OK.....i am not an electrician by any means, but with an electric train.......I often see what looks like just One Wire that powers the train.
Don't they need a second wire.?..... do they use one of the rails somehow.?
Thank You
Most railway overhead electrifications do use the rails to carry the return current.
Trolleybuses require a second overhead wire to complete the circuit.
Greetings from Alberta
-an Articulate Malcontent
OK.....i see. Kind of like my house power (usa).
I have 120-VAC on the Black wire and Very Little/No Voltage on the white wire. The Rail or white wire simply carries current.
Thanks Again
SD70Dude Most railway overhead electrifications do use the rails to carry the return current. Trolleybuses require a second overhead wire to complete the circuit.
Also, electric trains like the Acela require such systems.
ben SD70Dude Most railway overhead electrifications do use the rails to carry the return current. Trolleybuses require a second overhead wire to complete the circuit. Also, electric trains like the Acela require such systems.
I don't follow , where is the second wire in this picture?
There is only one wire contacting the pantograph. Compared to this trolleybus, with two wires and dual trolley poles:
There is no problem with modern high-speed trains drawing the equivalent of tens of thousands of horsepower from 50KV single-phase overhead wire using ground return via the track. Yes, you will want sensible rail bonding, and yes, there are concerns with 'galvanic effects' in the track structure, and yes, there are concerns with the signal system including harmonics of the traction-current frequency, but these have been successfully dealt with for well over a century.
The only part of the catenary that communicates with the train (via the conductive shoes in the pan) is the trolley wire. The initial Milwaukee Road 3000V system had two trolley wires, but these both carried the same voltage with the same polarity, to double the current-carrying capacity, and didn't act as supply and return. All the other parts of the overhead system that are energized (catenary/messenger and auxiliary wire, hangers, pulloffs, etc.) do not contact the train to transfer power.
Obviously trolleybuses need separate supply and return, as they are well-insulated from the road on rubber tires and in any case don't follow a fixed guideway with an electrical return channel.
There are some electrified rail lines overseas (and historically even some in the US) that had 2 wires, although not for a return wire. They are/were 3 phase systems which also use the rails to complete the 3rd phase.
Great Northern's original Cascade Tunnel electrification was the only US steam RR electrification to use three phase. Having to insulate the two overhead conductors from each other limited the voltage that could be used. N&W electrification about 4 years later made use of a phase converter that allowed three phase motors to be operated from single phase.
The original Cincinnatti streetcar system used dual overhead (AKA metallic return) due to an injunction by the local phone company prohibiting the use of the track as a current return. This was in part because many of the early phone systems used ground return on their circuits and the leakage current from a track return created an awful lot of noise. Shortly thereafter, phone companies converted to metallic return for a much quieter connection. In addition, the various electric railways made a great deal of progress in improving bonding and reducing electrolysis (AC electrifications have a much smaller problem with electrolytic corrosion).
Thank You.
NDG Trolley buses have two wires, the left one in direction of travel the live one, and the left wire can be shared by streetcars
Trolley buses have two wires, the left one in direction of travel the live one, and the left wire can be shared by streetcars
It can work in reverse, too. On Chicago Surface Lines, and probably other operators, trolley buses could be operated on streetcar routes by using one pole contacting the overhead with the other pole wired to a trailer with steel wheels riding on the rails to complete the circuit.
NDG kenny dorham OK.....i am not an electrician by any means, but with an electric train.......I often see what looks like just One Wire that powers the train. Don't they need a second wire.?..... do they use one of the rails somehow.? Thank You Track Return. For illustrative purposes. In the following video at time 2:30 the arc at the wheels is from current from trolley wire passing through the Controller, Resistances and Traction Motors and back to Power Source thru rails. https://www.youtube.com/watch?v=eT2o_X87JxM This locomotive has since been restored, as here. https://www.youtube.com/watch?v=IwPdOj9xAAs At certain turnouts in Toronto, arcs can be seen from streetcar wheels passing thru track switches. Trolley buses have two wires, the left one in direction of travel the live one, and the left wire can be shared by streetcars. Thank You.
kenny dorham OK.....i am not an electrician by any means, but with an electric train.......I often see what looks like just One Wire that powers the train. Don't they need a second wire.?..... do they use one of the rails somehow.? Thank You
Wow...when i first watched your link, i thought that was right by me. That place looks very similar to The Western Railway Museum (Rio Vista Junction) which is about 5-10 miles down the road from me.
It was part of our "Glory Days" of railroads and everyday transportation. It sits on the right of way of the old Sacramento Northern.
erikemHaving to insulate the two overhead conductors from each other limited the voltage that could be used.
Is this actually so?
First of all, don't you mean 'isolate'? And second, you have two of the three phases on the two overhead lines, so any directed corona will be only slightly greater between them at 'peak' phase difference than from a single conductor in free space ... meaning that the actual 'insulation' wouldn't be at the wires, but the parts of the contacting apparatus, pantographs or whatever, such as tips of shoes, that might have to be closer than the wires themselves are?
In any case I would not expect the achievable voltage to be particularly lower due to OHLE considerations than for single-phase AC, and I would expect that any "insulation" considerations to the actual three-phase motors could be handled with isolated transformers for each phase as needed.
(Remember that GE experimented with DC voltage at least as high as 6000V with very interesting voltage to the motors (they used two in series geared to one axle, I believe, in the highest-voltage test) but found no particular net benefit over 3000V for even the heaviest electrification. This may be true with actual motor voltage used in motor-generator sets: even Tom Swift's three-oughts-one only used 2300V to the three-phase AC part of the drive ... and that was with a nominal supply voltage of 100KV!)
Isolate, insulate... I'm leaning towards insulate as the concern is preventing current flow between the phase conductors. The issue is not so much what happens over a simple track section, as wire spacing can accomodate voltages of interest. The limitations come from turnouts as that involves the phase conductors crossing along with the means of keeping the current collectors happy. The GN used two trolley poles, where the Italians used dual bow collectors.
Both GE and Westinghouse experimented with 5 to 6kVDC electrical gear, with Westinghouse running an experiental installation at Grass Lake. GE had investigated using 5kVDC for the Milwaukee electrification, and the savings in copper were eaten up by the increase in locomotive prices. In addition, the motors for the original Milw locomotives were tested at 50% overvoltage (i.e. equivalent to 4,500V on the catenary) with no problems with commutation.
There has been work on making "solid-state" transformers for utility use, where the iron core is replaced by a high voltage and high frequency switching power supply. Such units have been made an tested on 14.4kV AC. The enabling technology are high voltage SiC FET's (10kV), IGBT's (15kV) and GTO Thyristors (29kV). These would make a 10kVDC electrification a piece of cake as standard traction motors could be used.
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erikemThe limitations come from turnouts as that involves the phase conductors crossing along with the means of keeping the current collectors happy.
Look for the mother of all dead sections in the area of any 'crossover' of conductors, perhaps involving multiple isolated sections of wire or even formed conductors doing the duty 'powered frogs' on some model railroads do, with internal phase-aware isolation or switchable insulation when one of the collectors has to dead-cross within corona range of a 'wrong' phase. I suspect there are pictures and perhaps even drawings of crossover arrangements and the like for these double-overhead systems, and it would be interesting to see how a combination of ingenuity and kludging would overcome the obvious shortcomings you mention.
There are some interesting examples of how to design effective commutation for very high brush voltages -- apparently this was one of Frank Sprague's areas of interest, because he had a collection of approaches in one of the 'file boxes' of his ATC company's records. Some of these were not exactly the most manufacturable commutator configurations ... but I did not see observed problems with commutation being given as a problem even for the 6000kV DC installation (of course, that one was designed to preclude the problem, but the solution used there would have worked nicely had there been substantial savings in 'other' construction details for, say, 9000V DC operation with dual trolley). That GE almost immediately abandoned any particular HVDC development is likely telling.
As you note, it's possible to make very capable DC-to-DC voltage conversion devices (some designs make weird noise, some can produce electrical interference if not correctly shielded at high frequency, but those are comparatively small details) and I think there are particular advantages when 'wayside power storage' from active regenerative braking is provided. The "better" piece of cake I see is to use the transverted DC, appropriately clamped and de-spiked, as the input to the DC link of a proper AC synthesis drive. Does anyone still want to use 'standard traction motors' in a modern locomotive or MU train?
One quick and dirty method of going for a higher voltage on a DC motor is to reduce the number of poles. Rule of thumb for a commutating pole motor is to limit voltage between segments (bars) to 20V, and thus a 1500V 4 pole motor would need at least 300 segments. Converting the same armature to 2 pole should allow for 3000V operation. The motors on the M-G sets used to provide auxiliary power on the locomotives were usually built as 2 pole machines for that reason. Same thing with the Grass Lake experimental, the armatures were built for 2 pole operation and two armatures per motor housing to get a 2500V motor.
The downside of two pole motors is that they require a thicker (read heavier) frame to conduct twice the flux to get the same flux density in the air gap as a 4 pole motor. A higher flux density meant a greater torque for a given armature current - Benjamin Lamme's redesign of the streetcar motor made the previous double reduction motors obsolete. FWIW, Lamme was the guy who turned Tesla's idea for an induction motor into a practical device.
I'd agree that a modern DC electric locomotive would use an inverter and some form of AC motor. One likely change is that by using SiC or GaN FETs instead of Si IGBT's, the switching frequency can be raised to the point where it would be practical to put a low pass filter between the inverter and motor. This would eliminate the need for special wire needed for inverter fed motors.
One major limitation on DC catenary is the amount of current which can pass the cable. At 500 Amp, with 3.000 VDC you get approximately 1.5 MW of power (most 3 kV installations had double catenaries, so you could transfer double that amount).
I *think* that the safe limit for catenary is at around 1.000 Amps per cable, so a double cable could feed nearly 6 MW of power to the locomotives/MUs involved.
N.F.
nfotisI *think* that the safe limit for catenary is at around 1.000 Amps per cable, so a double cable could feed nearly 6 MW of power to the locomotives/MUs involved.
Remember that erikem was talking about HVDC, not something limited to the 3000V vicinity by switch and motor capacity. I believe this can be up to whatever the safe limit for corona on DC is (which is probably higher than equivalent RMS AC as corona arc initiation is more likely at the effective duration of higher voltage around the peak of the waveform) so somewhere north of 12.5KV (it will be transverted to DC link voltage on the locomotive) which I think provides what Rolls-Royce and Bentley termed 'adequate' power per pantograph.
To the extent this current is in the outer portion of the conductor, it might be possible to incorporate a heat-pipe arrangement inside the trolley to dissipate any point-contact ohmic heating or arcing from slow-speed or high-draw operation.
nfotis I *think* that the safe limit for catenary is at around 1.000 Amps per cable, so a double cable could feed nearly 6 MW of power to the locomotives/MUs involved.
The Milwaukee could reliably deliver 4,000 amps to a train, and the Little Joe's could momentarily pull 3,000 amps each. At 3kV and 4kA, the Milwaukee could supply 12 MW to a train. A 1923 test at GE showed that 5 kA could be collected at up to 60mph using the same catenary as the Milw.
Overmod was right in that I was proposing going to much higher than 3kV, say 10 to 15kV. Cree/Woldspeed has made some experimntal SiC IGBT's capable of running off of 15kV. 2kA at 15kV is 30 MW, which is close to 40,000 hp.
- Erik
Chicago's use of a trailer behind trolley buses was limited to shop moves, particularly to West Shops, which was more than a mile from the nearest trolley bus barn. Milwaukee used a "skate" to accomplish the same thing.
In most cities the trolley bus wiring was kept separate from the streetcar wires to equalize wear. Market Street in San Francisco has shared wire used by both today.
Most trolley car systems I'm aware of used power switches activated from the trolley overhead. Basically, a 'power sensor' was placed on the trolley wire at least one car-length before the switch. If the trolley was 'drawing power' as the pole passed through the sensor, it would set the switch to straight ahead. Power off would cause it throw the switch to the diverging path.
My late friend Leigh Wright in Detroit used the exact same method on his O guage trolley layout. He also invented the 'Detroit 4-car control' system which allowed 4 cars to be individually controlled on the same segment of track. Most of the other modelers in Detroit in the 50's-70's used it as well. All it took was a single diode placed in each car and both trucks to have insulated wheels on the same side of the car. Using 24v AC on the overhead, the diode in each car blocked half the AC cycle resulting in 12 volts (60 hz pulse, if you will). A corresponding diode was in each controller. Car 1 and 2 shared the same wire/rail combination, car 1 with a diode one way, and car 2 the other. Repeat using the other rail for cars 3 & 4. Of course, the trolley pole had to be 100% functional, and sparked as expected. I used the same scheme in my HO trolley lines. The down side of this system is that a reverse switch ala normal DC operations is not possible. So, to reverse car direction, the secret trick was to have 'pole reverse', where the 2 poles on a car were insulated from each other and were connected to opposite poles on the motor. The 'tie down' hook on the roof of the car became the 'return' route to the rails by being grounded to the car body, which, in turn, was connected to the uninsulated wheels side of each truck.
An interesting aside to the use of the rails to provide the common return for trolleys is that the electric companies themselves would use the same rails for their common return. This avoided the need of a 'return' wire for the electric companys' circuits to businesses and residences. In many cases, the electric company and the trolley company were one and the same business. The Milwaukee WI trolley/interurban/electric company is a prime example until the laws were changed that prevented electric companies from owning trolley lines. The somewhat comical offshoot of this has been happening on and off for the past 80 years or so as trolley lines are abandoned. The city goverments these days are completely unaware of what streetcar rails buried under the asphalt are STILL used by the electric company! Every now and then, some contractor doing street repairs or improvements will dig up the rails and remove them, blacking out a portion of the street to all places that depended on that return conductor! While I was living in Dayton OH in the early 70s, I recall two occasions where that was reported on the local news broadcasts. 15-20 years later, while living in Milwaukee, the same thing happened! I'm guessing that even though the electric company most likely kept records of the rails used for return wires, the city governments were never advised of it, so they dug up the streets with surprise consequences many times over the years! I'm also guessing that these days, the paper records of such are over 100 years old and have been discarded by the electric companies.
One must be aware that although the rails carry the return current, the rails can be up to 70volts above ground, as they sometimes are on overhead trackways.....
What is single, two and 3 phase and how does it work?
Single-phase and three-phase are forms of alternating current. Three-phase electrification requires double overhead and the running rails to complete the circuit. Three-phase electrification was initially used on Great Northern and the Italian State Railways. There may be others.
caldreamerWhat is single, two and 3 phase and how does it work?
Now, for a moment, consider the effect of a two- or three-cylinder steam engine with the pistons moving backward and forward just as forces on the electrons in electrical wire do. Each piston is relatively 'peaky' in the thrust it produces, going in fact to zero where its motion reduces. If we combine the effect of two, or more piston motions, as in a typical double-acting 2-cylinder quartered locomotive, we have a resultant with much lower 'peaks' but more of them, giving more even thrust over time. If we now go to three 120 degrees apart, we can get smoother torque yet.
The phase of an AC current is a measure of how far its peaks are 'displaced' from a current of identical frequency that, as drawn, starts at a different time. As all three 'waveforms' are identical, it is only this time displacement of their rise and fall that is 'different' between them at any particular moment that currents might be reacting with each other.
With most three-phase current, you have three currents (in three separate wires) in the same relationship as the cranks in Swiss drive. Their relationship, among other things, is to produce a very even net voltage both in 'push' and in 'pull'.
Now an ordinary DC locomotive needs two conductors to run: a source of voltage potential, and a ground or 'voltage sink' that the potential difference can push electrons toward to make the actual current. In vehicles like trolleybuses this has to be two wires, but from streetcars on, engineers have realized that the actual ground potential is so low that track rails can be used as the return contact, so only one insulated wire is needed, and in most cases the only effects of the 'ground return' are galvanic/electrolytic chemical problems like accelerated corrosion.
The same arrangement can be used for two-phase AC, but as Erik notes you need two 'wire' conductors plus a third that can be provided via 'the track'.
To retain the torque advantage of three-phase, you need three conductors, which can also be provided using two wires and 'the track'.
Others here will likely give you a better and more understandable explanation, but at least this is here early as an option.
Overmod The same arrangement can be used for two-phase AC, again requiring only one overhead wire.
The same arrangement can be used for two-phase AC, again requiring only one overhead wire.
I believe you meant single phase as proper two phase requires a minimum of 3 wires.
The analogy of two cylinder and three cylinder steam locomotives is a good way of describing the difference between two phase and three phase. This can be extended to describing single phase as being equivalent to a single cylinder steam engine, namely that the single cylinder engine is incapable of supplying torque when the piston is at one of the extremes of travel. With two cylinders 90 degrees out of phase, when the piston in one cylinder is at an extreme and thus incapable of generating torque, the piston in the other cylinder is at the middle of its travel where it generates the maximum torque for a given force from the piston.
The reason why three phase dominates over two phase is that the common wire in a three wire two phase installation conducts 1.41 time more current than the two "hot" wires, whereas in three wire three phase (no common wire), all three wires are carrying the same current magnitude. In a balanced four wire three phase system, the common wire conducts no current, hence can be eliminated. Three wire three phase can be either delta or wye (star) connected, while four wire three phase is inherently wye (star) connected.
With properly configured stator windings and balanced currents, both the two phase and three phase systems provide a constant rotating field, that is free of torque pulsations.
Historical note: The two phase system came about from Westinghouse and Tesla using a pair of single phase generators shifted 90 electrical degrees apart to generate two phase power. GE went straight to three phase generators.
Erik_MagI believe you meant single phase as proper two phase requires a minimum of 3 wires.
A good read on how some of the electric power industry came about is Maury Klein's THE POWER MAKERS: Steam, Electricity, and the Men Who Invented Modern America. This was my source for the tidbit on Westinghouse going with two phase. Klein also did a good job of giving Benamin Lamme credit for what he contributed to the electric industry through his work with Westinghouse, one example was simplifying Tesla's design for induction motors.
Bought a copy ($2.07 delivered!) on your recommendation.
(In the meantime, can anyone confirm the number of poles in the traction alternator of a U34CH? I could swear that thing ran 720rpm at HEP idle, which I understand would correspond to 10 poles, but it's been ... well, 50 years since I saw demonstrated how you put the locomotive in that mode.)
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