More from Alan:
Click for larger image "An early 'Swiss Federal Railways' 4-6-2 electric loco., date unknown, 1920? 15000 volts single phase ac, overhead supply at 16.66 hertz. Speed 56mph with 480 tons drawbar pull, with 2 series-compensated motors by Derlikon. Note the use of connecting rods on the six driven wheels, as in steam practice, and no money wasted on frivolities like covers!
DC series motors are practically perfect for traction in every respect, and were used extensively by the London Underground [600vdc] & in tramwork. Transmission of power over any great distance favored ac motors. Even when the voltage was pumped up to 1500vdc [live rail], or 3000vdc [overhead], ac offered lower overall asset compexity and capital/running costs over long distance routes, even with the poor starting performance. Polyphase ac traction was not practical until solid state switching devices became available, due to pantograph complexity. All motor design for traction is dominated by the need for
variable speed, so synchronous motors are out. AC motors are, in fact, not wholly suitable for traction conditions- even 3-phase induction motors are primarily a constant speed type. The single phase motors used were therefore usually of the dc commutator type, with brushes on every pole and fitted with interpoles.
Now, simple dc motors [with field windings] rotate in the same direction, whichever way they happen to be connected to the dc system, [ field and armature currents are simultaneously reversed]. If a series dc type motor be supplied with ac of
low periodicity, say 15-25 hz, it will duly operate in the same direction. This is why low hz was required. Field
and armature irons must be laminated, and air gaps kept as small as possible to minimise leakage, which [in the ac case] affects the power factor. Even so, a motor like this will have a poor power-factor, and the general methods used to improve performance were; to have resistance strips between armature conductors and commutator segments; to introduce commutating and compensating windings; by distributed series windings carried in slots on the inner perifery of the stator; to run with a weaker field than in dc practice; and just to
make the motors bigger! Even with all this refinement, starting vibration, noise and pulsation of torque was still inferior to dc."
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Click for larger image "A typical early 600vdc electric locomotive used on certain sections of the London Underground. This is # 12, so it came from the second batch of locos built by The British Westinghouse Company Ltd, and was photographed some time after 1905. #1 thro' #10 were of similar general form - Ordered 1904, delivered 1905, weight 50 tons, 4 x 200 hp motors. Bodies and bogies by Metropolotan Amalgamated of Birmingham England, and electrical equipment by British Westinghouse. This type all seem to have all been scrapped in 1922, when they were replaced by 'Metropolitan Vickers' locos. "
-Alan
[This message has been edited by pauluk (edited 01-14-2006).]