London Underground 1995 Stock

The 1995 Stock is a type of train on the Northern line of the London Underground. There are 106 six-car trains in operation; they entered service between June 1998 and April 2001[2] replacing old 1959 Stock and (Mark 1) 1972 Stock.

It is the only deep-level tube stock to use selective door opening, for example at Moorgate, Charing Cross, Hampstead and Clapham Common.

It used to be operated mainly with a combined power controller, brake lever and 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 turned the top of the handle and pushed the red lever forward. However, London Underground introduced TBTC on the Northern line in sections since June 2013. The entire line became TBTC driven in June 2014.

Traction control[edit]
1995 and 1996 stock have similar bodyshells, but they use different AC traction control systems, more modern on 1995 stock since the 1996 stock design spec was ‘frozen’ in 1991.

1995 stock uses Alstom’s “Onix” three-phase insulated-gate bipolar transistor (IGBT) drive, 1996 stock three-phase induction motors fed from a single-source inverter using a gate turn-off thyristor (GTO), derived from those on Class 465 Networker trains.

Close-up of 1995 Stock cab
Earlier 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 three-phase AC induction motor is smaller and lighter for the same power. 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 used on 1996 stock achieves this by ‘chopping’ out short pulses of current, whereas 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 high 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 depending on the amount of voltage being dissipated.

Since October 2007 1995 stock has been fitted with the royal blue seat moquette used on the 1996 and 1973 stocks. The upgrade was completed in early 2008.

by simon schofield

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