Polyphase Power

I had been wondering about something lately, about the AC power on the M2/4/6s. With motors with huge horsepower they have to run on three-phase AC, but as there is only one wire overhead, the current that comes in is necessarily only single-phase. I'm curious to know how the train accomplishes that.

All New Haven Line MUs (M2/M4/M6) use DC motors powered by an AC to DC tansformer and have only 648 HP per car (4 motors per car).

Actually, the transformer doesn't make the DC, the rectifier does

Here's more to confuse everyone: The M7s have 3 phase AC motors, but the third rail is DC. Aren't power electronics fun?

capecodlocoguy wrote:All New Haven Line MUs (M2/M4/M6) use DC motors powered by an AC to DC tansformer and have only 648 HP per car (4 motors per car).

Oh yeah that's right! I forgot the M1-6 are DC traction.

Nasadowsk wrote:Here's more to confuse everyone: The M7s have 3 phase AC motors, but the third rail is DC. Aren't power electronics fun?

Yes, tremendously.

The fun thing about AC inverters is you can draw the basic circuit out on a napkin. Yes, the real thing tends to have a few more parts, but the heart of it's pretty darn simple:

* A converter to make DC - this is generally a rectifier, or GTO/IGBT setup. Pretty simple, really.
* A 'DC link', which is a big filter to get smooth DC at X voltage.
* A 'brake unit' or 'dump unit', this is a resistor and a transistor (on small inverters, it's litterally THAT SIMPLE, on bigger ones, it's pretty close). it presents a load on the DC link, when comanded.
* The inverter - this is little more than 6 IGBTs, or 6 groupings of them, in a so-called 'six pack' arrangement.

Industrial stuff, the brake unit's almost always an option. Just monitors the DC link, if it goes above X volts, it tries to keep it from rising anymore. Of course, any useful DC load can be inserted here, like other inverters (HEP!).

The converter puts out DC at X-Y volts, and shuts off above X-Y. That way the brake unit's not fighting it all day

The inverter just chops it up into AC, under control of the computer, which is anything from an analog circuit (yep) to a 32 bit with DSP and blah blah. The latter type can get <b>amazing</b> control, especially with an encoder. I've seen demos where the inverter plays music with the motor (FWIW, a class D amplifier is basically the same thing).

Somewhere, I had a pdf file with the main traction schematic of a recent Shinkansen series, somewhat simplified. There wasn't much to it..

if they did use poly phase that would make for one very Complicated Pantograph

Here’s how the Great Northern did it with trolley poles, 10th card, just like a trolley bus. The closest to pans was the bow trolleys used in Italy at 3300 volts. I’m not sure how the 2 sides are insulated from each other.

Bill

yeah that is interesting but for the most part for the all The high Speed Trains and Curves and Crossovers i think a modern day version of that would still be very complicated. and also when the wires break/wind/fall haveing 3 out of phase wires so close would really cause havok

There was a german electric coach, tested around 1905, that used three phase. The wires were vertically ordered, and there was pole mounted on the roof. Each wire had a mini-pan side mounted off the vertical pole. I don't think this would work outside of a test environment, as the pole was as tall as the coach and I don't understand how it would negotiate a switch. If I find a pic, I'll post or link it. However, the beauty of modern inverters and transformers is the ability to feed one phase in to three phase out, giving a cheaper traction motor and more controlable. Also, AC inverters are cheaper than DC inverters.