London Underground 1995 Stock
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The 1995 Stock used on London Underground's Northern Line is currently the newest fleet of trains on the network. There are 106 1995 Stock trains in operation, each of six cars; they entered service between 12 June 1998 and 10 April 2001. Despite the 1995 Stock designation, the step-plates found on the trains read '1996'. It is believed that the 1995 designation served solely to distinguish this stock from the similarly identified Jubilee Line 1996 stock.
The 1995 Stock shares many features with the 1996 Stock found on the Jubilee Line, and indeed both types were built by Alstom in Birmingham. The 1995 and 1996 Stock have different seating layouts and cab designs, and 1996 Stock trains are, at seven cars, one car longer.
The 1995 Stock is operated mainly with a dead man's handle at the driver's right hand. When the train is stopped the top of the handle is turned away from the position in which the driver holds it. To depart the driver turns the top of the handle and pushes the red lever forward.
[edit] Traction control
1995 stock and 1996 stock have similar bodyshells and were both built by Alstom. However, they use different AC traction control systems. The control system on the 1995 stock is more modern than the 1996 stock, reflecting the fact that the latter's design spec was 'frozen' in 1991.
The 1995 stock uses Alstom's "Onix" three-phase Insulated Gate Bipolar Transistor drive. For comparison, the 1996 stock uses three-phase induction motors fed from a single-source inverter using a GTO (gate-turn-off) thyristor, derived from those on Class 465 Networker trains. [1]
Earlier London Underground stock, like most electric trains until the 1990s, used DC motors. These are now regarded as inefficient, partly because they are traditionally controlled by resistors, and partly because a 3-phase AC induction motor can provide the same power at less size and mass.
However, in an AC motor, maximum torque can only be achieved when the resistance of the rotor windings equals the reactance. AC motors in an industrial setting tend to be operated more or less continuously and therefore large banks of resistors can be used on start-up to raise resistance and maintain torque. This would not be efficient in a small motor designed for stop/start operation.
As a result, it was only the invention of electronic control systems from the 1980s onwards that made AC traction viable for trains. The motor can be supplied using an inverter, and by varying the inverter's output frequency it is possible to keep the frequency of the currents flowing in the rotor windings constant, and hence the reactance (while resistance remains fixed).
The GTO thyristor used on the Jubilee line's 1996 stock achieves this by 'chopping' out short pulses of current. However, modern AC traction such as the 1995 stock uses the IGBT (which is actually an amalgamation of a MOSFET and a conventional transistor), which can switch very large currents very rapidly without damage. In consequence, the 'whine' effect is less noticeable on 1995 stock than on 1996 stock.
One disadvantage of IGBTs is that a higher voltage is dropped across them than other devices (high being about ¾ of a volt). As they conduct several thousand amperes, the resultant power dissipated by the IGBTs requires forced-air cooling. The cooling fans can be heard running and then shutting down when a train draws to a halt.
[edit] Train details
- Length per car 17.77 m
- Width per car 2.63 m
- Height 2.875 m
- Total seating capacity per six-car train 200, plus 20 perch seats and 48 tip-up seats. There are also 24 wheelchair spaces.
- Total passenger capacity per six-car train 914
Weight
- Driving Motor Car 29.4 tonnes
- Trailer Car 21.5 tonnes
- Uncoupling Non-Driving motor car 27.9 tonnes
- Per train 157.6 tonnes
[edit] External links
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