Re: Fate of 85/86 class

[David Johnson <trainman@ozemail.com.au>]

Greg Hunter wrote:

> Others have mentioned that the 1500V system limits the number of locos you
> can ultimately use and that the losses in a dc system are must larger than
> a high voltage system.
> The first bit is true - the Como- Heathcote section can handle 9 megawatt
> trains (12,000 HP) of 4 electric locos in series parallel.  And this is
> about as big as any 1500V system could ever go.
>
> The losses in the two systems, running big trains, are not all that
> different and not really significant in either case. Electrical systems are
> designed to be pushed to their limit by single big trains and ultimately
> this means big losses for short times. For the 4 loco trains on the Como
> bank losses on the dc system are about 13% but on the overall return trip
> less than 5% all up.
> An AC system would be designed for the volts to fall to maybe 18 kV so the
> losses would be not that much less than dc.

But remember basic electrical principles.  25kV is nearly 17 times the voltage
as 1.5kV.  That means for the same POWER, you require 1/17th of the CURRENT.
Just for an example, we shall compare the two systems, assuming 0.1 ohm
resistance between the substation and the train.  We will also assume 4.8MW of
power being consumed by the train.  This is equivalent to 3200 Amps (1500VDC X
800A X 4), or 192 Amps for 25kV.  So, when P=I2R...  On 1500VDC systems, the
POWER lost in the overhead in heat would be 3200*3200*0.1 or 1.024MW of lost
power.  On 25kV systems, the power lost would be 192*192*0.1 or 0.0036864MW of
lost power.  There is also the voltage drop issues.  V=IR.   So, on 1500VDC, the
voltage drop in the wire would be 3200*0.1 or 320 volts.  On 25kV, the voltage
drop would be 192*0.1 or 19.2 volts.  Of course, these figures are theoretical,
but are only to give an indication of the superiority of 25kV.

On an aside, I was driving a 4 car tangara to Carlingford yesterday, and noticed
at the end of the line in full power, I could get a maximum speed of 51 km/h
with 220 Amps in the motors and the line volts were down to 1122V.


--
David Johnson
trainman@ozemail.com.au
http://www.ozemail.com.au/~trainman/
------------------------------------
These comments are made in a private
capacity and do not represent the
official view of State Rail.
C.O.W.S. Page 11.