Re: Re(2?): Lights, Volts and Amps (Long geeky thread)

[Alan Cordeiro <cordeiro@avlna.com>]

>I may not fix Audis very well, but I've taught and lived electric power
>generation for 20 years. In 68 class aircraft carriers, the distribution
>system was designed for 4160V vice the industry standard 450V. The reasoning
>was that since the current would be so low, motors could be built much
>smaller and compact for a given power. This allowed them to have reactor
>coolant pumps (kinda like a BIG Audi aux cooling pump) only 15 feet high
>instead of 60 feet.
>
>And, of course, in the RealWorld(tm), everything works only within a certain
>band. If I raise voltage too high, then I get insulation breakdown and short
>circuits. If I lower it too much, my heat losses (IsquaredR) go sky high and
>I get hot spots and meltdowns. 
> 
>********************************AUDI FAN******************************
>EMCM(SW) Dave Head (nuclear grade electrician)

A motor is designed to provide a certain power, and that is what makes it
as big as it has to be. Torque comes from the radius of the rotor/stator gap,
and depends on field flux strength, and armature flux strength, the later
of the two depends on TURNS and CURRENT.

If all other things remain constant, and you double the voltage, and need
half the current to get the same power, you need TWICE THE NUMBER OF TURNS
in the windings ( field, armature, stator of a squirrel cage motor, whatever )

Guess what, you now need twice the number of turns to produce the same flux. 
But, since the current is halved, you need only half the cross-cestional area
 of wire i.e. this wire in the NEW, IMPROVED winding will be twice as long,
and ahve twice the resistance per unit length.

That's FOUR times the resistance, therefore the SAME LOSSES, ( I*I*R ).

In real life there is a small reduction but this is minimal, doubling the
voltage
will at best get you a 5% reduction in motor size ( over a REASONABLE voltage
range, say 50 volts:100 volts or 1000 volts to 2000 volts etc ).

To dramatically reduce motor size for the same power output, try other ideas
such as:

1. Doubling the SPEED.

2. Improve cooling methods, allowing motors to reduce losses, and run at
higher full
load currents. Typically large ( 100s of megawatts ) generators are HYDROGEN
COOLED,
to reduce overall size.


Now what you are talking about works GREAT for HYDRAULICS. Aircraft systems
use 5000+ psi
hydraulic systems, to dramatically reduce size AND WEIGHT. In fact. our
beloved Audi
( THERE !!! Finally !!!  sigh of relief, the mandatory Audi content )
uses 2200 psi ( instead of 1300-1500 psi like 'Murican types ) to get a more
compact
steering gear, power brake servo, and hydraulic pump. 

But then again there are TRADEOFFS to using a higher oil pressure........

Alan Cordeiro