Adirondacker wrote:People stopped using GTOs. IGBTs are easier to control. I seem to remember they are more efficient too.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.
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.
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?
Steve Sconfienza, Ph.D.