On Tue, 25 Apr 2017 07:49:22 +0100, Chris J Dixon wrote:

Johnny B Good wrote:

On Mon, 24 Apr 2017 12:44:26 +0100, Mike Clarke wrote:

I think they used to run pairs of motors in parallel on starting and
switch over to series at higher speeds. That was some time ago and I
don't know if they still do.

If true, the sequence would have been series for starting, then
paralleled for 'cruise speed'. Effectively a two speed "Electric
Gearbox"
created out of nothing more sophisticated than a cleverly sequenced
bunch of contactor switches.

It is quite true, though the actual method is somewhat more complicated.
I was about to start writing a description, but it is covered rather
well he

http://www.railway-technical.com/tract-01.shtml

Although the article mentions pretty well all aspects of DC motor
control as historically employed for railway locomotion, the use of a
journalistic style over a technical authoring one resulted in a confusion
of descriptive terms (eg currents instead of magnetic fields and the
impression that the [armature] windings on the rotor were wound *around*
the rotor shaft).

Although the author 'loses points' for the use of confusing terms, he
does gain points for his comprehensive coverage of the difficulties,
imposed by the limitations of the technology of the day, facing the
designers in their attempts to recover the braking energy for use by
other trains on the line.

--
Johnny B Good