HSTEd
Veteran Member
- Joined
- 14 Jul 2011
- Messages
- 16,636
So this occured to be a while back:
Most traditional DC loads found on trains would consist of resistive heating/lighting elements or series wound motors, either in motor generators or in traction systems. These loads are totally insensitive as to the polarity of the voltage applied to them and will function identically in either case as long as the magnitude of the voltage is correct.
The rest would mostly be shunt wound motors with the possibility of controlling shunt windings seperately using contactors.
So is there any particular reason to maintain a single polarity for the DC power voltage relative to the ground?
This would be especially useful on tramways and the like that use pantographs/trolley poles because you could have the up line at +750V and the down line at -750V, so that multiple vehicles on opposite lines will 'share' rail return current and hold down current leakage. Substation spacing could be improved significantly thanks to lower currents and lower earth voltage rise as the worst case would only have half the current flowing in the rails (max capacity on one line and no demand on the other).
This becomes more problematic on third rail systems because of the long train busses which prevent simple "phase brakes" as the two lines will short through the train bus.
But you could probably work something out involving coasting sections longer than the longest single unit (4 cars was the longest commonly used on SR I believe?) that would only be brought in in specific circumstances to assist stalled trains. For crossovers the coasting sections would obviously only be de-energised if the crossover was set to connect the two lines.
Most traditional DC loads found on trains would consist of resistive heating/lighting elements or series wound motors, either in motor generators or in traction systems. These loads are totally insensitive as to the polarity of the voltage applied to them and will function identically in either case as long as the magnitude of the voltage is correct.
The rest would mostly be shunt wound motors with the possibility of controlling shunt windings seperately using contactors.
So is there any particular reason to maintain a single polarity for the DC power voltage relative to the ground?
This would be especially useful on tramways and the like that use pantographs/trolley poles because you could have the up line at +750V and the down line at -750V, so that multiple vehicles on opposite lines will 'share' rail return current and hold down current leakage. Substation spacing could be improved significantly thanks to lower currents and lower earth voltage rise as the worst case would only have half the current flowing in the rails (max capacity on one line and no demand on the other).
This becomes more problematic on third rail systems because of the long train busses which prevent simple "phase brakes" as the two lines will short through the train bus.
But you could probably work something out involving coasting sections longer than the longest single unit (4 cars was the longest commonly used on SR I believe?) that would only be brought in in specific circumstances to assist stalled trains. For crossovers the coasting sections would obviously only be de-energised if the crossover was set to connect the two lines.
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