I am looking for an explanation of the differences between AC and DC power. AC power is now dominant with the newest locomotives. What are the advantages and disadvantages of each?
There are some older threads that discuss this subject very thoroughly, but in summary:
Initial gas-electric railcars, then diesel-electrics, the first streamliners, switchers, the EMC-EMD FT, all diesels through WWII and into the "second generation" used dc equipment. This meant dc generators and dc traction motors. This meant brushes and commutators which are maintenance item, and lots of relays and contactors for control.
When solid-state high-power-handling silicon rectifiers were developed to replace mercury arc rectifiers and rotary converters, diesel locomotive builders immediately took advantage of them to replace dc generators with alternators, which do not require brushes or commutators, and which produce ac power which is rectified to dc for propulsion applications. But such locomotives are still referred to as dc locomotives because they use dc motors. Still, the use of ac generators (alternators) was a step forward in control simplification and increased reliability.
With the development of computer technology, precise control of ac hysterises or squirel-cage motors became available, and the rotating bars of such motors can withstand far higher heat the the coils of the rotating armetures of dc motors. So for equivalent horsepower and weight and gearing, a locomotive equipped with such ac motors can lug at far higher current and thus torque and tractive effort (but with the same horsepower at much lower speed) than the same kind of locomotive equpped with dc motors. In addition, the maintenance requirements for brushes and commutators is eliminated entirely. I know of no advantage of dc locomotives over ac, except possibly lower cost, and this may evaporate in the future. AC locomotives are easier to maintain and can provide higher starting and hill-climbing tractive effort. They can eliminate helper districts and at the same time give maximum efficiency when used as helpers.
Except for upgraded versions of the PCC and other replica equipment, all light rail, electric commuter, and rapid transit equpment is also built with ac motors today. Ditto high speed trains and electric locomotives.
Great explanation.
If I may elaborate, the first generation of "AC" locomotives had only an AC "generator" (it is called an alternator as stated above). "Passive" electronic devices in the form of high current silicon diodes were the enabling technology.
The second generation of AC locomotives, what we now mean by a proper AC traction system, requires active electronic devices, much like the transistors in all of your consumer electronic devices but operating at much higher current levels. The AC applied to the motors needs to be at a frequency at which the current alternates to closely match the motor rotation speed to get the best torque and efficiency. So the AC from the "traction" alternator is first rectified using silicon diodes to turn it into DC as on the first generation of what people called "AC" locomotives, and then a high current electronic circuit turns that DC into AC of a different frequency than coming from alternator, a frequency that the control system changes as the locomotive varies in speed.
The electronic circuit that produces the high-current variable-frequency AC is called an inverter, and the concern regarding purchase cost of a true AC locomotive having both an AC traction generator (called an alternator) and AC traction motors has to do with providing that inverter. I am told that the cost of high current inverters is coming down with advances in the semiconductor electronic devices required to rapidly switch on and off current to make AC.
I am also told that the GE AC locomotives have a separate inverter circuit to supply the traction motor on each axle whereas the EMD AC locomotives have just one inverter for each set of traction motors in a truck (bogie). Having one inverter per truck might save on cost, but that means the wheels in each truck are made to turn at the same rotational speed: this means the wheel shop needs to make sure that each wheel in a truck is the same diameter after "wheel truing" -- putting the wheels in a lathe to make sure they have the correct profile for proper operation. This had not been needed with DC locomotives and with GE'S AC locomotives.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Thank you very much. Not being very mechanically oriented it make perfect sense. I remember when I was a kid my dad had a car with a generator versus an alternator. when the rpm's died down at a stop light the lights would dim. His next car with an alternator did not have the same issue. I guess that equates to DC versus AC.
As a train fanatic I love the magazine and the forums. I fulfilled one of the items on my bucket list this past summer, riding on the UP Denver Post steam train from Denver to Cheyenne and back. I even got to sit in a dome car, what a great experience. Next up, a ride in the cab of a diesel locomotive.
All the best.
Thank you!
One inverter per truck sounds like an electronic version of the side-rod or chain drive used on GE 45-tonners, which had one traction motor per truck.
The key to successful AC propulsion was the development of the "Gate-Turn-off" Thyristor. The early high-current thyristors required a separate subsystem to reverse polarity and force the thysristor to off position. I managed the first demonstration of AC propulsion on the Cleveland transit system in 1973, a. WABCO effort with funding from UMTA. The demo was a technical success but WABCO was forced out of the business by outside forces that I won't identify.. I felt we had left the CTA, a longtime customer of our brake equipment, out to dry and left the company in disgust. A few years later, the GTO Thyristor appeared, making the task much easier as well as less costly and the rest is history.
UMTA Reports "UMTA 0H-06-006-73-1. Single Car Operation, Demonstration of DC to AC Mass Transit Propulsion and UMTA OH-06-0006-73-1, Revenues Service may still be available
A good point was made in an eralier post that General Electric's AC traction motor equipped locomotives use one AC inverter per traction motor while EMD's current (no pun intended) AC drive offerings use only one inverter for each truck (i.e 2 inverters, each provides AC power for three traction motors).
Apparently the one inverter per axle system offers some advantages, because EMD is developing their own version and has demonstrator locomotives buit, the SD70Ace P-6 (6 inverters to 6 motors) and the SD70Ace P-4 (4 inverters,4 traction motors with one axle in each C truck unpowered).
Presumably that will be standard equipment for EMD in the near future.....
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
carnej1 A good point was made in an eralier post that General Electric's AC traction motor equipped locomotives use one AC inverter per traction motor while EMD's current (no pun intended) AC drive offerings use only one inverter for each truck (i.e 2 inverters, each provides AC power for three traction motors). Apparently the one inverter per axle system offers some advantages, because EMD is developing their own version and has demonstrator locomotives buit, the SD70Ace P-6 (6 inverters to 6 motors) and the SD70Ace P-4 (4 inverters,4 traction motors with one axle in each C truck unpowered). Presumably that will be standard equipment for EMD in the near future.....
Like it or not, parts on anything electrical or mechanical will fail. When inverters fail, on the EMD you lose 1/2 your tractive effort as you have now lost power to one complete truck. When the inverter fails on a GE, you only lose power to one axle. In many cases the five remaining working axles can pull almost as much as the formerly 6 working axles.
Never too old to have a happy childhood!
Locomotives with ac generators (alaternators) and dc motors might be considered first generation ac locomotives, but today they are called dc locomotives, with only the locomotives using ac motors considered ac locomotives.
Note again that the ac from the alternator is recrified to dc and then by gated thyristors or power transistors inverted to ac with the frequency precisely controlled to just be slightly ahead of the motor rotational speed, and the phases to the various field coils precisely controlled. This is also true of ac electric locomotives, today, and ac mu cars, with the ac from the catenary rectified to dc first, after a transformer has brought it to the required voltage.
One more way of thinking about this would be a DC locomotive accomplishes commutation in the motor using the physical motion of the motor to time the polarity switch (commutator and brushes). An AC locomotive accomplishes commutation using solid state switches (an inverter) external to the motor and adjusts the timing of the polarity switch using a computer (tries to maximize power by adjusting frequency)
This give the AC locomotive an adhesion advantage over the DC locomotive. The control for power on an AC locomotive is the frequency of the switching of the inverter. The control system can do this almost instantaneously. On a DC locomotive, it's voltage supplied to the motors, but those motors, with their coil windings, are big inductors, and inductors try to maintain constant current. It's the current that's making the rotating force, so any adjustment to power is very slow compared to AC.
On a bad slip, the DC motor will tend to run-away and overspeed, the AC motor will just drop power and spin at near synchronous speed to the frequency being fed to it.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
carnej1Presumably that will be standard equipment for EMD in the near future.....
EMD copying GE. The end must be near!
Some miscellaneous comments:
The real impetus for using traction alternators instead of traction generators was that 2500HP was about the limit for for a generator that would fit inside a hood unit. The GP-35 went through great lengths with field shunts to allow EMD's traction generator transmit 2500HP. Traction alternators only became practical after the development of high current silicon rectifiers as ignitron rectifiers ad a much higher voltage drop for a given current rating.
Jerry Pier was right about gate turn-off (GTO) thyristors making AC motor drives practical. An inverter could be made with standard thyristors, but the commutation circuitry added a lot of cost and complexity. While the circuitry for the GTO is simpler than a normal thyristor, the commutation circuiry still requires a hefty current to turn off the thyristor. Related to his post, Wabtec ha demonstrated a pulse width modulation inverter for traction motors in 1969.
EMD used a single inverter per truck as the Siemens inverter used GTO's and it was more economical to have a single inverter per truck as single GTO's could handle the voltage and current needed for the inverter. GE went with IGBT's for their inverters, which initially had to be paralleled to supply one motor. It was therefor simpler to use a separate inverter per motor (axle).
- Erik
I know that BNSF must love the ES44P4's, with over 500 rostered. BNSF is running them on all of their trains from Z trains to coal drages. I wonder why haven't other railroads purchased any?
Ira
If I were ordering power for "my" railroad, I would want one inverter per axle, because keeping wheel diameters exactly the same is more expensive and complicated than maintaining additional inverters. It means inspecting and truing wheels more often, and can thus acually shorten wheel life. Rod connected wheels as on steamers are different, in that there are self-equalizing friction forces at work that do not exist with individual motors per axle.
erikem EMD used a single inverter per truck as the Siemens inverter used GTO's and it was more economical to have a single inverter per truck as single GTO's could handle the voltage and current needed for the inverter. GE went with IGBT's for their inverters, which initially had to be paralleled to supply one motor. It was therefor simpler to use a separate inverter per motor (axle). - Erik
Early AC4400CWs had GTO inverters. EMD 268, its 3800-hp AC-traction research vehicle (converted from an SDP40F in 1988), had an inverter for each of its six traction motors.
I mentioned rod-conected drivers as not having the wheel size issue. Also, of course, gear-connected drivers.
For many years the French buiilt "monomotor" trucks for their B - B electric and diesel locomotives. Each truck had one motor mounted logitudinally, driving an axle at each end through hypoid or worm gears. Dusseldor Wagon Fabriken (Sp?) or Duwag, built thousands of European streetcars this way.
Any currrent practice?
JayPotterEarly AC4400CWs had GTO inverters. EMD 268, its 3800-hp AC-traction research vehicle (converted from an SDP40F in 1988), had an inverter for each of its six traction motors.
I remember GE engineers at Erie telling us that the GTO inverters were merely beefed up versions of what they were providing for their AC transit propulsion system. As I recall, the "beefing up" was primarily figuring out cooling for them.
GE's use of transit-car electrical components in its first AC4400CW was mentioned in the article "GE May Be Pulling Ahead of GM in Locomotive Race" that appeared in the 12-23-93 edition of The Wall Street Journal.
And its latest software for regulating wheel creep in the extremely low speed range is derived from software that it developed for transit cars. So I guess that GE cares more about how technology performs than it does about the origin of the technology.
The EMD ACeP-6 was the result of a dormant customer stating that they will not purchase EMD locos with one inverter/truck. I don't know if the demos resulted in any orders from that customer.
The Siemens AC drive is far more complicated than the MELCO drive that replaced it. There is a separate
Siemens cabinet in addition to the 'normal' EMD electric locker. On the MELCO drive, all of the components (save for the inverters) fit into the same size electric locker that would be applied to an SD70M-2. There are probably some economies of scope involved, as CAT uses MELCO inverters in certain earthmovers. MELCO equipment is IGBT-based.
With that said, the loss of one inverter on a 70ACe does not reduce HP by 50%. A more accurate figure would be in the range of 30-40%. In reality, the MELCO equipment has proven to be very reliable.
CPM500
interesting. While EMD was still in the GM family, before the GM "problem," I made the suggestion the EMD enter the electric commuter car, rapid transit car, light rail car, and electric bus business, so that GM could show broad support for USA transportation and not just sell cars and trucks. We know EMD did assemble the Amtrak AEM7's and I believe they still a Swedish or other European affiliate that does build light rail cars.
beaulieu I think the reason for the one inverter per truck was to reduce the control complexity of the locomotive. Remember the EMD SD60MAC was built well before the first GE AC4400CWs. Control software was and is the bane of modern locomotives. I believe that once GM got deeper into financial trouble R&D funding to improve the SD70MACs dried up. The only reason there was any money to develop the SD70ACe was that without it EMD would have had minimal value to a potential buyer. Siemens had a Eurosprinter running with one inverter per axle and using IGBTs rather than GTOs before GE did with their prototype AC4400CWs.
I think the reason for the one inverter per truck was to reduce the control complexity of the locomotive. Remember the EMD SD60MAC was built well before the first GE AC4400CWs. Control software was and is the bane of modern locomotives. I believe that once GM got deeper into financial trouble R&D funding to improve the SD70MACs dried up. The only reason there was any money to develop the SD70ACe was that without it EMD would have had minimal value to a potential buyer.
I do remember that EMD was demonstrating and selling AC motored locomotives before GE. My recollection was the SD60MAC being available before IGBT's large enough for locomotive use were being produced. As for EMD sticking with the GTO inverter design, there's a bit of an advantage of keeping commonality with the existing fleet, both for EMD and for their customers.
I suspect the next advance in AC drive technology will come when SiC MOSFET's are packaged in modules large enough to handle locomotive inverter duty. SiC technology should result in smaller and lighter inverters.
P.S. One advantage of the IGBT over the GTO is a higher switching frequency, a few kHz vs a few hundred Hz. This allowed for PWM operation over the whole range of motor speeds, which simplifies the design of the inverter. OTOH, GTO's are available in large sizes, Powerex has one rated at 4,000A and ~4kV, a rating that almost no IGBT even comes close to. The one exception is a Westcode device.
daveklepper interesting. While EMD was still in the GM family, before the GM "problem," I made the suggestion the EMD enter the electric commuter car, rapid transit car, light rail car, and electric bus business, so that GM could show broad support for USA transportation and not just sell cars and trucks. We know EMD did assemble the Amtrak AEM7's and I believe they still a Swedish or other European affiliate that does build light rail cars.
The current European affiliate is Vossloh, and they do build electric transit equipment. They also designed the body for EMD's proposed F125 passenger diesel.
http://www.vossloh-rail-vehicles.com/en/home/home.html
It's not so much a question of matching the inverter's frequency to the motor's RPM as the inverter frequency causing the motor to turn at a specific RPM in response to its frequency.
The AC traction motors are simple induction motors with one moving part and no brushes or slip rings. They turn at an RPM that is determined by the incoming power frequency. This happens pretty much regardless of the load on the motor (I know that for you pureists there is some electrical slippage with increased torque). This will happen within the acceptable load range of the traction motor and power source.
These locomotive drives are an outgrowth of similar industrial variable frequency drives that started appearing at lower voltages starting in the early 70's.
At any rate, the inverter drives and AC motors are far more controllable and reliable than DC motors and drives.
daveklepper Locomotives with ac generators (alaternators) and dc motors might be considered first generation ac locomotives, but today they are called dc locomotives, with only the locomotives using ac motors considered ac locomotives. Note again that the ac from the alternator is recrified to dc and then by gated thyristors or power transistors inverted to ac with the frequency precisely controlled to just be slightly ahead of the motor rotational speed, and the phases to the various field coils precisely controlled. This is also true of ac electric locomotives, today, and ac mu cars, with the ac from the catenary rectified to dc first, after a transformer has brought it to the required voltage.
Redore It's not so much a question of matching the inverter's frequency to the motor's RPM as the inverter frequency causing the motor to turn at a specific RPM in response to its frequency.
Each motor has a speed sensor-and ground speed is calculated by (in the case of EMD) a radar device.
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