AC traction: do 50Hz and 60Hz need different equipment?

[miyakoji]

I was having a look at the link below (2 more 2-car 521 series sets delivered, these from Kinki Sharyo), and I got to wondering if these were dual current, or actually triple current.  The venerable 485 series, for example, apparently runs on 1500VDC and 20kVAC at both 50Hz and 60Hz, while some other JNR-era equipment seems to be listed under one or the other.

 

I assume there has to be isolation in the catenary between two different frequencies, but must the train be switched when crossing from one zone to the other?

 

http://railf.jp/news/2015/02/07/191000.html

http://en.wikipedia.org/wiki/485_series

[katoftw]

switching mode tranformers can pretty much pick up a current from what it is being supplied and use it as long as the supply is within the tranformers limits.

 

a 485 built in 1964 would have the technology since it would have been around back then.

 

Much like a switch mode transformer for home devides, ie 100V to 240V, 50-60Hz. What every it is supplied, it just does it thing. The train's transformer is just on a bigger scale.

Edited February 8, 2015 by katoftw

[kvp]

Some older trains had to be manually switched between AC and DC, because in low voltage DC mode, you don't need the transformer in the circuit and it won't let DC through, 50 or 60 Hz doesn't mater that much as long as it's the same voltage. For modern switching mode power supplies, there are two ways to get it down to 1500V DC. One is the classical way of having a transformer between 25000V and 1500V and then rectifiying it so it can be feed to the 1500V DC traction control system. Another way is the conversion with transistors that match the phase and cycle of the overhead. A common way nowdays is to mix both and have a transformer between high and low voltage and a transistor based active rectifier to convert 1500V AC directly to variable voltage and variable frequency AC for the traction motors. This allows regenerative braking with relatively low voltage parts. On AC-DC ovearhead switches, the system just switches in and out the main transformer and changes the transistor switching pattern.

 

Going between various power districts can be done several ways. The oldest is having both districts next to each other with a single isolator. The train has to glide though the boundary with its pantographs down to avoid a short. The more modern version has a small unpowered section between each district and the train can leave the pantographs up and switch while under this section. For longer trains and emu-s with multiple pantographs the danger of connecting the two districts through the trainline is present, so they either don't use a train line or still have to pull down the pantos or at least turn on the pantograph isolation switches. Another way is to have the train stationery and manually switch a larger section of overhead. This is usually done at border stations, which even allows the swapping of single system locomotives. A more modern way is to have an active system that detects train position within a block and when the whole train is inside, switch the whole block while the train is running. If used between high and low voltage AC and DC systems, this needs in cab signalling or an autodetect system on the train to work to allow automatic switching between systems.

 

Lastly under an autodetect system i mean a system that can detect what kind of voltage is on the overhead and switch in the right system. This can be done by having the drive system disconnect and only the main transformer connected after a power outage and first listening to the incoming voltage. If the transformer has zero output, then the line voltage is probably DC. This can be tested with a dedicated (and protected) low current circuit and if it is, then the main driver circuit can be turned on. If the transformer has output voltage, then the line is high voltage AC. For multi system locomotives (running on 15-25KV) the peak voltage has to be checked too (for a fixed 1:10 transformer this will be between 1500 and 2500V). Direct conversion drives can skip the transformer and directly check the high voltage AC, but getting transistor based drive systems to directly support 25KV costs more than using an extra transformer. Using the arrangements above it's possible to support a wide variety of AC and DC systems with a single pantograph and drive system with relatively few parts. Very old multi voltage systems had a dedicated pantograph for every system and the right one had to be raised for each system. This was usually controlled manually. Nowdays the only need for multiple pantographs is when the lines need different pantograph cross sections, one using wider ones with more swing in the wire or smaller ones for a more precisely hanged wire that needs less space in tunnels.

[Sacto1985]

If you want to see an example of switching between DC and AC overhead power, YouTube member saremya posted back in 2013 a video of trip on the JR West Hokuriku Main Line on a 475 Series EMU:

 

 

As the train approaches Itoigawa Station going west from Naoetsu, saremya shows what happens to the train as its switches from 1500 V DC to 20,000 V AC overhead power--you see it at 33:32 to 34:20 of this video.