just call it 2 phase

Ah he was smarter than that. Let's see, you want to put a small transformer in every light fixture so that he won't sell as many light bulbs. General Electric (which at one time was Edison's company IIRC), sold lightbulbs by the millions.

Considering that few people are conversant with total cost of operation, they would have looked at Edison's more expensive light sockets and opted for someone else's product line.

daestrom

|> |> I don't like to call any AC system based on Edison in any way. Edison |> did |> |> not design around AC. He did DC. Thus he didn't split his power |> system |> |> in |> |> any way considering angles, because there were no angles. Edison would |> |> not |> |> recognize the power system coming into my home. Tesla might. |> | |> | Au contrarie, Edison might look at the three wires, measure 120/120 and |> 240 |> | and say, "Gee, that's pretty much how I did it except you're using that |> | 'deadly' AC crap!" |>

|> And, of course, that was Edison's downfall from being a big supplier of |> electric |> power to the country. But had he accepted AC back in those days, I |> believe it |> would have had much more influence on what we have today as electrical |> systems |> than anything we could possibly do today. If he had stepped AC down to 10 |> volts |> at the light socket, he would have been able to make electricity safer |> (because |> the light socket was the most dangerous part, being right up near where |> people |> worked), but also made his light bulbs more reliable (lower voltage means |> a |> thicker filament). His goal wasn't to sell light bulbs in greater |> quantity. |> His goal was to sell electric service as a replacement for gas lighting. |>

| | Ah he was smarter than that. Let's see, you want to put a small transformer | in every light fixture so that he won't sell as many light bulbs. General | Electric (which at one time was Edison's company IIRC), sold lightbulbs by | the millions.

Edison was not making light bulbs to sell light bulbs. If that were the case, he wouldn't have done what it did to actually try to extend the life of them. Remember, he did not invent the light bulb. He just improved on it. Instead, he was trying to create a greater demand for electricity than existed for a few motors here and there at the time.

| Considering that few people are conversant with total cost of operation, | they would have looked at Edison's more expensive light sockets and opted | for someone else's product line.

Either way, the business model was on creating a demand for electricity and selling an incrementally priced service. He would not have the market locked up on light bulbs for very long. His goal was the electric service business which would be a monopoly where it was deployed.

I disagree. ISTR one of his 'demons' driving him to produce a *practical* electric lamp was that he was concerned over the fire hazards of gas/oil lamps. The 'Edison Electric Lamp' was clean and much safer. My father had an advertising poster from that era that boasted no need to strike a match, this room was equiped with the new, cleaner and safer Edison Electric Lamp.

daestrom

|> Either way, the business model was on creating a demand for electricity |> and |> selling an incrementally priced service. He would not have the market |> locked |> up on light bulbs for very long. His goal was the electric service |> business |> which would be a monopoly where it was deployed. | | I disagree. ISTR one of his 'demons' driving him to produce a *practical* | electric lamp was that he was concerned over the fire hazards of gas/oil | lamps. The 'Edison Electric Lamp' was clean and much safer. My father had | an advertising poster from that era that boasted no need to strike a match, | this room was equiped with the new, cleaner and safer Edison Electric Lamp.

To some degree he was concerned over the gas/oil hazard. But he also knew the public was concerned over it and was merely taking advantage of it with a solution that was indeed safer. It's not any different than any other business recognizing a public concern and providing a solution because they can see a market in something safer. But this would be the same whether DC or AC power was used. The big issue was why Edison wanted to stay with DC. Did he genuinely believe AC was more dangerous, or was he just protecting his investment in DC and marketing AC as dangerous.

Most stepper motors are actually two-phase devices. They are often driven by two square waves with a 90 degree phase shift, but they can also be driven by sine waves to give a much smoother rotation.

Dave

On Thu, 26 Feb 2009 09:12:08 +0000 (UTC) Dave Martindale wrote: |>Single phase motors are definitely inferior to a polyphase |>motor, including a two phase machine- but two phase systems of any |>consequence died before either of us saw daylight - some recovered |>during the 40's as low power control motors and tachometers (phase fixed |>but voltage magnitudes variable). | | Most stepper motors are actually two-phase devices. They are often | driven by two square waves with a 90 degree phase shift, but they can | also be driven by sine waves to give a much smoother rotation.

Controlled motors can be done in a lot of ways. A couple years ago I did a forensic disassembly of a VCR that was hit by lightning, and notice the head motor on it was 12 pole. This certainly wasn't a power frequency driven motor.

For very basic motors, with little or no control, the practical choices have been made already.

You are right. I am showing my age in referring to 2 phase "variable voltage on one phase" 2 phase machines-which predated steppers-using sinusoidal excitation. In the same era, 3 phase devices for remote positioning (forget the name) were used. In that era, the modern digital electronic control was not available or was too bloody clumsy prior to transistors and integrated circuits. Since then both 3 phase and 2 phase steppers have been built and used. From your comments, it appears that the pros and cons of 3 vs 2 phase steppers have come down in favour of 2 phase. For sinusoidal excitation- they are equivalent except for a

And then there were syncrho-tranmistters that used single phase on the rotor and three 'phases' of output to drive receiver units whose rotors were excited by the same single-phase supply. If you swapped the single phase supply the receiver would point 180 out and if you swapped the 'three phases' on the stator you could get rotation in the opposite direction (or be off by 120). These may be the remote positioning units you're thinking of.

Of course these were not meant to rotate continuously but rather provide position/orientation information. And in that vein there were all sorts of variations including 'resolvers' that would provide the sin() and cos() functions of a ship's speed and direction and break it down into N-S speed and E-W speed.

Not exactly 'two-phase', but I always found them fascinating. Used for fire-control systems for ship's guns back before electronics. Also used for dead-reckoning plotter with a simple 'bug' under a glass table.

daestrom

I have a set of these that runs on 120V, 60 cycle single phase. (One side of two phase?) :-), and actually produces useful torque at the receiver. I used them to rotate my amateur radio antenna.

About the only recent use of the Scott-T transformer set is in converting from the 3-phase "synchro" data to the 2-phase "resolver" data. They're still used in some servo systems.

Of course now almost everything is digital. I have a patent on a high-precision "resolver" for machine positioning that used an electrostatic pick-up from a conductive pattern on the rotor. The output was digitized for machine control.

A variation of this is the electromagnetic deflection of the PPI radar display used for aviation traffic control.

Thanks- synchro is the word that I wanted ( and escaped me completely at the time of writing -so much for long term memory!). In another application, large versions were used in lifting wide "gates" using 2 motors and the transfer was such as to balance the lifting force on the two sides of the "gate" They could rotate continuously but this would be effective at low speeds only.

Are you referring to "Selsyn" motors?

Selsyn is the name, thanks. There were other motors but the selsyns were fascinating. The same concept was used to transmit speed information from the shaft of a generator to the flyballs of a governor.

I say a variation of that in shipyard cranes (the tall cranes that straddle the pier with trucks/train going under them). They used two wound-rotor motors, one on each 'truck' of the gantry. By tieing the three-phase rotor connections together, the two rotors would turn in locked-step so the gantry would go down the pier straight. Not exactly a synchro, but a neat way to control the motion of both sides of the gantry long before position indicators, digital feedback, or other 'hi-tech' tracking systems.

daestrom

There's no reason each socket would have needed a transformer.

Edison was a smart guy, but he was rather bull-headed. I've read many books about him over the years, as I've long admired him as well as others such as Tesla and Westinghouse. Edison was dead-set convinced that his DC system was superior, safer, and the only way to go, and he was unable to see it any other way. It was more about his stubbornness than any true technical or market advantage. The attitude is still everywhere, look at all the people who are fiercely loyal to a particular car manufacture or computer operating system. They will go to great lengths to push the superiority of their chosen one while insisting that everything else is worthless junk, regardless of the actual merits of one over the other. It's more a religious argument than anything else.

Bad move. More current causes fires, or makes them more of a problem in the home. Fires are a bigger issue than electrical shock ever was.

In Edison's case, it was a combination of hard headed preservation of his income source (as Daestrom indicates) and the bull-headedness that you indicate. While he pushed the "safety factor" (and it is true that human response to 60Hz is greater than to DC and higher frequencies- but Edison didn't know that!) despite lack of evidence to the contrary (I would rather break 100A AC at any voltage level than for DC at the same or lower voltage level for DC).

On Tue, 24 Mar 2009 17:18:08 -0700 StickThatInYourPipeAndSmokeIt wrote: | On Tue, 24 Mar 2009 13:23:51 -0700, James Sweet | wrote: | |> If he had stepped AC down |>>> to 10 volts |>>> at the light socket, he would have been able to make electricity safer |>>> (because |>>> the light socket was the most dangerous part, being right up near |>>> where people |>>> worked), | | | Bad move. More current causes fires, or makes them more of a problem | in the home. Fires are a bigger issue than electrical shock ever was.

For long runs of wiring, yes, more current increases risk. Stepping the voltage down right at the point of utlization is just not that kind of risk. Millions of low voltage lighting systems in use today would have to be ripped out if this were a problem. Fires due to overloaded circuits are a continuing problem even at line voltage. That's one reason my kitchen plans include 6 outlets (4x 5-15 on 2x 20 amp circuits, 1x 5-20 on its own dedicated circuit, and 1x 6-20 also on a dedicated circuit) at each point where outlets will be. Plenty of outlets means none of those triple tap things. I saw one catch fire when I was young. It had been pulled part way out of the wall outlet due to the weight of the cords plugged into it.

| In Edison's case, it was a combination of hard headed preservation of | his income source (as Daestrom indicates) and the bull-headedness that | you indicate. While he pushed the "safety factor" (and it is true that | human response to 60Hz is greater than to DC and higher frequencies- but | Edison didn't know that!) despite lack of evidence to the contrary (I | would rather break 100A AC at any voltage level than for DC at the same | or lower voltage level for DC).

As I understand it, Edison's DC system was actually pulsed DC from generators with brush contacts switching polarity. Or at least that's the illustrations I have seen. Perhaps he paralleled then at different locked phases to avoid the zero voltage point? If it was truly pulsed and went to zero volts, then shouldn't it have been as easy to break fault current as AC?

I presume, until I hear otherwise, that the generators used by Edison were fairly similar to modern dc generators except for refinements and bells and whistles.

Coils rotating in and cutting the magnetic field. Each is connected in series with adjacent coils as well as to commutator segments. Each coil generates ac that gets rectified at the commutator. The brushes contact the commutator at segments that have small potential differences between adjacent segments. That is because the attached coils happen to be moving mostly along the magnetic field.

Except for some commutator ripple as the brush shorts out adjacent pairs as it moves along the segments, the output is dc all the way. There is not series of pulses.

Bill