I mentioned in the 25kv thread the existance of booster sets, basically a 4 ton flywheel turned by a motor, turning a generator. Attached is the power schematic for a Class 71 DC locomotive (courtesy of Dennis). Have a look, can anybody suss it's operation?
Was its operation to cover the gaps at junctions in the third rail supply, where there is a gap or break in power,
so the flywheel turns the generator to supply power for a limited period to cover loss of power?
One function of the booster set, yes, but there was another function that was not obvious.
(21-06-2012, 06:24 PM)ed731pdh Wrote: [ -> ]Have a look, can anybody suss it's operation?
Just about, though the symbols take some working out.
Dual voltage 750Vdc/1,500Vdc?
Didn't these get sold to Holland or were they the Class 76's?
rob.
Your thinking of the 1500vdc Sheffield locos that went Dutch.
Yes, the booster set would allow seamless power notching from 0 to 650vdc (later upgraded to 750vdc, the Westinghouse DH27 compressors had a resistance loop fitted which can be seen on the side of the compressor to help them cope with the raised voltage), then would add it's output potential to that of the incoming DC voltage to double the available voltage being fed to the series wired traction motors. The most important parts of the set up were the booster motor start resistances and very important, the current limiting relay which would stop the booster dumping power back into the 3rd rail in the event of failure of the supply due to a short circuit or driving by accident into a current isolation. A roof mounted pan was provided to run off the tram lines within sidings when off the 3rd rail. For such a small loco, they had an enormous tractive effort and ran the Kent boat trains with ease. Cab "bog chain" operated the air powered whistle on the roof.
Eventual fate, some were rebuilt into what became Class 74 by removing the booster and fitting a Paxman diesel inside along with some very complicated electronics that just didnt work. Only one left now, 71001 at Shildon, part of the national collection. The booster sets from the locos went to Wimbledon Depot where they provided power for the depot on a feed seperate from the main line because if there was a short, it would usually happen within a depot.
Good old Wiki, a decent explanation.
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Reversible booster
Before solid-state technology became available, reversible boosters were sometimes used for speed control in DC electric locomotives. To avoid confusion, it should be explained that it is the electrical output of the booster that is reversible, not the direction of rotation.
The motor of the MG set was connected in parallel with the supply, usually at 600 volts, and was mechanically coupled, via a shaft with a heavy flywheel, to the generator. The generator was connected in series with the supply and the traction motors, and its output could be varied between +600 volts, through zero, to -600 volts by adjusting switches and resistors in the field circuit. This allowed the generator voltage to either oppose, or supplement, the line voltage. The net output voltage could therefore be varied smoothly between zero and 1,200 volts as follows:
Generator producing maximum opposing voltage, net output zero volts
Generator producing zero volts, net output 600 volts
Generator producing maximum supplementary voltage, net output 1,200 volts
To match the 1,200 volt output, the locomotive would have three 400 volt traction motors connected in series.[2] Later locomotives had two 600 volt motors in series.
When the locomotive was working at full power, half the energy came through the MG set and the other half came directly from the supply. This meant that the power rating of the MG set needed to be only half the rating of the traction motors. Thus there was a saving in weight and cost compared to the Ward Leonard system, in which the MG set had to be equal in power rating to the traction motors.
If the power supply to the locomotive was interrupted (e.g. because of a gap in the third rail at a junction) the flywheel would power the MG set for a short period to bridge the gap. During this period, the motor of the MG set would temporarily run as a generator.