Overhead wire current limit ?

[pre65]

I read somewhere that when electric locos are hauling freight in tandem (class 91) the traction motors on one bogie are disabled to limit the current draw.

Or keep putting £50 notes in the meter ?

[aussiesteve]

G'day Philip,
I am sure that BW can enlighten you as to the UK overhead limitations.
I can only comment about our woeful 1500 vDC system here in NSW.
The more locos utilized on a train, the more demand upon the overhead supply substations.
This demand also increases when changing the traction motor combination.
Here with DC traction, you start a train in Series motor combination.
Then as train speed increases and demand reduces, you engage series parallel combination.
TOP gear parallel (actually second stage series parallel) and or weakfields can only be utilized if the train load and track gradient permits.
The problem with DC traction system is requiring resistance to reduce the voltage going to each motor when at slow speed.
Also to prevent wheel spin.
Once the train has stepped out resistance after achieving the necessary speed, you don't want it going back into resistance.
Locos have resistance bank cooling fans to prevent the banks from cooking.
But, remain in resistance for too long and the fans become ineffective.
Our line volts average 1750 volts (though stated at 1500 volts).
Demand too much current from the subbie and that drops below the minimum 1050 volts.
You can trip out the subbie when line volts drop below 1000 - 950.
The subbie will also trip if you demand more than 5000 amps.
With a single loco demanding 600 amps, that becomes a total of 1800 amps draw from the subbie when in parallel.
Three locos demanding 600 amps when in parallel becomes a total draw of 5400 amps.
That would definitely blow up the subbie.
Our 86 class buzz boxes were tabled for a max train tonnage of 600 tonnes when climbing the 1 in 33 grade.
Three locos being required to lift a train up to 1800 tonne.
For that you could NOT engage in TOP gear only second gear (series parallel).
Second gear on such a grade with such a train would only provide a train speed of around 25 to 30 kph.
Get down below 20 kph in second gear and you risk going back into resistance.
Very much a balancing act of staring at the line volts and traction amps and train speed.
Bung four weasels on the same train and you stomp up the hill in style.
The subbie section can also be overloaded if a following train enters the section.
So, we had power margins to prevent a freighter from following an emu pas.
An 8 car urban regarded as two 86 class buzz boxes in oomph drain.
The 8 car urban will engage top gear to maintain sectional running time.
You sneak into the subbie section behind it with two or three 86ers and BANG, no power.
So, freighters had to await 20 minutes before entering the subbie section to ensure that the urban was clear of it.
GO weasels.
Steve.