On Apr 21, 4:16*pm, Mizter T wrote:
On 21 Apr, 13:55, Andy wrote:



On Apr 21, 12:46 pm, Mizter T wrote:

On 21 Apr, 11:39, "Paul Scott" wrote:

"thoss" wrote:

At 03:06:46 on Mon, 21 Apr 2008 Boltar opined:-

Change the traction package on a unit, and put it back in service.
Don't see why that would require closing the line. * It might not
be possible to couple a DC unit and an AC unit in a single train,
but they can certainly share the same track and power rails.

If they do it lets hope they don't reduce the performance of the
central line trains to the slug like performance of the northern line
ones. The central trains are the only ones on the tube that have
anything approaching the decent acceleration that you'd expect on
a modern metro.

Is that because of the trains or because on the tube sections
acceleration is aided by gravity?

It is because the Central line has full automatic train operation (ATO),
which for normal day to day use drives the trains nearer the limits than
manual driving can. *There is no reason why a change from DC to AC
traction would make performance worse, in fact it ought to improve
AFAICT.

Indeed - it's all about the ATO.

There is also the fact that all axles are motored on the Central
line's 1992 stock, whilst the Northern line's 1995 stock has two
trailer cars per 6 car unit. This means that the '92 stock is lighter
(per car). Having every axle motored generally allows better
acceleration, if all other things are equal. It will be interesting to
see how the Northern stock performs, once the trains are used to their
full potential.

Though, speaking from a position of total ignorance here, might the
motors on the Northern line trains be more powerful, thus less are
needed?

Indeed they are, but the problem is more putting the power to the
track. The size of the wheels on tube stock means that best
performance was gained by using small, less powerful (DC) motors under
the cars in the 1992 stock and powering every axle. By the time that
the 1995/6 stock came along, the AC motors became suited to the task
and these are more powerful for their size than the DC ones. I think
that when the 1992 stock was being designed, there was no motor
powerful enough to both fit under the cars and give the desired output
per train, without having every axle powered.

There is also the consideration of adhesion. Having more axles powered
spreads the 'push' out and gives less chance of wheelslip etc., when
it is wet or icey.