Re: Newbie - Current, Voltage, Resistance, Power and Transformer theory

Sorry, the first word is "Imagine" and not "Image".

I always screw that word up the same way and my work processor never helps.

Dorian


"Dorian McIntire" <dorianmc@xxxxxxxxxxxxx> wrote in message
news:6bSYg.17093$O65.16967@xxxxxxxxxxxxxxxxxxxxxxxxx

"hdjim69" <hdjim69@xxxxxxxxxxx> wrote in message
news:1161016103.078589.288050@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I'm self-teaching myself electronics and the only place to ask
questions is in this forum so please excuse me if this has been asked
a million time already but after reading several books, I still have
questions on these topics. No real values, just theory in this
question.

I'm just starting the section on AC and the book is explaining why we
(homes and industry) use AC instead of DC and the use of transformers.
Now, the books says the reason we use AC is to minimize power loss.
That homes and industry need a lot of current and if we were using DC
we'd need to push a huge amount of current through the transmission
lines and the higher the current the more we'd lose in heat loss. OK
fine. But now let's see what happens in AC. Rather then pushing a
huge amount of current we have a very high voltage say 200,000 to
600,000 volts and a low amount of amps (current). But how can we have
this HUGE amount of "pressure" (the typical explanation of what
voltage is) and hardly any current ? I've been reading that voltage
and current are proportional - the more voltage the more current.
Ahh... but this isn't the case really since current is a variable
value. It depends on the amount of resistance. So getting back to the
transmission lines, if we have HIGH voltage and LOW current then
resistance MUST be high. E = I * R that is, if I is low R must be high
to get a high value of E. And resistance is what cause heat which
causes power loss. So how can we have low current + low resistance =
high voltage ?

In summary, if we have very high voltage and low current we must have
very high resistance which would eliminate just about all the current
so loss would be almost 100%.


TIA

J


Image you wanted to transmit 1Kw (kilowatt) of power from point A to point
B. This 1Kw of power could represent 1V (volt) at 1000A (amps) or it could
represent 1000V at 1A or an infinite number of other possibilities. The
product of volts and amps in all cases is 1000 watts. If your transmission
line has 1 ohm of resistance there are two scenarios:



A - You push 1000A of current of through the transmission line and
experience a power loss of 1000A * 1000A * 1Ohm (I squared * R) =
1,000,000W (watts) or 1000Kw of power dissipated as heat in the
transmission line. You will measure a 1000V voltage drop across the
transmission line and will require 1001V to get your 1V at 1Kw to the load
on the other end. You lose 1,000,000 watts of power in the process of
transmitting 1000 watts.



B - You push 1 A of current through the transmission line and experience a
power loss of 1A * 1A * 1Ohm = 1 Watt of power dissipated as heat in the
transmission line. You will experience a 1V voltage across the power line
and so will need 1001 volts to get your 1000V at 1Kw to the load on the
other end. You lose 1W of power in the process of transmitting 1000 watts.



You will need a transformer on the other side of the transmission line to
convert your transmitted power to the voltage required by your load.
Transformers make converting power from high current - low voltage to high
voltage - low current and back again trivial in AC systems and with very
little power loss in the transformer itself.



The down side of high-voltage transmission is the possibility of
insulation failure, arc-over and, of course, accidental electrocution of
personnel.



The same power transmission problems exist in other systems such as
hydraulic and mechanical systems. If you were transmitting a large amount
of hydraulic power over a long distance you might convert it to high
pressure-low flow using an appropriate hydraulic converter. When
transmitting a large amount of mechanical power a long distance you might
use the gear-train (mechanical equivalent of a transformer) to transmit
the power in the form of a high torque - low rpm. The power robber in all
these systems is friction due to movement of some medium whether its
fluid, rotating parts or moving charges.



Hope this helps.



Dorian





.

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