What is the advantage and disadvantage of both?
Do you think AC will be the only thing available in the future?
Other than AC has better low speed control, and has no copper windings to ‘bird cage’? DC traction motors do not cost as much and for high speed service, they work just fine. As AC costs go down, I would suspect we will see more AC traction motor locomotives.
GE offered a AC ‘A-1-A’ trucked solution to BNSF last year for the same price as C-C trucked DC units. I have heard nothing about the results of the locomotives BNSF bought, but I have heard nothing bad from the field. My understanding is that the units were wired for 6 AC traction motors, but only have 4 invertors/traction motors, and GE claims they can be upgraded to full C-C units at some later point if desired.
Jim Bernier
Cost of AC motors and control equipment will come down as more are built. Advantages for the AC motors are lighter for an equivalent power, and AC motor adhesion control is much more effective, and no commutators to wear and need replacing.
The following are the advantages:
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No commutator or brushes to need maintenance.
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If the pinion breaks or something, the motor will essentially continue on at its current speed instead of over revving and blowing apart (birdcaging).
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Easier to seal and cool the motor.
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More horsepower and torque is possible from a given volume of motor. Traction motor space is very limited.
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More precise control of drive wheel speed which can increase available tractive effort.
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The motors themselves are actually simpler and less expensive with only two bearings and one moving part.
The main disadvantages are:
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More complex electrical and electronics gear with inverters and so on.
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More cost because of the extra cost of the drives.
IMHO, DC drive technology is very mature without much room to grow. AC drive technology is much more open to expansion. As the cost of the drives comes down it is likely that AC drives will become more predominant, especially on heavy mainline locomotives. Current high horsepower variable frequency drives (inverters) are about 1/10 the cost they were 15 years ago.
I suspect that the cost of high power inverters will continue to decline. Current state of the art are IGBT inverters with induction motors, 5 to 10 years down the line we may be seeing GaN or SiC FET’s driving permanent magnet synchronous motors. The new FET’s should result is smaller inverters due to faster switching times leading to less heat being dissipated in the active devices. A permanent magnet synchronous motor can produce at least 25% more torque for a given frame size than an induction motor, as well as being more efficient at low speed high tractive effort conditions. Cooling will be even simpler than with the induction motor as very little energy will be dissipated in the rotor - it might be simplest just to use water cooling.
- Erik
Some advantages noted are a comparsion of AEM-7DCs vs AEM-7ACs. To meet AMTRAK acceleration specifications for the NEC; they usually limit DCs to 7 - 8 cars and ACs to 10 cars.
But as mention in the above post the abilty to seal the motor is the most important one. The thread about the CAL Z problems of running in the snow and having ground faults on the DC locos is very important. As a CEO of any RR that operates in snow country I would want all my trains to operate in snow without grounding out. The severe winterwe are having maybe many RRs will change their orders to AC and order a much higher percentage of ACs. The reduction of inverter costs may also be a factor?
I think the only Class 1s that are ordering DC 4000-4400hp 6-axle GE and EMD locos are CN, NS, and BNSF, though BNSF has a large AC-traction fleet too. UP, CSX, KCS and CP have seem to gone exclusively with AC with recent orders. Mostly DC-traction NS has a handful of former-CR SD80MACs and 24 ES44ACs, while CN has no AC-traction whatsoever. Even regional Class 2s IAIS and MRL have AC-traction on their rosters. And the Indiana RR now has SD9043MACs too.
The Chicago Transit Authority is also beginning to go with AC traction motors, some other transit operations may be doing the same thing.
I think they generally only need to power one truck when they do this. There’s enough adhesion provided to achieve the acceleration they need for service.
That seems to be a good idea for CTA since they do have some winters with a lot of snow.
Yes - it’s already happened in Europe - as far as I know, the big rolling stock suppliers on this side of the pond (e.g. Bombardier/Alstom/Siemens) only offer AC traction on their light rail/EMU/locomotive/HSR products.
I’m sure EMD & GE are only continuing to build DC equipment because some US/Canadian railroads still want to buy it, not because they believe it has a long-term sales future.
Tony
Could it be that the tax situation in the US and Canada (CNRR esp) is different than Europe.? Probably ACs will increase in price less than DCs. There also could be a sudden closing the price gap if some component(s) becomes much cheaper . Maybe when the price of AC locomotives gets to a certain point closer to DC locos then all RRs will order all AC lcomotives? At that point the remaining DCs will slowly become orphans with the DC specific item costs gradually esculating faster than the other loco costs. Have no idea what this year’s awful winter snows will do to future equipment decisions? May come to the point where in winter DCs will be sent to south away from snow storms.
GE has been implying of late that they would like to only offer AC power, the new A-1-A 4 motored ES44ACs for BNSF are being touted as a replacement for 6 motored DC units…
BNSF has covert their ES44AC to 4,000HP from 4,400.
Thank you all for the great responses.
After searching Google for AC traction motor- images, I’m surprised. What i saw was nothing more than a squirrel cage induction motor, although with a rather massive shaft. I expected a wound rotor motor, which I had been told was in use in Europe. Has the wound rotor and its rotor current limiting device been made superfluous by solid state electronic controls?
Yes.
The advantage of a wound rotor motor was that it would give both high starting torque and high running efficiency for operating at a fixed line frequency. The disadvantages are that it is heavier, more expensive and less rugged than a squirrel cage motor.
With a variable voltage variable frequency drive, the drive frequency can be set low enough to ensure good starting torque (peak torque is a function of slip frequency for a given motor design).
My understanding is that most ac traction motors currently in use are not true synchronous motors completely dependent on the input electrical frequency for speed control. The dissadvantage of such motors is that an instant overload where there is insufficient torque means loss of all power. What I have been told (and correct me if I am wrong) is that they are “hysterises non-synchronous” motors. The operating principle is pretty much the same, but the difference in construction is that the rotor bars are not perfectly horizontal but are slanted, all parallel of course, but not at right angles to the direction of motion and not parallel with the rotor shaft.
The great advantage over wound rotors is huge ability for high thermal overload allowing short-time overload ratings to be longer in time and larger. This and the absence of brushes associated with both dc motors and ac-commutator motors (GG-1, EF-3, etc.) drastically reduces maintenance requirements.
Really? I’m aware that some of UP’s GEVO fleet has a software system that reduces tractive effort when the units are used in DPU mode but I didn’t think they had derated the whole fleet…
Thank you all for the responses. I didn’t know this kind of traction system could be so interesting.