Q.E.D. and Desmostylus I can see that i’m going to have to invest in that typing tutor program after all… i wasn’t near fast enough with my armature/pole explanation. And Desmostylus, thank you for being so succinct.
I think most everyone else has answered correctly. I’m not all that familiar with polyphase hardware, so I’ll just sum it up the best I can.
There are two basic flavors of generators: a) generators that produce AC, b) generators that produce DC. The former is often called an alternator.
In a generator you (basically) have two things: a stator and a rotor. The stator is the outer stationary part, and the rotor is the rotating center part.
The basic idea is that you want a set of coils to “see” a changing magnetic field. When this happens, a voltage is induced in the coils. This is called “Faraday’s Law of EM Induction.” This set of coils, in which voltage is induced by the changing magnetic field, is called the armature.
There are two ways of producing a magnetic field: permanent magnets and electromagnets. Electromagnets have two advantages over permanent magnets: 1) They’re lighter, 2) You’re able to easily control the magnitude of the magnetic field by varying the current through the electromagnet. So DC electromagnets are almost always used to generate the magnetic field in generators.
So where do you put this stuff? Based on the above you have two choices:
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Put the electromagnet in the stator and the armature in the rotor. When the rotor spins, the coils in the rotor will “see” the magnetic field produced by the electromagnet in the stator. And because the magnetic field is directional, each coil in the armature will see a continuously changing magnetic field as it makes a complete revolution.
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Put the electromagnet in the rotor and the armature in the stator. When the rotor spins, the coils in the stator will “see” the magnetic field produced by the electromagnet in the rotor. And because the magnetic field is spinning, each coil in the stator will see a continuously changing magnetic field as the rotor makes a complete revolution.
Theoretically, either method will work. But for practical systems it has been found that #1 is best suited for DC generators and #2 is best suited for AC generators.
The number of coils in the armature dictates the number of phases. Multiple coils are used to produce polyphase power. Two, three, and six phase generators can be found, with three phase being the most common for power production.
Note that there is a lot more to this science. I haven’t even begun to talk about damper windings, synchronous generators, exciter circuits, regulator circuits, salient/nonsalient pole configurations, delta vs. wye configurations, armature reaction, and induction generators.
A side note in case anybody is interested, In my experience,
In a 3 phase circuit the third phase will have a higher voltage about 180 vs. 120. A 3 phase motor can be reversed by simply reversing the first two wires.
Here’s one site http://www.phaseconverter.com/static-converter.html that sells three phase converters. So it can be “manufactured at the users end” so to speak. It’s my understanding that units like these are often used where a particular piece of equipment can only use three phase power and it’s not otherwise available or too expensive.
If you had a shop at home and need three phase for a milling machine or the like, this would be one way to go.
Your local utility would not normally be able to provide three phase power to a residence.
Hmmm. It’s been my experience that each phase has the same RMS voltage.
In 3-phase power as generated and distributed by nearly all utilities all phase voltages are equal. I say “nearly all utilities” only because there might be some local utility that is not connected to the power grid that has some bastard system.
A single phase to three phase converter might very well be a motor generator. I.E. a single phase induction motor driving a three phase generator.
The phase converter described in the post above is used for starting 3-phase motors only. Once the motor starts it will run single-phase but at an output power that is reduced from the 3-phase nameplace power rating. An analagous method is used for capacitor start, single phase induction motors. A centrifugal switch connects a capacitor in series with a special starting winding which is provided in addition to the normal field winding. The series capacity-inductor connection of the starting circuit provides a current that is at some phase angle relative to the run field current and so furnishes a rotating magnetic field for start. Then when the motor gets up to speed the centrifugal switch disconnects the starting circuit and the motor runs on single phase.
David Simmons
If you look a little further in the site I posted, you will find the rotary converters that provide 100% power. They have a 150 HP rated unit for just under $8000.
http://www.phaseconverter.com/
What you say about the static converters is 100% correct. I wanted to point out to the OP that there is a way to get 3 phase power other than from the utility
Methinks you have been measuring w.r.t the centre point of a star connection.It would not be such a surorising thing to have one phase carrying more load, its desirable to have things balanced but the reality is that on systems where each phase is used seperately, one of them is usually dragging more current from the supply and one is dragging less.
On polyphase systems the use of complex notation, instead of trigonometric notation is quite elegant and demonstrates the power of imaginary quantities as compared to vectors etc.
IMHO I think EE’s should be taught complex notation and completely ignore other methods, its easier to understand and when it comes to closed loop systems and feedback it tends to yield more manageable results, it’s almost as if they teach trig as an obtsicle course before you get to use the useful tools.
You might be right, I am not an electrician but I do what is necessary. My experience is that one phase will have a higher voltage to ground and this is the third phase, I use this method many times in identifying and hooking up three phase equipment.
“A 3 phase motor can be reversed by simply reversing the first two wires.”
Actually, ANY 2 wires…and how do you figure out which wires are the “first two”, anyway?
Virgo: (Aug. 23-Sept. 22): Certain shortcomings in your education and upbringing cause you to read meaning into the relationships among various celestial bodies.
Uhh… the first two you can reach?
Yes, I know that “rotary converters” were listed in your cite. My post could have been clearer on that point.
Somehow this picture from your cited literature of the rotary phase converter sure looks to me a lot like a specially built motor/three-phase-alternator all on one shaft and in one housing.
I used to provide calcs for 3 phase load analyses in lighting & power distribution at work. My boss asked me to show him how to do the math. I started explaining real and imaginary components and he said, " You mean you just make 'em up?" I never convinced him otherwise.
“Imaginary”. THAT needs a bit more elegance.
I can’t even work out what that sentence means. Are you trying to say that a rectangular coordinate system is complex, but a polar coordinate system isn’t?
I think he’s trying to say that in this application, complex notation (think direction & magnitude) is more easily used and understood than the trig involved in using the rectangular system.
And I suspect that what you just said, hammerbach, is actually the opposite of what casdave seems to be saying.
herman_and_bill What you are referring to is the ‘wild leg’; an unavoidable result of a 3Ø delta connected light and power bank. The transformers are connected per a normal 240v straight power bank with the exception of one transformer having the center tap grounded. The voltages phase to phases would read: AØ-BØ = 240v, AØ-CØ = 240v, BØ-CØ = 240v. The voltages phase to ground would read: AØ-grd = 120v, BØ-grd = 120v, CØ-grd = nominal 208v. The curious thing about straight power delta connected banks is that, because there is no reference to ground, phase to ground readings can vary substantially depending on how well the load is balanced. This has thrown off many an electrician as they expect voltages to read evenly across all phases. There is no similar problem with wye connected banks as each transformer has one side of the coil grounded; the problem is that you can’t use wye connected banks to feed a 120/240 3Ø service as the reading are (Ø-Ø/Ø-grd) 120/208.
When Crafter_Man states:
this is what he means: In straight delta configuration, voltage measurements are taken phase to phase as there is no reference to ground to allow for phase to ground readings. Wye connected configurations are grounded so phase to ground readings are appropriate.
I think what he’s saying is that complex (i.e. phasor) notation should be used when mathematically analyzing AC systems. I guess I agree.
I guess that you are trying to say that synchronous reference frame analysis is preferable to pure time domain analysis. In some situations that’s true.
I reckon I’ll wait to hear from casdave about what he actually meant.
IMHO EE’s should express themselves clearly. Please define “complex notation”.
Complex notation in the Electrical environment employs the use of j which is used to represent the term sqrt-1, which you will know is a value that cannot exist in the real domain as you cannot obtain a negative value for a square root.
To get round this, the out of phase component is described as having two parts, the real value, multiplied by an imaginary one (sqrt-1)and this is what gives rise to the term complex number, a number comprised of two parts.
In pure mathematics i is used instead of j but EE’s have a conflict with the use if I which is used to represent current.
The effect of using imaginary quantities is that it is effectively a rotational operator, so you can multiply two complex terms together and get the resultant vector far more easily than you would if you were trying to multiply trigonmetrical quantities.
Obviously you can do far more than just multiply, you can carry out all the usual functions.
You have to first remember that j*1=j
j*j = j[sup]-1[/sup] = j[sup]2[/sup]
jjj= j*j[sup]2[/sup]=-j
and jjj*j = 1
so that when you try to multiply two terms
[1+2j][1+j] for example
you get
[1+j+2j+ -1] or [-1+3j]
and when you then plot this on an Argand diagram just the same way as Cartesian coordinates(the axis are labelled differantly) this is far easier than trying the whole exercise using trig.
You can obtain the resultant phase angle of Cos phi quite easily, if you wish, by using Pythagoras on the Argand diagram.
It has been many years since I tried this in trig and I would have to do a good deal of reading to remember all the circular functions, but it is enough to say that complex notation is so much better, and when you start to design circuits using complex matrices with more than a handful of elements it gets unwieldy enough as it is, using trig it would be a nightmare, though I guess that’s why we have computors.
One thing that complex notation is especially useful for is determining the phase angle of feedback when used in the frequency domain, it is not always obvious that designed negative feedback can change phase w.r.t frequency and can become positive and result in an unstable system.
More than this will see me having to return to my notes for a better illustration.