50 hz VS 60 hz and a 120 HZ question

Don sez:
"As for the transmission lines, I notice that the high-tension three phase lines every few towers appear to interchange two of the
three wires -- I guess making the equivalent of twisted pair wiring to minimize radiation by canceling from the other phase. I suspect that the interchanges would have to be redone to be closer together at a higher frequency."
Power lines interchanging location among themselves are part of a "transposition" scheme. All open wiring, be it power or communications, employs some sort of transposition. Open wire pairs with high frequency "carriers" are transposed at regular intervals in a scheme (crosstalk elimination) that protects them against other wire pairs on the same pole line, and protects other wire pairs from the carrier (s). The transposition scheme on a large open wire lead can be quite complex. It is closely related to common mode rejection of audio disturbances on balanced lines or the inputs of operational amplifiers.
A more apparent transposition is that found in communications cables. All the pairs are twisted at varying pitches throughout the cable and the locations of individual pairs is moved around within any individual group, or bundle, of wires within the cable. Bundles are twisted around each other.
Bob Swinney
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In this instance, it's to equalise inter-phase and phase-earth capacitance and also phase inductance. Doesn't need to be done very often at all, but does need to be done once in a while.
Mark Rand RTFM
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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.
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Don Foreman wrote:

Why, yes, into the 1950's, Ingersoll Rand made direct-drive air compressors with these HUGE 300 RPM synchronous motors. An interesting feature of synchronous AC motors is that varying the rotor field allows the motor to draw leading or lagging power factor, so a device was put on the motor to read the plant's power factor and set the field to correct the whole plant's PF. This saved them a BUNCH of money on the electric bill.
Jon
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Randy wrote:

Others have addressed the true efficiency topic, so I won't. But, there are a bunch of downsides to the use of 400 Hz power. First, ever heard the whine of 400 Hz equipment in aircraft? If you think "hum" in your Hi-Fi is annoying, just imagine what 800 Hz (full-wave rectified 400 Hz) would sound like! And, for equipment powered by 3-phase, that would be 2400 Hz, totally piercing.
Another annoying point would be the speeds of motors. An ordinary 2-pole AC motor would run at 24,000 RPM! Now, of course, you can add a bunch of poles in the winding, and get to more reasonable speeds, but that all costs money, as do the much thinner motor laminations. The power companies would be horrified, as they'd have to build those transmission substation transformers with many feet of thin iron laminations, and use Litz wire for the windings to control eddy currents. Also, the radiation from the lines would be greatly increased. That's a major reason why really long transmission lines have gone to DC.
Jon
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Maybe in this context, this question is not too off topic. When people talk about electric cars being non-polluting, they often fail to consider that steam generation plants have smokestacks, and they DO put CO2 into the atmosphere. Of course, they run MUCH cleaner than auto exhausts. so I'm sure they pollute less. But, how much of the power they generate is lost in transmission? You have to know this in order to estimate the environmental benefit of electric cars. I would like to know: what is the efficiency of the power grid? Just a ball-park figure, for purposes of discussion will do.
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Leo Lichtman wrote:

There are many things to consider including idling at a stoplight with a gas engine while with a electric motor it is not using any electric. Also a motor can be a braking device and put power back into the batteries. No oil is needed for lubrication of the motor other than an ocasional shot of grease. The down side is that the batteries have an finite lifespan and they will be expensive to change. Another problem is heat in the winter , unless there is a small aux engine generator.
John
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"john" wrote: There are many things to consider including idling at a stoplight with a gas engine while with a electric motor it is not using any electric. Also a motor can be a braking device and put power back into the batteries. No oil is needed for lubrication of the motor other than an ocasional shot of grease. (clip) ^^^^^^^^^^^^^^^ John, I am well aware of the advantages of electric and hybrid cars. I am trying to get a handle on HOW MUCH advantage they offer in terms opf air pollution. An electric car is not totally non-polluting. The electricity to run it comes from a power plant somewhere, connected the power grid. We know how much CO2 is released when a pound of fuel is burned. The comparison that needs to be made is, how much fuel does it take to run an automotive engine, vs how much does it take in a power plant to drive it the same distance? Some of the power fed into the grid is wasted as heat. How much?
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Leo Lichtman wrote:

Roughly half.
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Randy wrote:

The lower frequencies are more efficient but the motor size increases. At 400 cps the size of a 1 hp 3 motor is one sixth of the size of the same HP motor run at 60 cps. With the higher frequencies you need less core material since the time of one cycle is proportionately less and with the smaller core it will saturate in 1/400 of a second. Proportionately 400 x a smaller flux is equivelent to 60 times saturating a larger core for the same HP but the hysterysis and eddy current loses go up with the higher frequency, but the core size goes down. The weight savings on an aircraft are tremendous. The wiring on many aircraft is 3 115 volt Wye. All the lighting is fed at 115 vac.through transformers thus saving on wire size.
John
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