50 hz VS 60 hz and a 120 HZ question

I seem to remember reading that Westinghouse-Tesla AC was chosen over Edison DC because the AC could be transmitted with lower power loss over distance. So was AC more efficent at the distances it was being transmitted at the begining of the electrical revolution? Thanks Karl

Reply to
kfvorwerk
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Yes, because unlike DC it could be transmitted at high voltage (requires more cheap insulation or spacing) , low current (requires less expensive copper) and easily transformed to low household voltage at the end. The Edison system needed small, inefficient power plants in every neighborhood. Also Edison wasn't as clever a businessman and promoter as George Westinghouse, and I think the propaganda film of AC killing a horse backfired on Edison.

Reply to
Jim Wilkins

An interesting side-bar comment may be in order here:

One of the requirements of a transmission line is that it be able to transfer most of the power applied to it. Transfer is attenuated by the inherent losses of the transmission line. The ideal transmission line has only resistive loss (R=E / I) and that is, of course, a "DC" line - or simply a piece of wire carrying only direct current.

Early on in the formative days of telephony it became apparent that a pair of wires, unto itself, made a lousy transmission line for "voice currents". Oliver Heaviside, showed that a telephone wire pair could be equalized for voice transmission over long distances. This was done via introduction of "lumped constants" in series at strategic locations along the transmission path. Inductance was inserted to offset the effects of capacitance, wire to wire, wire to pole, and wire to ground. Later, this became known as "loading" and found its way into cable pairs carrying high frequiency, multiple conversation, telephone carriers. A modern application is DSL.

Little known communications trivia: Transcontinental telephone service was available in the U.S. long before the advent of electronic amplification. It was made possible by loading ("Heavifying") ordinary open-wire telephone pairs. AFAIK, only the pair of wires separated by the telephone pole, known as the "pole pair" were loaded. They were of heavier gage wire than ordinary and loaded with series inductances. Thus you could talk coast-to-coast over an unamplified open-wire telephone pair. The toll rate was pricey.

Bob Swinney

Reply to
Robert Swinney

most of the power

transmission line. The ideal

"DC" line - or simply

wires, unto itself,

that a telephone wire

done via introduction

path. Inductance was

wire to ground.

carrying high frequiency,

available in the U.S.

loading ("Heavifying")

the telephone pole,

ordinary and loaded with

pair. The toll rate was

The reason this worked is because it altered the characteristic impedance (Zo) of the line to match the Z of the telephone transmitter (carbon button mike) and receivers, which were about 600 ohms. Optimal power transfer occurs when source, line and load are impedance-matched. Zo of a line is sqrt(L/C) where the L and C are per-unit-length as henries per meter and farads per meter. Adding distributed series L's in the line raises the Zo of the line.

If you look inside an ordinary telephone of later vintage, one with a dial on it (disc with 10 holes), you will find an inductor. Same idea.

>
Reply to
Don Foreman

OK, we rule out 400HZ as too high, due to the 1/4 wave length problem, would it be worth while to go to 90 or 120HZ?

Standard around here anyway is uninsulated high tension lines, would insulation help?

I read a few articles in my welding mags where a shop bought all new inverter type power sources and paid them off in less than a year in power savings, so there has to be less power drawn by the supply, correct?

Thank You, Randy

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Reply to
Randy

The difference wasn't AC vs DC per se, but voltage. Power can be transmitted more efficiently at higher voltage and lower current. AC enables use of high-voltage transmission because voltage of AC can be efficiently changed up or down with transformers. Consider that the feed for 200 amp 220-volt residential service is 2/0 copper, .365" dia. This same size wire at 500,000 volts would serve 2273 residences. "Wire" to handle these 2273 residences at 220 volts would be over 17" dia!

The DC equivalent to a transformer at the time would have been a large motor-generator set which was expensive, inefficient, required maintenance and would eventually wear out.

Reply to
Don Foreman

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

Reply to
Jon Elson

But, the core losses per volume (or weight) goes up with frequency. Hysteresis loss is directly proportional to frequency, you are forcing the iron atoms to realign their spins at twice the frequency. Eddy losses are not so simple, they have to do with Bmax and the lamination thickness, mostly.

Jon

Reply to
Jon Elson

Yes, because there WAS the AC transformer. You could use a DC-DC motor-generator set, but there are practical limits to how much voltage you can get out of a generator armature. It was pretty easy to see that with no moving parts, you could get much better insulation in a transformer, and thus transmit power at kilovolt levels with existing technology in the late 1890's.

With Edison's DC system, you couldn't convert voltages without rotary machines, and nobody was going to have a 1000 V line feeding a rotary converter in their basement. And, nobody wanted 1000 V on their bulbs and light switches. So, if the entire system ran on 120 V DC, then you had large currents flowing all through the system, and got pretty big voltage drops in just a couple city blocks. You could fight this with big wire on the poles for a while, but it soon got out of hand.

The pole transformer outside my house runs on 7200 V, so it is a 30:1 winding (primary to the full 240 V secondary). So, if I am pulling the full rated load for my house (200 A) the transformer is pulling 6.67 A from the high voltage distribution mains. MUCH easier to send 6.67 A down a wire than 200.

Jon

Reply to
Jon Elson

The really big advantage of a 3-phase hermetic refrigeration compressor is it needs no starting relay. Just apply power and the motor spins up. I doubt there is a great increase in efficiency of small 3-phase motors over single phase. If you want REAL efficiency gains, permanent-magnet synchronous motors run by variable frequency drives do show large gains in efficiency. All variable-speed air conditioners use some sort of VFD, and permanent magnet motors are now replacing the induction motor, as they ARE more efficient.

The liklihood of the US adopting 3-phase wall outlets for all appliances is about as high as (your favorite never-will-happen scenario here). There would be REAL costs to extending 3-phase power to all residences, and putting in 3-phase meters and 3-pole breakers in the panels. With a basement full of machine tools, I'd love it, though. I do NOT have

3-phase power on my pole, but it is "only" a block away.

I can just imagine having a 5-prong Harvey Hubbell twist lock plug on those lamps with three bulbs and a 1-2-3 switch on them!

Jon

Reply to
Jon Elson

One could argue the thing forever, as the losses are almost all engineering tradeoffs when the various components are designed.

But, it is all moot. I have no way to calculate the cost, but if the entire generating, transmission and distribution system had to be replaced, plus most appliances more complicated than a toaster or light bulb, the cost would certainly be in the trillions of $. Couldn't possibly be any less than that. Maybe some power house alternators could be re-wound for 4-poles to get the required frequency, and the transmission lines could probably stay, but that's about it. Now, for the transformers, the Bmax would go down at the higher freq, assuming same voltage, and that would help the hysteresis loss, but I think some of the other losses would go up.

But, this stuff gets REALLY complicated. For instance, the leakage inductance of the transformers is carefully engineered to control fault currents. At higher F, the fault currents would go down, but that means voltage regulation would get worse, too.

Absolutely not at all. You could try shielding them, but that would be insanely expensive. But insulation does not prevent the electric and magnetic fields from radiating, so it would have no effect.

it could just be that they were getting blitzed by the power company for having terrible power factor with the old welders. The way old welding power supplies were made, they really were quite efficient, but the transformers were intentionally made to have huge leakage inductance, that's how they limited the welding current. Thus, the native power factor was insanely bad. So, they had to add power factor correcting caps, which drew a huge leading PF when not welding, and were a compromise that only corrected the PF at one welding current.

Depending on the type of meter they had, they could get dinged REALLY hard for the bad PF.

Jon

Reply to
Jon Elson

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

Reply to
Jon Elson

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.

Reply to
Leo Lichtman

With 3 phase distribution you would not have 5-prong Harvey Hubbell twist lock plug on your lamps.

In this country (Australia) 3 phase is available in most places but only delivered to those homes that need it for Air con and such bigger loads. Small things, lamps etc still run of one simple plug similar in size to the US 120volt plug. The area served by a pole transformer or roadside box seems to be larger than is common in the US but the single pase homes are distributed across the three phases. Pole pig outside my house was changed last week from 40KW to 60 KW (I think).

And Gloat!! I could have 3pahse in my workshop if I needed it but a table saw is the biggest thing I have.

John G.

Reply to
John G

They didn't have the "marvolus" solid state and wonderful gas switching devices back then so DC was at a SERIOUS disadvantage. :-) ...lew...

Reply to
Lew Hartswick

the efficiency comes with varying loads. A single phase motor is designed to operate at a certain load, any load more or less the phase shift to the second winding through the cap is not at optimum and will generate more heat.

If you want REAL efficiency gains, permanent magnet

I would look more to supplying houses with DC. With today's electronics everything can easily run on DC with built in VFD's that run directly from dc rather than first rectifying it inside the drive. The same goes for any appliance. Using DC would make it much easier to eliminate switching noise from vfd's and other similar devices by just adding a couple of caps on only one circuit.

John

Reply to
john

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

Reply to
john

The lower frequencies are more efficient but the motor size increases. At 400 cps the size of a 1 hp 3Ø 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 3Ø 115 volt Wye. All the lighting is fed at 115 vac.through transformers thus saving on wire size.

John

Reply to
john

Interesting, a mate who used to work in Germany said that most houses there had 3 phase and different floors were supplied with different phases to balance the load. Here in the UK a house typically has 1 phase and the phases are distributed around other houses to even the load. My mothers house does have 3 phase wired to it though as the previous owners built the house and said that having 3 phase wired at that time was only a small additional cost, all the house need is a 3 phase meter to use it as the meter is currently single phase, a small cost seeing as the 3 phase is in place already.

Reply to
David Billington

No, it would be "abnormal", but I COULD do it if I had 3-phase in the house! It would definitely WOW people, though.

I just happen to be in a funny spot where it was inconvenient to run the low voltage cable to other houses. Many of the houses around here have a couple of them sharing a transformer. A subdivision of really fancy ($500K - $1 million) houses have pad mount trandformers in tasteful green camouflage for every 3 houses or so. We live on a full acre, most of the houses have that much or more in this area.

OOOhhhh, pleas DON'T rub it in! Since the ready availability of these VFDs, it is a lot less of a problem.

Jon

Reply to
Jon Elson

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