Earlier we had a discussion about present day locomotives. Oops no diesel is a locomotive, a motor (yes) but I'll save that argument for another day, time and location. If TB will allow me to download a video by Jaw Tooth., from You Tube. In the approximate middle of the video, you will see a helper unit go by. On the cab it is marked as DC to AC. Proving it can happen. Railroads will shop their engines/motors and rebuild them swapping them out from DC to AC. I did say that earlier but someone had to make some noise about it and prove me ...well...it is what it is. Just like us hobbyist, in our changes from DC to AC with DCC. Did you get that? The railroads are doing something similar with the DPU's and remote control. I'm not sure who is keeping up with who? Despite everyone else knowing more then I do. LOL !! I'm laughing at my own statement. There are those who do know more then I do. Especially those who follow NS and keep up with what they are doing. Surprised they didn't jump into the previous discussion. That or I missed it. Here it is: Now don't go bother McGoo over in Bakersfield about this. He doesn't see NS go through Tehachapi and Bakersfield that often. It works. The video plays, nicely I might add. I love it when something comes together as planned. Don't rue the day. Just enjoy it.
In model trains, it was AC to DC to DCC (which really is AC to DC). And, as an aside, I linked to a MRL video which featured SD45s! Also some SD35s! It sure is neat to see/hear them still operating. Doug
... and rather than "Radio Equipped" as made famous on Erie locomotives and cabooses, DCC users can instead use "Decoder Equipped" decals.
CN sent off some of their C44-9W's back to Erie recently for the same treatment. Waaaaaay cheaper than a new AC loco, and they come back as good as new, a lot better than they were on DC traction just on pulling power alone. That's what I do when the DCC gizmo refuses to work on my DC layout. Oot ye go!!! Nice video.
IINM, 1:1 AC locomotives use a diesel prime mover to spin a DC generator. Then AC power is created from said DC by power electronics to run AC traction motors. AC traction motors provide improved torque/slippage control with improved efficiency, and much less maintenance. Don't forget, it was the greatly reduced maintenance of diesel (DC) locomotives over steam locomotives that doomed the steam engines.
Here is a write-up on AC vs DC traction locomotives by Republic Locomotive: https://www.republiclocomotive.com/ac-traction-vs-dc-traction/
Earlier locomotives used DC generators, but anything now is equipped with an AC alternator. DC units have the AC rectified to DC for use in the traction motors. AC traction motor-equipped units have AC alternators which are rectified to DC and then use inverters to convert the power back to usable AC.
The big advantage of AC traction is the ability to control not only current going to the traction motors but also the frequency. Synchronous AC traction motors can have their rotation speed changed by changing the frequency of the AC power but leave the current alone. I messed around with a synchronous AC motor back in my college days, early 1980s, and saw that it always stabilized at the same speed, given 60 Hertz. Varying the frequency changed the RPMs but current draw was more or less constant. Controlling both current and frequency makes it possible to improve tractive effort by increasing the adhesion. The less the axles slip, the more tonnage can be pulled. Remember where it all started: CP 4744, MLW M640 4000 hp locomotive, with 18-cylinder Alco 251 engine. After a fire in the early 1980s, it was rebuilt by CP, MLW and Brown-Boveri with AC traction, the first diesel-electric to have that. It went from six DC motors (C-C) to four AC motors (A1A-A1A), and kept its 4000 hp rating.
And clocks. Like those big wall clocks we had back when I was in grade school and high school (plus or minus 50 years ago...). Around here, if there's one constant about electricity, it's the accuracy of the 60 Hz frequency. Those clocks kept good time.
Large AC motors are also much cheaper to purchase and rebuild than DC motors. AC drive technology has made amazing advancements in industry too. In my work, we converted large scale machinery drives to AC and replaced the AC drive systems several times over the decades to keep pace with the latest technology. Yes, it's all pretty cool stuff.
Hi All! I know a couple of Locomotive maintainers and here is what they say.... DC generators were used at the start of diesel electric locomotion. But as the HP grew, the electrical system need to grow more complicated to ensure proper transitions as the current grew. If they were not constructed correctly the electrical systems were prone to "flashovers" causing havoc with the electrical systems. GP-35s had a tendency to flashover if not maintained correctly. AC alternators were then used as they would not flashover. So the locomotive would then rectify the AC to DC for the traction motors. But if a DC traction motor overloads, esp. at high current and slow speed they would "birdnest" where the windings would come apart. EMD would limit current at slow speed, where as GE and Alco did not at first. AC has several advantages. The motors can be used to hold a train on a grade without birdnesting the traction motor. The Alternators do not flashover, and the AC components seem to have a longer life between needing overhauls. In AC locomotives the AC power is converted to DC for control, then inverted to AC for the traction motors. EMD uses 1 inverter ( converting DC to AC) per truck. Simple design but lose an inverter you lose power to the entire truck. GE uses one inverter per axle, so if you lose and inverter you only lose that axle. The main complaint engineers had with AC at first is that AC is slower to "Load UP" than DC Hope this helps... Bruce
The "birds nesting" that I know of are/were from TM's AC or DC, over-speed where the windings would come apart upon spinning too fast too long. DC TM's when operating under heavy loads in the "short time rating" would burn the commutators and brushes when severe arching took place when turning too slow with too much amperage to same. AC TM's can be under heavy load down to stall without severe damage. Late model DC units have thermal protection, that basically de-rate the amperage to the TM's to prevent severe damage. Some rebuilds of older units had these protections added, SP had what they called "Maxitrax equipped" units that allowed DC units to be ran with AC units in heavy drag type service. Some other carriers had similar equipped units. I was never really impressed with these units as they basically de-rated themselves into switch engines and didn't pull much, they sounded like they were wrinkling the rail behind them, but look at the amp meter and it was way down.
I always wondered why locomotives switched over from DC to AC, but never got around to looking it up. And now I know the ins and outs of AC vs. DC. Procrastinate and ye shall eventually receive anyway!
AC motors are much cheaper to manufacture and repair than DC motors, and AC drive technology has made extraordinary advances in the last few decades. ACs are/were said to be more slippery, but I'm not sure if that still stands.
With the newest of the newfangled computerized traction control systems engines have incredible tractive effort and adhesion figures that would have been unimaginable in the time of the first generation diesels. Motors can be controlled by either current or frequency changes in the power supply. It's very hard to make them slip now - but that being said, someone somewhere always manages to do it...
