Western Railroad Discussion > AC vs. DC Locomotives

From reading other threads, I get the impression that both AC and DC versions of locomotives are still being ordered/produced/delivered. I'm curious, what are the advantages to each, and/or why are both needed? Seems like I heard one was better for coal service; Can't recall if that was because of the slow loading speeds or what... I'm sure many of you can help explain why the railroads haven't settled on one or the other.
Thanks!
-Wes

Short simple answer is AC is best for coal/heavy loaded trains cause they can pull harder at lower speeds and can't burn out there traction motors.
JC

JasonCNW Wrote:
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> Short simple answer is AC is best for coal/heavy
> loaded trains cause they can pull harder at lower
> speeds and can't burn out there traction motors.
> JC

Then what is the advantage of DC that keeps them from making them all (all *new* locos) AC rather than having two standards?
Thanks for the reply,
-Wes

DRGW Wrote:
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> Then what is the advantage of DC that keeps them
> from making them all (all *new* locos) AC rather
> than having two standards?

They're cheaper, and work just as well as AC locomotives in applications that require horsepower but little brute force pulling power, such as on high speed intermodal trains.

Do all AC locos have AC traction motors?

That might sound like a dumb question, but I remember reading somewhere that there were some locomotives with alternators that produced AC that was then converted to DC.

colehour Wrote:
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> Do all AC locos have AC traction motors?
>
> That might sound like a dumb question, but I
> remember reading somewhere that there were some
> locomotives with alternators that produced AC that
> was then converted to DC.


late model GP38's and SD38's were offered with an optional AC alternator in place of the DC generator and these units were called GP38AC and SD38AC by EMD, the AC designating the use of an alternator. The AC alternator was standard on the DASH 2 line so there was no need to use a GP38-2AC designation. Likewise the MP15 switcher was offered with the choice of AC alternator or DC generator, with the resulting units classed MP15AC or MP15DC.
speaking in more recent terms (last 10-15 years) when you hear of an AC locomotive, it is regarding AC traction. SD70MAC, SD70ACe, SD80MAC, SD9043MAC, SD90MAC-H, AC4400CW, ES44AC, AC6000CW, yadda yadda yadda.

Bryan Jones
Brooks, KY
Bryan's Train Photos

Thanks guys!
-Wes

colehour Wrote:
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> Do all AC locos have AC traction motors?
>
> That might sound like a dumb question, but I
> remember reading somewhere that there were some
> locomotives with alternators that produced AC that
> was then converted to DC.

In addition to the list of locomotives and comments in the previous post, keep in mind that on AC traction locomotives, there is an alternator, but the power is first converted from AC to DC and then inverted back to AC. This is necessary because the raw frequency produced by the alternator varies with engine speed, and to get full traction, it is necessary to vary the frequency to be close to that of the traction motor speeds.

Thanks for the responses to my question. Now I understand why AC is converted to DC and then back again.

Now for another question: The NE Corridor uses AC current, correct? With those electric locomotives, does the frequency of AC traction current get changed through some kind of power converter to match the speed of the traction motors? Or perhaps the AC is converted to DC?

Depends on the equipment. Back in the day, the entire PRR fleet used AC traction motors but one must make it clear that these were motors very similar to DC motors (they had windings on the part that turns--the armature--and commutators and brushes to get the power there as well as on the part that doesn't turn), just optimized in the way it was built to take AC. Low frequency power (25 Hz) was used for this reason. This is completely different than the type of AC traction motor referred to on modern AC traction systems which is an induction motor and it does not have its advantages. In fact it has some disadvantages which is why there were locomotives on other lines which went to the trouble of changing AC into DC even when the only practical way of doing so was a huge motor-generator set aboard the locomotive. Nevertheless even the hugely successful and famous GG-1's used brush/commutator AC motors. (PRR did buy some surplus GN engines with the M-G system for pusher service when GN's electrification shut down.)

Later the PRR had some experimental units with a complex type of mercury vapor rectifier for converting AC into DC called "ignitron." Later on they ordered the E44's (C-C hoods) using this system--but not until after the Virginian had ordered a 3300 version, later sold to the New Haven. By the time the E44 order was nearing completion the technology had advanced and the last units were built with silicon diode rectifiers instead of ignitrons. The rest of the fleet was eventually converted.

The next level of technology used still involved rectifiers and DC motors but the rectifiers would be thyristers. If you know how a typical household light dimmer works...by controlling what portion of the AC sine wave the device is "on" for then you know how a thyrister locomotive works just on a bigger scale. The E60C's and the AEM7's work this way.

The newer system is AC traction which involves induction motors and they need AC power whose frequency varies with their speed, or rather, their speed varies with the frequency so the AC has to be developed by a computer-controlled inverter system. Inverters begin with DC so the AC from the catenary has to be rectified first.

The Acelas are AC traction as are some rebuilt AEM7's.

In all cases there is a step-down transformer to reduce the high voltage coming from the catenary. (On older engines like the GG-1 the transformer secondary had multiple taps which were switched to obtain varying voltages as part of the control system of the locomotive.)

In a seminar at the Mercantile Library in St. Louis a few years back, some one made a presentation on the evolution of diesel-electric locomotives. This also explains some of the growth in available horsepower.

First generation power had DC generators feeding DC traction motors. The higher the horsepower of the generator the larger the physical size of the generator. Apparently the generator size reaches a maximum for mounting in a locomotive at about 1500 HP.

The development of the silicone rectifier permitted the use of an AC generator (alternator) to generate electrical power which was rectified to DC for application to the DC traction motors. This system reached its size limitation at about 3000 to 3500 HP. I am not sure what changes allow current HP levels to be even higher.

The next development was the variable speed (frequency) AC traction motor. As noted above, AC power is generated with the diesel engine driving an alternator. This power is rectified to DC, but is then converted back to AC at a controlled frequency. The key difference here is the controlled frequency. The speed of the DC motor is basically uncontrolled. That is how we get wheel slip. When the resistance to the motion of the motor goes away, the motor spins freely. In an AC motor the speed of the motor is regulated by the frequency of the AC current. Therefore an AC traction motor avoids wheel slip on individual motors because all of the motors are turning at the same speed. For one to slip, all must be slipping. This difference is what allows a consistent application of the a higher percentage of the available power into motion, particularly at lower speeds.


Ron Zimmer

Let me ask the question another way....is their any DISadvantage to purchasing AC-technology (other than cost)? Is there more maintenance? Or, if there was no cost differential between AC and DC, would AC win hands down?

Thanks....

jofegan

jofegan Wrote:
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> Let me ask the question another way....is their
> any DISadvantage to purchasing AC-technology
> (other than cost)? Is there more maintenance?
> Or, if there was no cost differential between AC
> and DC, would AC win hands down?
>
> Thanks....
>
> jofegan

Maintenance, the additional DC-AC turn would be more parts with more things that could go wrong, shutting down a locomotive. I know you wish to ignore cost, but upfront cost is a good heck of a lot more for AC than DC. Considering some railroads cheaped out on buying isolated cabs for crews, the much bigger cost of AC when it is unneeded would definitely be avoided.

Also, on CSX there is a powered axle rule. No train, except for the Pope Creek coal trains in Maryland and the Q090-Q091 produce express train I believe, may have more than 24 powered axles. A DC unit is counted straight up (i.e a six-axle ES44DC is six axles), but an AC locomotive is counted at 1.5 axles per physical axle (a six-axle SD70ACe counts as 9 axles). This limits CSX to having at most two AC locomotives running on power on one train (9+9=18, which is good, but 9+9+9=27, not good). A DC locomotive may be added with the two AC locomotives (i.e. an ES44DC-AC4400CW-AC4400CW [6+9+9=24, good], which I mention because I have physically seen it). However this wastes the advantage of the AC motors, as the DC motor cannot operate at the low speed of the AC motors without burning up, so this would be used mostly on faster trains such as intermodals and auto racks, which wastes the slow speed capability of the AC motors. The best thing for the railroad is to buy AC for slower coal trains and things like that, and use the cheaper DC where the AC does not have advantages, such as over-the-road intermodals.

jofegan Wrote:
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> Let me ask the question another way....is their
> any DISadvantage to purchasing AC-technology
> (other than cost)? Is there more maintenance?
> Or, if there was no cost differential between AC
> and DC, would AC win hands down?
>
> Thanks....
>
> jofegan

If there was only a small difference in price, the AC locomotive would win hands down. The DC motored locomotive has higher maintenance costs.

The basic difference between AC and DC locomotives is that the commutation on the traction motors. As redneckrailfan and others pointed out all of the newer locomotives have alternators and produce an alternating current and then convert it to DC. The AC produced is not really the same as household AC as the frequency is dependent on the speed of the diesel engine.

There are a number of different ways to build motors but the common DC motors and so-called "universal" motors that are used in electric drills, vacuum cleaners, etc use brushes and mechanical switching or commutation to synchronize the magnetic field with the speed of the motor. As the motor turns the magnetic field has to stay ahead so it always trying to pull the motor around. AC motors use sensors to detect the rotation of the rotor and electronic switching to accomplish the same trick of keeping the magnetic field ahead of the motor. The other major difference is that the switched coils on the DC motor are on the rotor or the moving part while the switched coils on the AC motor are on the outside. There are several advantages to the so-called "AC" traction motors which are really just inside out electronically commutated DC motors. The electronic commutation is more reliable than the mechanical commutation and does not produce arcing and the coils that produce most of the heat are on the outside stationary part of the motor where is easier to cool them. This is why the AC locomotives can run at slow speeds with higher traction forces for a longer period of time than the DC locomotives. Traction force is directly proportional to the current running thru the motor and, unfortunately, so is resistance heating. Power is speed multiplied by force. So if the diesel engine produces 4500 HP, the power available stays the same regardless of speed. Therefore, at full power, as the speed drops the traction force goes up until either the motors overheat or the wheels spin. As the traction motor cooling is more effective on the AC motors, the AC locomotives can produce more low speed traction force for a longer period of time without overheating the motors. At higher speeds, the only advantage of the AC is the added reliability of electronic commutation. Overall, the only disadvantage of the AC is the added cost of the high power electronics required for the electronic commutation.

Different railroads have different needs and desires. BNSF is using AC locomotives for coal and probably grain trains but DC locomotives for intermodal, etc. I believe that UP is currently only buying AC locomotives and is using the new AC SD-70 ACes and GEVOs for intermodal and general freight use and the older AC locomotives for coal and grain trains. I am not sure about the other railroads other than I believe that Norfolk Southern is strictly DC.

"However this wastes the advantage of the AC motors, as the DC motor cannot operate at the low speed of the AC motors without burning up ..."

This isn't entirely accurate. The newer locomotives (I believe GE started with the Dash-8's, not sure about EMD) have built in protection that will derate the traction motors if they start overheating, so they can be operated with AC's in drag service, they'll just cut back their output if their minimum continuous speed is not being maintained.

sdrake Wrote:
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> Different railroads have different needs and
> desires. BNSF is using AC locomotives for coal
> and probably grain trains but DC locomotives for
> intermodal, etc.

BNSF buys and uses AC locomotives exclusively in coal service. They haven't been able to economically justify their purchase in other types of service.

> I believe that UP is currently
> only buying AC locomotives and is using the new AC
> SD-70 ACes and GEVOs for intermodal and general
> freight use and the older AC locomotives for coal
> and grain trains. I am not sure about the other
> railroads other than I believe that Norfolk
> Southern is strictly DC.

UP seems to consider that AC locomotives have better economics in all services, as they seem to use them indiscriminately, though they also do buy some DC traction locomotives. Their control of motive power doesn't seem to be as tight as BNSF. They and CP and the Mexican railroads are the only ones that exclusively purchase AC traction.

NS and CN have yet to buy any AC traction locomotives, though NS inherited the SD-80MACs along with their part of Conrail. KCS has been buying AC traction for use in coal services. Like BNSF, CSX has been buying both types, with AC traction being assigned to drag services.

run8 Wrote:
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> sdrake Wrote:
> --------------------------------------------------
> -----
>
> > Different railroads have different needs and
> > desires. BNSF is using AC locomotives for coal
> > and probably grain trains but DC locomotives
> for
> > intermodal, etc.
>
> BNSF buys and uses AC locomotives exclusively in
> coal service. They haven't been able to
> economically justify their purchase in other types
> of service.
>
> > I believe that UP is currently
> > only buying AC locomotives and is using the new
> AC
> > SD-70 ACes and GEVOs for intermodal and general
> > freight use and the older AC locomotives for
> coal
> > and grain trains. I am not sure about the
> other
> > railroads other than I believe that Norfolk
> > Southern is strictly DC.
>
> UP seems to consider that AC locomotives have
> better economics in all services, as they seem to
> use them indiscriminately, though they also do buy
> some DC traction locomotives. Their control of
> motive power doesn't seem to be as tight as BNSF.
> They and CP and the Mexican railroads are the only
> ones that exclusively purchase AC traction.
>
> NS and CN have yet to buy any AC traction
> locomotives, though NS inherited the SD-80MACs
> along with their part of Conrail. KCS has been
> buying AC traction for use in coal services. Like
> BNSF, CSX has been buying both types, with AC
> traction being assigned to drag services.


You can put KCS in the AC locomotives for all services category now. The 250 newest locomotives bought for KCS and KCSdeM have been AC traction. They have 150 more on order. Their last purchase of new DC motored locomotives were the SD60s delivered in 1991 or '92. They don't run that many coal trains. I also wonder if CSX switched back to DC just to increase the number of new locomotives on the roster, they had so many old leasers.

No one really questions the great advantages of AC traction. It's just that it's much more expensive so it seems that at least since its introduction into the North American railroad scene up until now it has only been found to be cost effective for heavy haul applications. I don't know if something like the SD-70ACe will change that. As I have said before I think it's curious that the railroads came to this conclusion yet transit operations find AC traction to be the way to go.

filmteknik Wrote:
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> I think it's curious that the
> railroads came to this conclusion yet transit
> operations find AC traction to be the way to go.

That's simple. New transit vehicles are getting away from simple resistor control, and moving toward power electronics. That means the difference in price between DC and AC traction is much smaller than for diesel-electrics. That smaller increase in price for AC is justified by increased reliability, lower energy cost, and lower maintenance cost. It's an easy choice.

A DC traction diesel-electric, on the other hand, has no power electronics, except for the rectifiers, and to go to AC traction requires the addition of the electronics package. That is where the price difference comes in, plus there is a slight fuel consumption penalty in higher speed operations with AC traction, further adding to cost.