Railroads have used (or at least tried) the following approaches to using AC to drive their traction motors:
I think the GG1 used 2 above, DC traction motors run on AC.
More modern railroads tend to use solid state rectifiers, or more frequently, SCR's (thyristors).
With regard to mercury arc rectifiers, My dad has some in his old ham radio transmitter -- when he's got it on, they glow with a marvelous blue glow, and the voltage is high enough that little bits of dust that naturally collect on high voltage DC terminals cause arcs, so in addition to the blue glow, you get a faint whiff of ozone and an occasional pop.
Doug Jones
jones@cs.uiowa.edu
===============================================================================
Henry van Cleef, vancleef@netcom.com
Wed, 27 Jul 1994
Prewar (WW II for the younger folks) technology simply ran the motors on AC, stepped down to a nominal 600 volts on most systems. Any elementary DC-AC machinery text discusses differences in methods for winding armatures and running DC-type machinery on AC.
My recollection is that the Virginian and New Haven, in the 1950's, used mercury-vapor (vacuum tubes, but with some mercury in them) rectifiers to drive DC-configured motors.
Detroit, Toledo, and Ironton, when Henry Ford owned it (1920's) had at least one locomotive that used a motor-generator set in the locomotive.
I also recall something about the Virginian using AC motors with multiple pole fields to give 2-speed operation on AC, but have forgotten what configuration the AC motors were to give starting torque. You can get multiple near-synchronous lock-in speeds from AC motors by switching the stator pole windings.
There were several traction systems that used 3-phase and two trolley poles plus the track to carry the three lines. I think most of these ended up being converted to standard DC setups very quickly.
Most traction systems used 660 volts DC with motor-generators to convert commercial power in substations. Current losses with low-voltage DC systems running from an overhead trolley was huge, and most traction systems had substations and feeder lines going out to provide power to the trolley every few miles. Interurban lines, which had less density of cars running simply accepted greater losses between substations, but the voltage could get quite low starting a car from a location other than a scheduled stop, which generally got a boost from a feeder.
Third rail systems, such as that used by the New York Central, used rails to carry the currents, so were not subject to such high losses,, and could supply power to run larger units pulling trains.
=================================================================== Hank van Cleef The Union Institute History of Science E-mail vancleef@netcom.com or vancleef@tmn.com ===================================================================Dave pperson, pierson@cimcad.enet.dec.com
In article vancleef@netcom.com (Henry van Cleef) writes, in part:
My recollection is that the Virginian and New Haven, in the 1950's, used mercury-vapor (vacuum tubes, but with some mercury in them) rectifiers to drive DC-configured motors. Yep. And a few PRR units. Later converted to solid state. I also recall something about the Virginian using AC motors with multiple pole fields to give 2-speed operation on AC, but have forgotten what configuration the AC motors were to give starting torque. These used 'phase converters', the AC equivalent of the motor generator sets used elsewhere. A big single phase AC Machine was started from a small dc battery machine. The AC machine, once running, had good torque. It ALSO had a set of three phase windings. These OUTPUT three phase, constant frequency power to the three phase traction motors. (Some locos had scott transformers instead, another way to trick single phase AC to three phase...) You can get multiple near-synchronous lock-in speeds from AC motors by switching the stator pole windings. If there are multiple sets (or subsets) of poles. Another trick is to take power from the _rotor_ of one machine and send it to the stator of another (added complexity...), the so called cascade connection (used on, but not named after the GN Cascade Tunnel, in the early days) The two sets of motors interact to give half synchronous speed. There were several traction systems that used 3-phase and two trolley poles plus the track to carry the three lines. I think most of these ended up being converted to standard DC setups very quickly. And the first (highest) GN Cascade Tunnel, for about 10 years. Overhead was a bear... And lots of lines in Italy. Most traction systems used 660 volts DC with motor-generators to convert commercial power in substations. Current losses Wellll, to be picky, thats voltage loss/drop... with low-voltage DC systems running from an overhead trolley was huge, thanks dave pierson |the facts, as accurately as i can manage, Digital Equipment Corporation |the opinions, my own. 200 Forest St |I am the NRA. Marlboro, Mass 01751 |pierson@msd26.enet.dec.com 'He has read everything, and, to his credit, written nothing.' A J Raffles
