I haven't read the book on Sprague but I intend to soon, but what's the reason AC is not used on the third rail, and has it ever been used on the third rail? What about AC at lower voltages? Physics?
Thanks.
aegrotatio I haven't read the book on Sprague but I intend to soon, but what's the reason AC is not used on the third rail, and has it ever been used on the third rail? What about AC at lower voltages? Physics? Thanks.
while I am no expert on rail electrification, I know there are many subway systems running with AC traction motors so it is very doable..
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
Even AC propulsion does not run of AC power directly, on AC propulsion power gets rectified first.
then inverters are fed of the DC busbar.
High voltage AC is normally banned from anywhere within human reach.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
These days even on DC systems they no longer order equipment with DC motors.
The days of series wound traction motors are history
DutchrailnutThese days even on DC systems they no longer order equipment with DC motors. The days of series wound traction motors are history
The series wound TM with voltage controlled by series resistors was the whole game from the first electric street car right up through the MetroNorth/LIRR M1s - for transit/suburban operation, anyway.
even the M3 and M4/6 have DC motors but the M-7/M-8 do not
Dutchrailnut Even AC propulsion does not run of AC power directly, on AC propulsion power gets rectified first. then inverters are fed of the DC busbar. High voltage AC is normally banned from anywhere within human reach.
To continue, the power on the DC busbar is fed to a computer controlled bank of Power Transistors that can produce an AC sign wave at any needed frequency. At a low frequency the motor RPM is slow and increases as the frequency is raised. Torque is controled with voltage. VVVF (Variable Voltage Variable Frequency) gives complete control of starting torque and speed control. We started to build AC VVVF elevator motor control systems in the 1980s, today almost all new elevators by all the manufactures are AC driven.
Very Big AC Motor, 30 tons. The DC elevator motors in the Twin Towers (World Trade Center) weighted 26 tons.
The two "ears" on the near end are two Blowers, the covered part on the far end is the machine Brake, the area covered with cardboard is the groves for the hoist cables.
Don U. TCA 73-5735
I totally dig the DC-to-AC conversion, but I'm assuming there's no third rail AC that anyone knows about.
I'm not aware of any ac third rail electrification in any conventional sense, but I believe conductor rails are/were used for (3-phase) ac for (a) some of the unconventional people movers found at airports, etc. and (b) the ship moving locos in the Panama canal: but whether these could accurately be described as 'railways' is debatable.
I think the general consideration has been that if you had a low enough voltage to permit third rail, then traditionally dc had an overwhelming advantage over ac. It's only relatively recently, with the development of power electronics that ac traction motors have become generally preferable for railways.
In a sense, the linear motor could be considered a fourth rail with a kind of ac, but even there the power rail for the onboard equpment and matching stator (or is a rotor that doesn't rotate) on the car is DC. (Vacouver Skytrain, Toronto Scarboro line, and Kennedy Airport).
On a slightly different bend of this subject I was on the board (before moving from the area) of a vintage street car system using 600 volt DC in the overhead trolley wire. Our problem was the burning out of the DC trolley motors and the very expensive rewinding jobs that followed. We came up with a variable speed AC motor and a Baldor conversion box whose size was a cube of about 18 inches on each dimension. Instead of the traditional control stand the motor is controlled by a small pot about 1 inch in diameter. We did keep the historic control stand but the only working item inside is the pot. Our biggest problem is connecting the heavy cast iron control arm to the pot.
The nature of AC vs DC needs to be understood. 600 V DC has an absolute value of 600V. AC 600 Volts has a higher peak voltage. Think of an AC sine wave. So
1. More insulation for the same voltage of AC vs DC.
2. Not as much power available on a 3rd rail AC vs DC.
aegrotatio I totally dig the DC-to-AC conversion, but I'm assuming there's no third rail AC that anyone knows about.
Why is AC so much better thn DC?
TrainsRock Why is AC so much better thn DC?
No commutators to wear or arc or otherwise be damaged. The Commutator is the carbon brushes that carry the power into the rotating armature. They wear down, and if they are carrying high power when the locomotive or LRV hits a bump such as a switch or crossing you will get arcing which destroys the brushes even faster and can ignite the brushes if there is carbon dust build up. Finally a 3-phase AC traction motor can be more finely controlled allowing the motor to produce more pulling power without as much risk of wheelslip.
Good answers all. Again, the peak voltage for ac is 1.414 times the given voltage, so the insulation must be better for ac than the same voltage dc.
I don't have it with me but I believe in Middleton's The Interurban Era there are a couple of AC third rail installations mentioned, one as high as 1200 volts. I seem to recall that the superior transmission characteristics of AC encouraged use at higer voltages, which complicated the safe use of third rails.
I do not recall any third rail high voltage AC installations in the Middleton-Drew Interurban book, but I may be mistaken. I do recall only one AC third rail installation, forget where it was, and it was less than 1000V, and it did not last long.
Central California Traction used 1200V on their third rail from inception until de-eletrification in the 1940's - the CPUC was concerned about the safety hazard. The Michigan Railway had a stretch of third rail energized at 2400VDC for maybe a year, the problem of arcs between the third rail and journal boxes demonstrated that wasn't a good idea.
Traditionally there were two advantages to AC, the use of transformers allowed use of a much higher supply voltage than was possible with DC and the use of taps on the transformer allowed for lossless speed control. The disadvantages are that AC series motors were heavier and less efficient than DC series motors and the overall weight of electrical equipment was higher with AC.
As others have mentioned, at the voltage limit imposed by third rail, there is really no point in going to AC. The AC drives used by contemporary locomotives and transit equipment involved the use of inverters, so it is simpler to design the inverter to run off the third rail potential.
- Erik
Yes, but Central California Traction's third rail was 1200V DC, not AC. 848V AC would give the same insulation problems as 1200V DC (0.707 times 1200)
Very true, which is one of the advantages of HVDC for long distance power transmission. This goes back to the point that you, I and others have been making that there is no advantage to using AC on third rail due to the inherent voltage limitations and and several disadvantages which include the peak vs rms voltage problem that you pointed out.
One small caveat: 1200VDC wasn't really practical before the development of the commutating pole ca 1906.
erikem Traditionally there were two advantages to AC, the use of transformers allowed use of a much higher supply voltage than was possible with DC and the use of taps on the transformer allowed for lossless speed control. The disadvantages are that AC series motors were heavier and less efficient than DC series motors and the overall weight of electrical equipment was higher with AC. Erik you are correct. With modern inverters and a-synchronos traction motors instead of AC series wound the traction motor weight disadvantage is no longer applies. Also supply voltage is no longer a problem. As others have mentioned, at the voltage limit imposed by third rail, there is really no point in going to AC. The AC drives used by contemporary locomotives and transit equipment involved the use of inverters, so it is simpler to design the inverter to run off the third rail potential. And inverters are designed to take large voltage differences so once all the rolling stock has inverters the DC voltage could be increased to 750, 1000 etc. They can still output the AC voltages needed for traction and car auxilaries use. A possible thoery; 3rd rail may have been used so there was not the requirement to provide overhead clearances for CAT or trolley wire?. Most heavy rail is in tunnels and the short distance between DC power substations is probably cheaper than building taller clearances? Even Boston's blue line changes from overhead at the first station of its tunnel. Anyone ever noted the difference in height of that station vs the rest of the Blue line? That does not explain the conversion of MN of 2 miles from 11.5KV overhead AC to the underunning DC? - Erik
Erik you are correct. With modern inverters and a-synchronos traction motors instead of AC series wound the traction motor weight disadvantage is no longer applies. Also supply voltage is no longer a problem.
And inverters are designed to take large voltage differences so once all the rolling stock has inverters the DC voltage could be increased to 750, 1000 etc. They can still output the AC voltages needed for traction and car auxilaries use.
A possible thoery; 3rd rail may have been used so there was not the requirement to provide overhead clearances for CAT or trolley wire?. Most heavy rail is in tunnels and the short distance between DC power substations is probably cheaper than building taller clearances? Even Boston's blue line changes from overhead at the first station of its tunnel. Anyone ever noted the difference in height of that station vs the rest of the Blue line?
That does not explain the conversion of MN of 2 miles from 11.5KV overhead AC to the underunning DC?
DC better than AC: More power for the same insulation requirements. Peak voltage of AC is 1.414times that of the rms (root mean square) of stated voltage. Also, the latest development in electronics allows non-mechanical conversion of dc to ac and back, without transformers and with a change in voltage with efficiencies matching transformers. So high voltage dc transmission is in the works.
AC better than DC: (1) With convenitonal tehcnology, the ability to transform voltages up and down by transformers, thus allowing transmission at very high voltage and relatively low current for minimum resistive line loss, with ability to use optimum voltage for whatever load is served. (2) Computer controlled ac motors vary speed and meet load characteristics by computer and electronic control, and do not require brushes and commutators which wear and require maintenance and replacement.
WRT "AC better than DC":
I did neglect one advantage of AC over DC - electrolysis. Electrolytic corrosion has been a concern since the first street railway electrifications. The corrosion with AC is typically 1% or less than DC for the same current. The problem can be further reduced by use of "booster" transformers to force the return current to stay in the tracks - the modern equivalent are the ferrite beads on signal and power cables coming out of computers.
The ability to use transformers with AC is an advantage that applies to transmission of power as opposed to application. All but the earliest third rail installations were run electricity that was originally generated as AC, transmitted as AC and then converted to DC at lineside substations. (N.B. I'm very sure that Dave already knows this and could probably list the third rail installations that were fed by DC power stations).
The AC polyphase motor with variable frequency drive are proving to have a lot of advantages over DC commutator motors. AC series motors and fixed frequency AC induction motors are generally considered to be inferior to DC commutator motors for railway applications.
My recollection is that the first use of variable frequency AC traction motors was for transit, WABCO was working on PWM (Pulse Width Modulation) inverters in the late 1960's. Inverter technology became a lot simpler after the development of high-power IGBT's. The efficiency variable frequency drives is high enough that the improvements in efficiency are being driven much more by reducing cooling than by saving energy.
Perhaps the next phase is development of high voltage (>3000V) DC electrifications. Use of DC would eliminate the phase imbalance problems with commercial frequency single phase. A further advantage is that DC to DC converters are smaller and lighter than commercial frequency transformers of the same power rating. The downside is that electrolysis would still be a problem.
Why do they not bring Commercial 3-phase into the Railway substations, rectify it to DC then invert it to Single-phase AC for output to the Cantenary, this would balance the phases as far as the Commercial Grid is concerned.
beaulieu Why do they not bring Commercial 3-phase into the Railway substations, rectify it to DC then invert it to Single-phase AC for output to the Cantenary, this would balance the phases as far as the Commercial Grid is concerned.
Although I am not famaliar with the how there has been development of 3 phase balancing transformers. Can this be converted to single phase AC output I am not quite sure that that is a possibility. It may have to do with using 2 separate isolation step down transformers (instead of auto transformers) built as a single transformer but do not quote me on that feature.
Anyone know more??
On the very-heavy-traffic PRR electrifications, the solution was to have phase breaks along the line. and actually segment different parts of the electrification to each of the three phases. I am unsure if this arrangement is true today. There is no reason, today, why three-phase power cannot be converted directly to single-phase via electronics and capacitor storage.
Funny you mention electrolysis. Perhaps based on prior experience or maybe just plain theory, the London Underground uses a third and fourth rail to keep the return current from causing the iron tubes to corrode prematurely.
While not strictly to prevent electrolysis, the Cincinnati street railway used dual overhead to prevent problems with ground currents - the local phone company obtained an injunction requiring dual overhead to prevent interference to the phone circuits.
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