• Dual Mode: 3rd Rail + Catenary locomotive

  • General discussion about locomotives, rolling stock, and equipment
General discussion about locomotives, rolling stock, and equipment

Moderator: John_Perkowski

  by docsteve
Adirondacker wrote:People stopped using GTOs. IGBTs are easier to control. I seem to remember they are more efficient too.
There's at least three ways to skin the cat. Taps on the tranformer, switching the inverters between serial and parallel operation and designing the AC-DC-AC magic to operate on a variety of voltages. I'm sure there are others. It's not 1915 anymore.
You are correct, and just for the uninitiated, GTOs are Gated Turn-On devices, IGBTs are Insulated-Gate Bipolar Transistors, both of which may be considered decedents of GE's late-1950s SCRs, Silicon Controlled Rectifiers.

The point that I see here is that it is also not 1935, either. The Pennsy's AC motors in the GG1 used transformer taps to for varying the voltage to control motor speed. This is a sloppy way to control AC motors (but in 1935 it was state-of-the-art), as it still uses brushes in the so-called universal motor. Rather, a purely AC motor, an induction motor, has no need of brushes, and it uses frequency to control speed. That is where the AC-DC-AC magic comes in, which means whether you have 25 Hz or 60 Hz you just rectify it and then invert back to AC, varing the frequency to control the speed (noting that voltage also needs to be controlled as the motor has higher impedance at higher frequencies). At this point, it doesn't really matter if the input is DC (going right to the DC-link) or AC (let's design it so that the rectifiers produce 700 VDC to match 3rd rail voltage). Using DC motors, it is just AC to DC, and vary the voltage to control the speed.

Off hand, I don't see any need for using a transformer-based system in any locomotives operating in AC distribution systems (whether or not the motors are AC or DC). I don't know: are there any reasons to stick with a transformer?

  by Adirondacker
docsteve wrote: Off hand, I don't see any need for using a transformer-based system in any locomotives operating in AC distribution systems (whether or not the motors are AC or DC). I don't know: are there any reasons to stick with a transformer?
I'm not an electrical engineer. I'm gonna assume the people with decades of experience at Bombardier and the other vendors know what they are doing.

Transformers are durable and efficient. They help even out the transients that the inverter produces. I can imagine all sorts of other goodness that comes from using sophisticated transformers ... but I'm not an electrical engineer.
  by EDM5970
As early as 1964, the French had a locomotive, the 40100 class, that would run on four different voltage and frequency combinations. They were 1,500 and 3,000 VDC, 15,000 VAC at 16 2/3 cycles, and 25,000 VAC at 50 cycles. These units had four pantographs, one for each power supply, but (for sake of this discussion) a set of third rail shoes could replace the pantographs for DC service. Designed for international service, they rode on six axles; the transformer supplied AC to silicon rectifiers, which then fed DC to the traction motors. Running with a DC supply, the transformer and rectifiers weren't used.
  by docsteve
Just thinking (two months later),

(1) Transformerless power supplies would be referred-to as switching (or switched-mode) power supplies, using -- in this application -- high-current solid-state devices.

(2) There are 1:1 transformers, isolation transformers, that can be used to physically (but not electrically) isolate one circuit from another; nevertheless, we have to assume that the line voltage, feeding the local distribution system, is well regulated, just like local systems are from the local electrical utility, so, for example, a 25,000 volt system is reliably delivering a well-regulated 25,000 volts (caveat: if the load gets too great the current will plateau and the voltage will begin to drop). Transients should blow the breakers on the distribution system, as happens when the home lights flicker during a thunderstorm. It is certainly possible to take an arbitrary AC and rectify it, transformerlessly, [the DC link] and then invert to an AC to feed to AC motors (or just feed the DC to DC motors).
  by JayBee
BandA wrote:MBTA blue line subway cars have or had catenary + 3rd rail

How big is a transformer for these electric locomotives, and how much do they weigh? I can't wrap my head around why electric locos aren't much smaller than diesel-electric which has to carry a big diesel + generator

Part of size is due to cooling systems and the desire for a certain amount of weight to increase pulling power(not horsepower)

For an example of an electric locomotive with light weight, Rhätische Bahn Ge 4/4 III locomotives work under 11kV/16.7 Hz cantenary produce 3220 hp. continuous using 3-phase AC traction motors and weigh just 62 metric tonnes(68 US tons). Dutch is correct, the heaviest part of any electric locomotive is the transformer.