer variable speed AC circulation motors and compressors are driven by either single phase or 3 phase inverters. What a change that is occurring daily with variable speed motors and their inverter supply. Efficiency is so high that they are getting smaller by not heating so much.
RRR RailRetardedRevWow. Sophisticated technology that, I'll bet it costs a fortune.
As noted in other posts here (and elsewhere) the general idea at the time the engineering was done was to secure an AC locomotive that would 'cost the same' as the equivalent 6-motor DC locomotive in production. (Presumably that is 'net' of the additional cost for the center-axle deloading mechanism).
Perhaps significantly, EMD's current version of the idea does not have any such mechanism.
It may be instructive to look at some of the comments on the Web about what services these locomotives are suited for, and where they are more trouble than advantage. Most of these are rather predictable, and they confirm basically what has been said here already: the retention of six axles is for weight, and for accommodation of the required length for 'common' production economy and certain "packaging" issues that require a longer locomotive than would conveniently ride on B-B trucks.
blue streak 1 A local HVAC man informs that newer variable speed AC circulation motors and compressors are driven by either single phase or 3 phase inverters. What a change that is occurring daily with variable speed motors and their inverter supply. Efficiency is so high that they are getting smaller by not heating so much.
The inverters almost certainly put out 3 phase, single phase induction motors don't take too kindly to wide variations in operating frequency. Going from a single phase inverter to 3 phase inverter needs 50% power circuitry and gets rid of all of the starting circuitry needed for single phase motors. These inverters can be run off of single phase or three phase.
With fixed frequency induction motors, there is a tradeoff between good staring torque and running efficiency. With an inverter drive, the starting frequency can be set low enough that a high efficiency motor can generate high starting torque. Modern inverters are efficient enough that the loss in the inverter is less than the running loss for a fixed frequency high starting torque motor.
There is continuing progress being made in the electronics used in high power inverters. The improvements in inverter efficiency translate into less cooling needed for a given power output, which then can lead to smaller and less expensive packaging. I suspect that cost per axle with AC will drop below the cost per axle with DC within a decade.
- Erik
Eric: Any idea if at this time the same HP DC motor costs more to build than the variable speed AC ? That certainly enters into the cost equation.
Streak, I would be surprised if a DC motor would cost less than the bare variable speed AC motor of the same HP if bought are bought new without trade-in. Add the value of a traded-in DC motor along with the cost of the inverter for the AC motor and that probably accounts for the price advantage of the DC motors.
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