Re: Fate of 85/86 class

["Greg Hunter" <huntergpmj@bigpond.com>]

David Johnson <trainman@ozemail.com.au> wrote in article
<38A29E9E.A4F12928@ozemail.com.au>...
> 
> 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.

But an AC system won't have a resistance of 0.1 ohms.  To start you use
smaller conductors, so resistance is maybe double that of a dc system.  The
on an AC system you have to contend with the inductance of the overhead
which causes more voltage drop, depending on the loco power factor. Overall
the impedance per km is around 0.05 ohms on a dc system and 0.5 ohms on AC
- 10 times bigger.
Then the reason you went to AC is the need to have longer distances between
substations. (50 km instead of 10)
Ultimately, as I tried to get across before, the worst volts will be about
25% drop in either system because that's an economic way to do it.

> 
> 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.

Yep - sounds right.  OHW resistance is 0.4 ohms between Granville SS and
Carlingford.  220A/motor = 880A traction plus say 100A for Air con etc and
the voltage drop is 400V.  Volts at Granville SS should be about 1550 so
that gives 1150 at the train.
The line does what it's designed to do - just run a 4 car train as cheaply
as possible.

Greg