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…
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.
With DC, you get to use series wound traction motors, which give good acceleration and are easy to control. You just put resistors in series with the motors to control the voltage going to the motors.
These days even on DC systems they no longer order equipment with DC motors.
The days of series wound traction motors are history
Yes, solid state power conditioning equipment is a real game changer.
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
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.
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
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More insulation for the same voltage of AC vs DC.
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Not as much power available on a 3rd rail AC vs DC.
There is, but I don’t think it counts. Vancouver BC Skytrain uses an two conductor AC third rail, but the propulsion is from linear induction motors in the track structure…
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