I think that you are mixing up size and efficency. The higher the
frequency the size for a certain power output decreases. But ! the
losses from eddy currents and the like increase. There is a crossover
point which was below 50 hz in the old days ....most of the old
generating stations were 25 hz and even 16 2/3 Hz.
Today with modern steels and thinner laminations the cross over point
is much higher. But that is not the only point to consider ! with high
voltage transmission lines the losses climb with frequency and
distance. The very big long ones even use DC !
Regards Bob
"bob" sez:
"I think that you are mixing up size and efficency. The higher the
frequency the size for a certain power output decreases. But ! the
losses from eddy currents and the like increase. There is a crossover
point which was below 50 hz in the old days ...."
There is no specific coorelation re. size, frequency and efficiency. The
efficiency of any
electrical device is a function of its design. i.e., efficiency is always a
trade-off against cost,
performance, weight and a host of other things. Each device is designed to
operate with a certain
efficiency (not always the maximimum efficiency) within certain boundaries, cost
usu. being
foremost, among them.
Bob Swinney
most of the old
generating stations were 25 hz and even 16 2/3 Hz.
Today with modern steels and thinner laminations the cross over point
is much higher. But that is not the only point to consider ! with high
voltage transmission lines the losses climb with frequency and
distance. The very big long ones even use DC !
Regards Bob
This group seems to have alot of electrical knowledge, sooo.....
The US is 60 HZ and Europe is 50 HZ, a 60HZ motor is more efficient,
so, what would happen if the US or the world for that matter would
switch to 120 HZ or maybe even as high as 400HZ (which is common in
aircraft). Motors and transformers would be much more efficient,
power savings could be enormous, both in transmission and use.
I realize this would have to be a 50 to 100 year task. Laws would
need to be passed and electronic devices would need to be sold that
would work on both frequencies. (many switching power supplies such as
in computers do not care what freq. power is input.)
I've read a bunch of articles about the newer inverter type welding
power sources that use 400 HZ and use small transformers and use much
less input power for the same output power.
I'm guessing it would take a panel of "experts" a few years to work
out the costs VS savings for this one. I've heard of some really
stupid things being studied by our government, maybe someone should
look into this.
I bought some old machinery from the Bethlehem Steel plant that was 25
cycle, WOW, talk about inefficient!! I had a 5hp motor that was built
on a 15 hp frame size.
Ideas, thoughts??
Biggest problem I think would be the generating end of things.
Thank You,
Randy
Remove 333 from email address to reply.
"Interesting you mention DC transmission lines. My neighbours were in
China last year and saw the new big dam, they were told the generating
station produced 500kV AC for local use and 500kV DC for long distance
transmission. Subsequently I have been told that some UK electricity is
supplied by France as DC. My question is what sort of gear is used to
convert the DC to AC. A big motor generator unit?."
Naw ! That would be . . . . too inefficient ! They would have done it that
way in the old days.
Now it is done with solid state devices.
Bob Swinney
Interesting you mention DC transmission lines. My neighbours were in
China last year and saw the new big dam, they were told the generating
station produced 500kV AC for local use and 500kV DC for long distance
transmission. Subsequently I have been told that some UK electricity is
supplied by France as DC. My question is what sort of gear is used to
convert the DC to AC. A big motor generator unit?.
But that is not the only point to consider ! with high
Aircraft use 400 Hz stuff because it weighs a lot less. And
aircraft can get away with it because they are "small."
When a transmission line becomes a significant part (more
than a few percent) of a wavelength, interesting things start
to happen with the greatest "strangeness" occuring at a
quarter wavelength. You can easily make a transmitting
antenna that will radiate most of your power. And, if you
put a significant load (low impedence) a quarter wavelength
down the transmission line, it will reflect an open at the
other end of the line and you won't get any power to go down
it at all.
In free space, a quarter wavelength at 60 Hz is 775 miles
(somewhat less in a transmission line...). So, a line of up to,
maybe, 50 miles can be constructed with little or no attention
paid to the details. For longer lines, an engineer is going to
have to start paying attention to the design, which, of course, is
pretty much standard practice. But a wavelength at 400 Hz
is only 15 percent of that at 60 Hz, so the typical line lengths
found, say, running around an average city would have to
be carefully engineered...
(And, yes, some of the numbers I used are SWAGs. But I'm
a radio guy. A power guy might use a sharper pencil. YMMV,
but you get the idea...)
Jerry
Inductive reactance =2pifL, so, as frequency goes up, the reactance goes up
in direct proportion. Not what you want when you're sending power to a
load. Capacitive reactance = 1/2pifC, (that's 1 over 2pifC), so reactance
goes down as frequency goes up. This is leakage path between the
transmission lines and to ground. Also not what you want.
Leo,
And your point is ?
Bob Swinney
Inductive reactance =2pifL, so, as frequency goes up, the reactance goes up
in direct proportion. Not what you want when you're sending power to a
load. Capacitive reactance = 1/2pifC, (that's 1 over 2pifC), so reactance
goes down as frequency goes up. This is leakage path between the
transmission lines and to ground. Also not what you want.
efficiency of any
trade-off against cost,
operate with a certain
cost usu. being
A transformer's (or inductor's) core volume goes down as the reciprocal of
frequency for any given power level. Core materials can hold a nearly
constant amount of energy per unit volume, and the energy they must hold
is pretty close to power / frequency. This is why switching supplies (and
amplifiers) run as fast as the output transistors and catch diodes can go
-- they're trying to drive the transformer size down.
I believe that a motor's ability to transmit mechanical power follows a
similar trend, although the energy is stored in the gap between the
armature and coils -- I haven't done the math on this one yet.
So I certainly disagree as it pertains to transformers, and I think I do
for motors.
=========
what a group!
thread hijack [sort of]
Given the increased efficiency in terms of both size and power,
how would 3 phase residential power service (in new homes) affect
power distribution cost/efficiency and the life-cycle cost of
high reactive consumption units such as residential air
conditioners and possibly refrigerators if these were 220
3-phase?
Unka' George [George McDuffee]
============
Merchants have no country.
The mere spot they stand on
does not constitute so strong an attachment
as that from which they draw their gains.
Thomas Jefferson (1743-1826),
U.S. president. Letter, 17 March 1814.
George sez:
"Given the increased efficiency in terms of both size and power,
how would 3 phase residential power service (in new homes) affect
power distribution cost/efficiency and the life-cycle cost of
high reactive consumption units such as residential air
conditioners and possibly refrigerators if these were 220
3-phase?"
This poses a very difficult problem from an analytical viewpoint. On one hand
wire sizes could be
made smaller to deliver the same amount of power. But it would take 3/2 X more
wires, albeit of
smaller size. This get's into the labor - material continuum. Well designed
3-phase motors have no
better efficiency than their single-phase counterparts of the same power rating,
although they can
be made smaller. The labor - material continuum again. But 3-phase motors are
simpler to build
than single-phase motors and thus are a little more economical in scale.
Transformers at the
residential power drop would be 3-P to 3-P but there again, complexity would
increase cost.
What this boils down to is pretty much the answer to the original question:
There is no specific coorelation re. size, frequency and efficiency. The
efficiency of any
electrical device is a function of its design. i.e., efficiency is always a
trade-off against cost,
performance, weight and a host of other things. Each device is designed to
operate with a certain
efficiency (not always the maxim efficiency) within certain boundaries, cost
usu. being
foremost, among them.
Bob Swinney
Perhaps I can answer the question about the DC transmission of power.
The Dalles dam on the Columbia river transmits power to the LA area
using DC. In the late 1970's, I think, I was in the Portland Amature
Radio club. A member was an engineer for a local power company. He set
up tours for us to several interesting power generating facilities,
and one was the DC converter station at the Dalles dam. The station
takes 18 phase AC power, yes, 18 phase, at several thousand volts. The
power is then fed into several rows of mercury vapour controlled
rectifiers. They stand on ceramic insulators about 10 feet above the
floor. The output of the rectifiers feed power at 250,000 volts over a
pair of wires all the way to LA. The origional plan was to use both
lines at 500,000 volts and use the earth as a return. Law suits
stopped that because of the havoc caused by the ground currents.
An identical installation is on the LA side of the transmission lines.
There the controlled rectifiers convert the DC back to AC. The system
is symetrical in that when the river flow is low in the winter and the
people in LA are not using their air conditioners, the excess power is
sent back to the Northwest using the same AC to DC system.
The entire transmission system is controlled by computers. Even back
in the 1970's.
A very interesting and educational experience.
Paul
"Robert Swinney" wrote: Leo, And your point is ?
^^^^^^^^^^^^^^^^^
If it's not clear, then I may be wrong about something. The OP was
proposing to raise power line frequency from 50 hz or 60 hz to 400 hz. It
looks to me like that would raise the inductive reactance and lower the
capacitive reactance of the network, both of which would be wasteful. If
that's not correct, straighten me out, please.
Sorry Leo. I think the OP was trying to strike some sort of a connection
between line frequency and
efficiency. Equipment designs can be varied in numerous ways to maintain or
even improve
efficiency. As you suggest, reactance's can be manipulated as a way of
accommodating design changes
and maintaining efficiency. Changing line frequency, up or down, without taking
reactances into
account would certainly lower efficiency in any network. The rotary phase
converter comes to mind.
Although frequency is unchanged, reactance changes in the converter-load, viewed
as a network,
change with a change in idler motor size. Adding series cap. reactance is a
method of offsetting
(broadly tuning) increased inductive reactance as the size of the idler motor
and/or load is
increased.
Bob Swinney
"Robert Swinney" wrote: Leo, And your point is ?
^^^^^^^^^^^^^^^^^
If it's not clear, then I may be wrong about something. The OP was
proposing to raise power line frequency from 50 hz or 60 hz to 400 hz. It
looks to me like that would raise the inductive reactance and lower the
capacitive reactance of the network, both of which would be wasteful. If
that's not correct, straighten me out, please.
Regardless of the frequency, the laws of physics do not change. Power is
"consumed" (turned into heat, mostly) in a resistive load. And the IR
losses
will be the same (assuming the same voltage and current) regardless of the
frequency. There is no true loss due to reactance, only an opposition to
current flow. But, inductive and capacitive reactances are 180 degrees out
of phase with one another and, hence, if they are equal in magnitude,
cancel.
Thus, their contribution to the net impedence is zero; only the resistive
component has any effect on current flow and this effect is, of course,
minimized by using as high a voltage and low of current as practical. Now,
if the capacitive and inductive reactances are not equal, the dominant one
will twist the phase of the current with respect to the voltage and the
result
will be a loss of efficiency (a power factor of less than unity, to use the
buzz-
word...). So, the distributed inductance and capacitance of the
transmission
line become primary considerations of the engineer designing the line. And
the design will be different at different frequencies. But, in properly
designed
transmission lines, efficiency is independent of frequency.
This explanation is something of an over-simplification, of course. There
are
other things that come into play as the frequency goes up. But, at low
(power
line) frequencies, they can largely be neglected. (As one goes up into
radio
frequencies, things get complicated real fast...) And, of course, in the
trivial
case where the frequency is zero (DC), all of these concerns simply go away.
The bottom line is that, as the frequency goes up, the trickier the
transmission
line becomes.
Jerry
"Jerry Foster" wrote: (clip) Regardless of the frequency, the laws of
physics do not change. Power is
^^^^^^^^^^^^^^^^^^^^
I think you meant to say I^squared.
^^^^^^^^^^^^^^^^^^^^
There is no true loss due to reactance, only an opposition to
^^^^^^^^^^^^^^^^^^^
If the current is out of phase, the quadrature* component will have
I^squared R loss associated with it.
*I'm amazed I remember this term from college--I graduated in 1952.
^^^^^^^^^^^^^^^^^^
(clip) the distributed inductance and capacitance of the transmission
^^^^^^^^^^^^^^^^^^
We must realize, of course, that this is not the same as saying that a
transmission line can be designed whose efficiency is independent of
frequency. Transmission lines have to be designed for the particular
frequency that they will carry. I think this is a point that needs to be
added to the list of obstacles to the idea proposed by the OP.
Thanks, Jerry and Robert for the clarification/expansion of my comments.
A big NEGATIVE on the transmission. The higher freq means more
rediated power. (lost)
Yes a LOT less iron in all the machinery and transformers.
The REAL long transmission lines are now very high DC for
this reason.
...lew...
Higher frequency motors can be smaller for given power because they
run at higher speeds. A 4-pole 400 Hz induction motor would run at
about 11,500 RPM. System efficiency would probably be less because
speed reduction by gears etc would usually be required, but system
weight is what counts in aircraft.
As Bob Swinney points out, effiency is a matter of design, traded off
against other parameters like size, weight, cost, etc. In one sense,
lower frequency motors, though larger and more costly for given power,
could be more system-efficient where their lower speeds might reduce
or eliminate need for speed reduction kit which also has losses. Gear
trains are often considerably less efficient than ordinary AC
induction motors.
Power transmission over significant distance is generally more
efficient at lower frequency, with DC being most efficient. Further,
DC distribution would have the same advantage as 3-phase in that it
can deliver power continuously while single-phase AC delivers power
intermittently and thus requires energy storage in most useful
devices. In an ordinary motor this energy storage is in kinetic
energy and in the magnetic field. In electronic gear it's in
capacitors.
Developments in power electronics have made huge advances in the past
decade, and this will continue in terms of lower cost, higher
effiency and higher power levels. Power elex makes line frequency
about irrelevant because the power can be modified to suit the load --
as is already done in VFD's and brushless DC motors.
Power elex makes the notion of a "DC transformer" possible. The
actual xfmr is AC, usually at frequencies considerably higher than 400
Hz, but the functional block is DC in and DC out.
Inverter-type welders don't draw less power, but they do have much
better power factor than copper-iron machines so they draw less line
current, and thus result in lower losses in the distribution system.
A further point is the line length.
Speed of light = 300,000 km/sec
So, at 400Hz, 1 wavelength = 300,000/400 = 750km
And 1/4 wave is just less than 200km.
This is by no means an unheard-of length for a transmission line.
At 1/4 wave, all sorts of interesting transformer-like effects occur, as
any radio ham will tell you!
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