Re: AC locomotives & HEP [was Re: XPT's Reduced]

[matthew@mail.usyd.edu.au (Matthew Geier)]

In article <3afb7d65@news.alphalink.com.au>,
Adam Dunning <adamdunning@start.com.au> wrote:
>

>Firstly, Isn't the electricity generated by the generators now AC in all new
>locomotives (and most since the late 60's early 70's)?

 Some time in the 60's, but they had intergrated rectifiers so DC came out of
the 'Alternator'.  There is probably no locomotive in regular commercial service
in Australia with a DC generator, they are all alternators with rectifiers. 

>  If so, why does the
>electricity need to be rectified to DC before being converted to AC again?
>This would result in a loss of power.  Why isn't it just directed straight
>to the traction motors as AC?  Voltage? 

 Control and cost. AC motors speed tends to be regulated by the frequency of the
supply. The 'power' bands off the diesel don't match that of the AC motors. Its
really only since the advent of solid state micro-processor controled inverters that
AC traction motors have become practical in many applications. (Not just in the
railways - any where a variable speed motor is required).
 Im sure a pole-changing transformer tap changing AC locomotive could have been
(and probably were) built, but the complexity of the control system wouldn't have
been worth it.

> A simple transformer could easily
>be made to change the voltage to that used by the traction motors, couldn't
>it?

 Transformers only work effciently at the frequency at which they are designed
for. The further from the ideal frequency, the higher the loss. 

 Solid state converters with a 'DC' link a pretty effcient these days and are
probably now cheaper than transformers and certainly lighter. Its probably not
long before a VVVF drive package is designed that will take 25k AC straight off
the overhead with out needing a step-down transformer.

 Amtrak have a class of locomotive were the HEP for the train is tapped of the main
alternator - the locomotives control system takes care of taking the constant
voltage from the main alternator and doing what ever is needed to control the traction
motors. They found this wasn't very fuel effcient, running the 3000hp main engine at
the speed required for the HEP all the time....

 The XPT is an odd setup. They tapped the AC off the main motor and then added
extra complexity to each trailer car to deal with the variable nature of the 
supply.