Why Is there 3-Phase?

There is basically only one reason:

A more even load is placed on the prime mover and more even power supplied to the load.

Chuck P.

The fact that 3 phase motors are simpler is just a side benefit.

Economics.

You save money on generation and transmission because materials are used more effectively. But there are other advantages as well. All these advantages are achievable in other ways at higher cost.

The sheer amount of electrical power used overwhelms the advantages of other sources. For example, high voltage dc would be of great advantage for x-ray production but it is not economically feasible to drive x-ray tubes from a suitable dc source.

Bill

-- Fermez le Bush--about two years to go.

I'm not trying to pull an Edison. Specifically, what makes 3-phase more efficient than single phase?

Is it that you can kinda, sort of have a complete circuit with only one wire? Or, that you can have three complete circuits with only three wires instead of six?

Remember, I am pre-admitt>>

What? Nearly ALL X-ray tubes are driven by DC!

I have made several hundred supplies for such devices.

One was +90kV and -90kV for a 180kV DC electron beam splashing onto a Palladium target in a $900 German made X-ray tube which was/is used at every airport in the country!

The cleaner the DC E-beam, the cleaner the X-ray flux.The cleaner the X-ray flux, the better the contrast ratio of the imagery.

Where did you ever get the idea that X-ray tubes were AC driven?

PURE DC.

Even the 50kV supply that we sent to LANL to look at nukes was ALL DC ALL THE TIME.

There are dc driven x-ray equipments for things like portable hand held devices. I am willing to bet that most of the things you worked obtained power over three-phase lines--even at airports. You just conditioned it to be dc.

As a kid, I remember going to a dentist in the New York City region supplied by dc. The dc was not used to power x-ray tubes directly. A motor generator set was used to convert to ac which could be stepped up AND RECTIFIED to produce dc for the tube.

As far as I know, that may have been the only time I had an x-ray taken without a three-phase power being involved. In retrospect, I now believe that three-phase power was sent into this area to be converted to distributed dc.

Bill

-- Fermez le Bush--about two years to go.

My memory from working in manufacturing a long time ago is that 3 phase motors were very easy to reverse -- just switch any two wires.

Wrong. They were DC POWER SUPPLIES. It had nothing to do with the source power. THEY WERE DC POWER SUPPLIES.

NOTHING to do with any "handheld device" either.

The Los Alamos unit was a 50kV DC supply.

DIRECT CURRENT.

ALL X-ray sources are DC fed. I do not know of any that are not.

The electron beam has to STRIKE the target, not emit from it.

The X-ray flux then emits from the STRUCK target media, typically Palladium, but there are several other metals that are used as E-beam targets for X-ray generation. ALL utilizing DC E-beam streams.

Even a TV electron gun is a DC device! Have that hit a polished palladium ingot at a 45° angle of incidence, and you get an X-ray flux emission from it.

What part of DC *IS* what results from rectified HV AC do you not understand?

That is also true for two phase if you switch the two hots. Two phase was not as efficient in the use of materials.

Bill

-- Fermez le Bush--about two years to go.

These days, best practice is to drive x-ray TUBES with dc that is probably filtered some. X-ray equipment is almost always run from ac lines.

Although I do not know the history, I would be very surprised if some x-ray tubes were NOT run on ac. It is no trick to apply ac and have the tube be its own rectifier. The hardness of outputduring the cycle.

It was common practice for some radio transmitters to be operated with unrectified ac because of low cost. It was outlawed by treaty because of interference. Also, good operators could not receive as well with the rough tones.

Bill

-- Fermez le Bush--about two years to go.

------------ Cool it. Salmon Egg is considering the for your DC power supplies. Do you have a 50Kv circuit supplying wall sockets for airport X-ray machines? These "supplies" take AC as input and convert to DC at the desired voltage level. Even those that use batteries as supplies go through generation of AC as a step in production of DC at the desired voltage level. These "supplies" are an integral part of the unit- not something external and independent of the unit. No-one is trying to say that the X-ray unit itself is not DC driven but the whole package X-ray and its included power supply (or more appropriately power conditioner) , is driven from AC-just as my computer with 12 and 5 V DC busses, depends on my household 120V AC supply. You are thinking on a much smaller and specialized scale of things.

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

as was explained to me in collage it's 3 times the transmitted power with one third the copper... but then a lot of things they said in collage turned out to be wrong.

for a much more elaborate explination see:

Economics as Salmon Egg indicates: a) cheaper transmission-systems and lower losses per Kw delivered b) better use of iron in generators, motors and transformers c)Better operation of motors as well as being simpler than single phase motors (single phase motors are less efficient than 3 phase motors and are not inherently self starting). d) Although not initially recognised, lower electric and magnetic fields from transmission lines than would be the case with single phase. e)The ability to supply single phase loads as well as 3 phase loads. f)The ability to use 3 phase, 6 phase or 12 phase rectifiers to get smoother DC without filtering where needed.

Shall I go on?

Sure the load on the prime mover etc. is more even but that is actually an insignificant factor considering the mechanical time constants involved.

BULLSHIT!

That's not what he said AT ALL.

No shit?

I would never have known what the word RIPPLE referred to without your special inside info.

They are external. They are specifically made for each type of tube used. Some have 15 volt filaments, some have 6.3. Some have variable filament voltages.

We were contracted to make a supply originally Mfgd by ECG for the airport X-ray units. Both OURS as well as ECGs were SEPERATE units, NOT integral... totally INTEGRATED third party devices!

Theirs had a lead lined steel box, and an oil overflow recovery vessel.

Ours had a BRASS tank and a recovery vessel. BOTH were separate generation driver stages, and in the box was the final multipliers, and transformers for the final DC outputs, all in oil bath, along with the tube itself. The flux exited through a fiberglass wedge and got focused by a huge Aluminum lens, which diffracts X-rays.

That was a nine inch long, 2.5 inch diameter tube that was $900 each. It was made by a famous German doctor that now only makes X-ray tubes. The target was Palladium, and ingot that was around 20 grams in weight.

The supply we made for LANL was ALSO a custom, built to order replacement for an existing setup. They sent us the tube, and we made the supply for it. It is currently being used to examine nuke warheads for long term aging viability specs. That was 50kV.

We also made supplies for a gas pipe inspection device made in Boston, IIRC. It was 4kV, and had a ripple of 0.00006% and was a DC to DC converter.

No. That is exactly what he said. He even said that "rectified AC could be used to directly feed the tube".

Bullshit.

The airport unit was a box that measured over two feet by one and a half feet by nine inches thick, and weighed over 90 Lbs. WITHOUT the oil.

The driver was external to that.

The 50kV unit for LANL was 50W, IIRC, and we even made one unit that was a half rack at 50kV 250W.

The gas pipe unit was a little bigger than a pack of cigarettes.

So three out of four of those were bigger than any unit you or he ever saw in any hospital or chip x-ray station.

Perhaps like the spelling of the word COLLEGE.

You are a collage of insanity! Bwuahahahah!