What are motor "phases"?

[davidtu]

I see "single-phase" motor mentioned, etc. What are phases? How many are there? What are the advantages of one over the other? What does it require for household wiring, if anything?

Thanks!

[JimD]

David,

Power is generated in three phases. The electrical generator has poles on the rotor and the stationary coils are connected in 120 degree segments. When the electromagnets of the rotor pass the stationary coils of the stator, you get electricity. You get three sine waves that are shifted in time or phase 120 degrees from each other. You may have also heard that electricity in the US is 60 Hz or 60 cycles per second. That means that the sine wave for each phase transitions from positive to negative 60 times a second. This is driven by the fact that the rotor of the stator in a fossil plant is turning 3600 rpm. On a nuclear generator (the best kind of plant), the turbine rotor is bigger and turns at 1800 rpm but the rotor has two positive and two negative poles so the power is still 60 Hz (the steam in nuclear units is lower temperature and pressure for safety reasons and thus the turbine has to be larger to make a given amount of power).

House wiring is 120V phase to ground and 220V phase to phase. Most of your loads are 120V but big loads like an electric water heater, stove, electric furnace, etc. are 220V. That is just convenience. What you need to do work is Watts. Watts are Volts times Amps. At 120V, a water heater would draw too many amps for the size of the electrical conductor (the wire) to be reasonable in size. The easy solution is to up the voltage so the wire size can be reasonable.

Probably more than you wanted or needed but hope it helps.

Jim

[LinuxRandal]

General rule of thumb,
Three phase is provided to commercial area's not residential. If you live in an area that was residential and being knocked down and converted to comercial, and you see they have uped the number of transformers on the pole to three, you MIGHT be able to get three phase. Otherwise, if it is SMALL enough, you might get by with a really expensive phase convertor. (hence, Not worth your time looking at)

If I had three phase, I have seen commercial air compressors, for as little as $50, up to around $1000.00 for a rotory screw compressor (about three times the air volume that the home ones provide, on max).

[LCHIEN]

I hate this question, it involves thinking and explaining in another dimension involving vectors and phasors. Westinghouse was a very clever man.
If you understand it, then you know why there is one-phase and three-phase power but no two-phase power.

Just let it suffice to say that virtually all residential power is single phase and only large industrial machinery uses three-phase. The kind that uses more than 5 HP. Not for your average Joe Woodworker.

[leehljp]

I hate this question, it involves thinking and explaining in another dimension involving vectors and phasors. Westinghouse was a very clever man.

I am probably wrong about this but I for some reason I was thinking that Nikoli Tesla was the creator of that deminsion - 3 phase.

[BobSch]

I am probably wrong about this but I for some reason I was thinking that Nikoli Tesla was the creator of that deminsion - 3 phase.

Tesla came up with the idea of alternating current — as opposed to Edison's DC — but Westinghouse was the one who commercialized it.

[LinuxRandal]

Also, wasn't Tesla working on a wireless distribution system for "free energy" delivery, as compared to Westinghouse/


Always heard the stories about how Grandma's house was the second one wired in her town, the first one was her boyfriends (richest man in town), who she broke up with to marry Gramps.

[Jeffrey Schronce]

Just let it suffice to say that virtually all residential power is single phase and only large industrial machinery uses three-phase. The kind that uses more than 5 HP. Not for your average Joe Woodworker.

There are a lot of tools out there that are <5 HP that are 3 phase. I have no idea what the benefits are but I guess shops that have 3 phase get some benefit from using 3 phase for smaller items.

Examples :
Power feeders - 1 HP - 3 Phase
Bandsaws - 3 HP - 3 Phase
Edge Sanders - 3 HP - 3 Phase

I have been knocked out of a couple of good deals on DP's that were 3 Phase and had to pass on a nice used Powermatic jointer that was 3-Phase, 3 HP.

It seems anything larger than 3HP, ie 5HP must generally be on 3-phase.

[leehljp]

At one point, Westinghouse owed Telsa more in patient royalties than they were worth. Tesla, somewhat of an eccentric, cared more about his inventions being used and credit for that invention than for the money he would gather from it. Westinghouse was about to be taken into bankruptcy until Telsa forgave them the near million dollars that was owed him.

Other interesting notes on Tesla:

He invented a radio controlled torpedo that worked in the 1890s and demonstrated it to the US Navy. They rejected it.

He DID invent the radio; successfully sueing Marconi, and proving that he had invented the radio 2 years before Marconi did.

My favorite: He had a small pocket sized motor that was battery powered. He would occasionally attach it to a major support beam in a building and turn it on slowly. Then turn the rheostat up slowly trying to figure out the harmonics of the building.
Only problem was that it would cause mini-earthquakes in the surrounding acres and buildings. He didn't realize for a while that he was creating panic among his neighbors.

[Stick]

I hate this question, it involves thinking and explaining in another dimension involving vectors and phasors. Westinghouse was a very clever man.
If you understand it, then you know why there is one-phase and three-phase power but no two-phase power.

Just let it suffice to say that virtually all residential power is single phase and only large industrial machinery uses three-phase. The kind that uses more than 5 HP. Not for your average Joe Woodworker.

Hehehehe! I hate the question too, and I work with it every day! And then power factor and Vars come into it too.

Turbine synchronous speed is indeed related to the number of field poles. Hydro units have varying numbers of poles. At the plant I'm operating today, the main units have 64 poles and turn at 112.5rpm. Then I have a small one that just suppies station service that has 16 poles and turns at 450rpm. Other units I've worked with have 80 poles and turn at 90 rpm. There can be any number of poles as long as they are in pairs. The formula is f = rpm/60 x P/2 where f is frequency and P is the number of poles. For 60hz frequency, this can be simplified to rpm = 7200/P or P = 7200/rpm

One thing I'll add about having 3 phase power is it's expensive to install and expensive to use! There are usually demand charges as well as usage charges. You get charged for the peak demand when a machine starts, as well as for the time it's running virtually unloaded. So let's say I have a big lathe, milling machine, welder, shaper etc all on 3 phase. it takes a lot of current to start the lathe or the welder, but not much to keep them running once started. And if one is running and another is started, that only adds to it. I have it on the farm for my machine shop and barn fans and such, and most often my demand outweighs my usage. I wouldn't have bothered having it installed as the cost of putting it in is quite astronomical, but it was already there when I bought the place. The local utility told me it would cost 20 grand to have it installed today, due to the distance from the line to the yard pole.

BTW, Most people don't realize that even the power generated by a modern car's alternator is 3 phase AC, rectified to DC and regulated down to the needed voltage.

I know, I know....more than anyone needed to know.

[LCHIEN]

I'll let someone else explain it. See this article:

http://en.wikipedia.org/wiki/Three-phase_electric_power

Virtually all main power distribution is three phase. Look at any of the tall metal power transmission towers and you will see three main wires on big insulators. Those are three phase power lines.

[Stick]

The reason for it also has somewhat to do with the cost of the generator and motor windings. Three phase units are much, much smaller, lighter and cheaper than equivalent sized single phase. Example, we have 250hp motors at work. They are 600V 3 phase and are about 3 feet long by 2 feet around. A 250hp single phase motor would have to be huge, probably 10 times that size and cost.

it makes sense to keep the transmission lines 3 phase to avoid the cost of conversion for the large industrial customers.

BUT......AC power only travels on the outside of the conductors (it's called skin effect), so a solid conductor (needed for strength on a flexible line like a transmission line) is more than half wasted, and it also has tremendous line losses over long distances. In fact, conductors in utility high voltage switchyards were often hollow aluminum pipe for this very reason. Utilities will sometimes convert the AC power to High Voltage DC, or HVDC where they have long distance to cover. DC power uses the entire conductor, and only needs two conductors rather than 3. But, you need a very expensive converter station at both ends. The only disadvantage is that you can't easily tap off it to change voltages. Where I work, we have transmission lines that run 650 miles between converter stations at &#177;500kV. I can expound later if anyone cares.

[dlminehart]

"There are usually demand charges as well as usage charges. You get charged for the peak demand when a machine starts, as well as for the time it's running virtually unloaded. "

I can't vouch for it, but someone at our newspaper's printing department said that it costs $200 in electricity each time they start up our big (approx. 10'W x 25'H x 60'L) web press, and that its current draw at startup is the largest single use in the county.

[Stick]

On a nuclear generator (the best kind of plant)

Jim

Don't want start anything here, but I am truly curious, being a hydraulic power station/HVDC converter station operator myself. The media never once asks the question how does one go about decommissioning a nuke unit/plant when its useful lifespan is passed, and what IS the life expectancy? 40 years? Coal burners usually have an expected lifespan of around 40-50 years, hydro closer to 100 years. Obviously, the turbine parts can be replaced for many, many years, but does a reactor itself have a finite lifespan? The area surrounding it definitely would. Would there not radioactive waste and contaminated areas of the reactor that will have to be guarded for decades or centuries? Most likely at least 60 years according to this regulation. http://www.nei.org/doc.asp?catnum=3&catid=278 yes, certain parts can be decontaminated, but not all.

And...........the cost of storage of spent fuel is not regulated at all! Would the cost of that not add astonomically to the cost of any power produced by that "best kind of plant" if it were regulated? Or is that just a bunch of hooey by nay-sayers?

A gas turbine or wind generator is obviously the easiest to decommission, simply shut it down and remove it. Followed by a coal burning unit (coal pile to clean up, etc). Hydro plants are a bit more involved with the massive concrete structures, but normally you can just open the spillway and divert the water back to its original course, then remove the mechanical parts. Then if you decide to still use that water you just build another plant in another location. Lots of hydro and fossil plants have been decommissioned with few concerns for future generations.

Unknown to most people, hydro power is not technically cheap to produce either, due to absolutely ridiculous water rental taxes imposed by governments to force it to compete with much more expensive coal and nuclear generation. But it is renewable and clean. Wind and solar are not yet viable for large scale generation with today's technology. Burning oil or natural gas to produce power is simply insane!

What is the "best kind of plant"? Only future generations will be able to tell us where we went right or wrong. They ALL have their problems, it becomes a lesser of evils.

[JimD]

I admit I was trying to start something at least a little bit. I used to sell turbine generator parts for fossil, hydro and nuclear units and I currently sell nuclear fuel. I know a little about all the traditional style of plants but little about wind and solar. One of my customers used to be the Corps of Engineers on the upper Missouri. I like all kind of plants but I think nuclear does not get a fair hearing most of the time - it is better than most people think.

In terms of spent fuel disposal cost, the bill is already being paid in the United States. The federal government collects 1 mil per kilowatt hour generated with nuclear fuel for the ultimate disposal. We consumers haven't gotten anything useful for this fee yet but ultimately the federal goverment is promising to open Yucca Mountain. Most of the reactivity in spent fuel goes away fairly fast (i.e. has a short half life) but there are dangerous materials that need to be protected from exposure to the general public for tens of thousands of years. This kind of thing is not totally unique to nuclear energy but anything nuclear tends to sound scary. You can recycle spent fuel but it is not economic. Fresh fuel (like we make in the plant I work at) is only an alpha emitter - keep it out of you body and it won't hurt you. Spent fuel gives off lots of gamma and beta radiation so you can't get near it - have to do lots of things robotically which is expensive and difficult.

In terms of decomissioning it has happened a dozen times or more so far. Yankee Rowe, for instance is decommissioned and I think Big Rock Point is (we could check the NRC website to be sure). Shippingsport is too. These were all small plants that were decommissioned because they became uneconomic to operate. Trojan and Rancho Seco were decommissioned because the people who lived around them did not want them to operate. I don't know all the decommissioned plants but can remeber these off-the-top. Most of the materials in a nuclear plant are either essentially not radio-active or decay quickly enough that they can be sent to a low level waste landfill within 10 years of the time the plants shut down. The parts go the same place that waste from generating nuclear power like clothing worn by workers in radioactive areas and similar pieces from nuclear medicine go. Nothing highly radioactive like spent fuel can go to a low level site. It is buried in a way that prevents water leaching the materials out of the landfill. It is not cheap but the money is also by law required to be collected during the plants operation and kept in a special fund to make sure the money is available when it is needed. The highly radioactive material is stored in a special vault on-site until Yucca opens.

Nuclear power plants pretty much do not wear out. They develop problems that may be uneconomical to repair but if you were willing to spend enough, you could probably operate them forever. The worst thing I can think of is the embrittlement of the reactor vessel (the part that the fuel is in during operation) that happens to some older plants. Neutron bombardment will reduce the ductility of steel if it has trace amounts of copper in it. The damage can be reversed by heat treatment, however, so again if you want to spend enough.....

The plants will probably operate 60 years, possibly more if the NRC allows it and it is economic. A lot depends on how well the next series of plants runs. If they are good enough, some of the older plants could get retired earlier. Ours uses natural or passive systems to eliminate things like safety grade diesel generators and improve both safety and costs - at least on paper. Now if we can get one built.....

Jim