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 !
"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 !
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.
"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.
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...)
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.
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.
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.
"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.
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.
"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 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.
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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