In most cases, from generation to delivery, passing by transmissions and distribution, power is conveyed by three-phase circuits, encompassing two and single-phase circuits. On the other hand, power circuits with the number of phases higher than three are rather rare. Maybe a six-phase could be the only exception. What is the reason for this, after all?
Hello Sérgio Henrique Lopes Cabral ,
There is a terse answer to your question in the following reference book:
Donald G. Fink, H. Wayne Beaty (editors); Standard Handbook for Electrical Engineeers, Thirteenth Edition; McGraw-Hill, Inc.; 1993; p. 2-26, Section 88 - General Note.
Regards,
Tom Cuff
Power systems are based on a three-phase structure because it offers several advantages over single-phase systems, including higher efficiency, smoother power delivery, and reduced equipment size and cost.
One key advantage of three-phase power systems is their ability to deliver more power compared to single-phase systems for the same amount of conductor material. This is because three-phase systems provide a constant and balanced power transfer, which reduces the size of conductors required and minimizes losses.
Additionally, three-phase power systems result in smoother power delivery, with less voltage fluctuation and better voltage regulation compared to single-phase systems. This is particularly important for industrial and commercial applications where stable power supply is critical.
Moreover, three-phase power systems allow for simpler and more efficient motor operation. Three-phase motors are commonly used in industrial applications due to their high efficiency, reliability, and ability to deliver high torque.
An example of a three-phase power system is the electrical grid that supplies power to homes, businesses, and industries. In this system, power is generated and transmitted as three separate phases, typically labeled as A, B, and C. Each phase is offset by 120 degrees from the other, resulting in a balanced and efficient power delivery system.
In summary, three-phase power systems offer higher efficiency, smoother power delivery, and reduced equipment size and cost compared to single-phase systems, making them the preferred choice for a wide range of applications, from residential to industrial.
Three phase is the simplest geometry that, if I switch two of the phases, I have no loss of useful power.
Say I'm driving a motor - I don't care about the direction of rotation - it's mixing dough or stirring water. If I switch a pair of phases (ooopsy) then the motor continues to spin.
This is not true for any number > 3.
James Garry, do you mean the three-phase induction motor is why the whole power system has three phases ?????
Sérgio Henrique Lopes Cabral I'm not suggesting that it's the only reason, but it's one that comes (readily) to my mind.
James Garry I see it. Nonetheless, I would not risk to consider this as being the reason.
Hello Sérgio Henrique Lopes Cabral ,
The following undergraduate textbook gives the same two reasons as found in the reference book by Fink and Beaty but with a little more clarfication on the second reason: 1) the three-phase system delivers power at a steady rate as opposed to as a function of time in the case of the one-phase system, see the proof on pp. 555-557; and 2) the transmission of the same power in the three-phase system is more efficient in terms of the size of the conductors than in the one-phase system.
J. David Irwin, Chwan-Hwa Wu; Basic Engineering Circuit Analysis, Sixth Edition; Prentice-Hall, Inc.; 1999; pp. 554, first page of Chapter 10 - Polyphase Circuits.
I really like your question, but the reasons you seek are set forth in books, if you know where to look.
Regards,
Tom Cuff
Thank you so much, dear Thomas Cuff, for clarifying what the mentioned book brings. Nonetheless, I reinforce the essence of my question is not a comparison between single and three-phase circuits. In a few words, the essence is: Why power systems circuits are not based on four-phase or five-phase, for example?
Hello Sérgio Henrique Lopes Cabral ,
There are two criteria: 1) steady power delivery, and 2) material efficiency. Even if four-phase or five-phase or six-phase systems satisfied the first criterion, they would not satisfy the second criterion because they would require more transmission conductors. I mean why stop at six-phases? Why not ask why 60-phases would not be better? Note, just because books do not explicitly mention four-phase or five-phase or six-phase systems does not imply that they have not implicitly answered your question.
Regards,
Tom Cuff
Namaste Professor Lopes Cabral: I believe the three phase system is popular or exists is because three phases could be, or are, generated by the windings and the core in the same space occupied by an electric machine/generator. Then each phase could be distributed for extra revenue but the issue remains to keep the phases balanced to prevent/minimize the unbalanced/inter-phase current, which is not billable.
The concept is similar to binary addressing, such as in computer memory (2, 4, 8, 16 MB/GB or similar).
Thank you, once more, Thomas Cuff !
Usually, power engineers mention more efficiency or similar reasons, based on their feelings. I´ve never found a work with calculus or statistics to prove this. That´s why I make this question that remains without any convincing answer.
Six-phase arrangement was extensively analyzed (say 30-40 years ago) for UHV transmission lines (Lionel Barthold and alii). The advantages, with respect to the three-phase arrangement, can be expected in the insulation, RI, and AN performances. As far as I know, no any industrial application was attempted.
Hello Sérgio Henrique Lopes Cabral ,
The post by Giulio Santagostino reminded me that I had not looked in any books on EHV and UHV systems. The book by Khalifa mentioned that there are two types of EHV and UHV transmission systems: AC and DC, with DC systems being the "more economical (p. 3)".
M. Khalifa (editor & contributor); High-Voltage Engineering, Theory and Practice; Marcel Dekker, Inc.; 1990; pp. 2-3.
Regards,
Tom Cuff
We have to balance cost and efficency .The three-phase system is a good compromise, that all .
Dear Amit Singh , thank you for your interest in my question. From what you mention, four-phase system could be the most popular, but it is not...So, the question remains...
Dear Giulio Santagostino , thank you for your interest in my question. In fact, six-phase is a sort of exception. That´s why I mention this in my question.
Thank you once more, Thomas Cuff . It reminds me AC versus DC is another reason for a new question.
Dear Maillot Patrice , thank you for your interest. Would you , please, show us any analytical development to support your question ?
For AC/DC : with DC, reactive impedances are negligible .It is very interesting to carry current under sea or under rivers (Canada ).But you have high voltage rectifiers etc.
At the beginning of the AC electricity , they missed motors ( one phase systems).Then, they noticed that with two phases or more, synchronous and asynchronous motors can be made. Adding phases diminishes the currents in the cables. But transformers and motors are more expensive ( more coils, more difficult to build).
they stop to 3-phase systems for historical reasons as for 50Hz and 60Hz ( violins ancestors had a lot of trings ; four is a good compromise ). 6-phase systems are interesting for making aluminum : less ripple after rectification and very high power capacity .
Namaste Professor Lopes @Cabral: The memory-size example is related to binary arithmetic, not true for a multiphase electrical machine.
Thank you, dear Maillot Patrice . The question is exactly about why they did stop at the number three for phases. Some additional analytical proof may give fine support.
Two (90-deg shifted) phases allows to run motors without need to generate phase shift.
However, in two phase system the 'null' conductor current is not 0.
It becomes 0 in symmetrical 3-phase system.
By combining voltage from 2 or 3 phase systems one can create arbitrary number of phases, that is sometimes used to reduce ripple in power rectifiers - rectifying more phases diminishes the ripple and moves it to higher freq where it is easier to filter.
The two phase system is made with the one phase and a capacitor or an inductance. Now, we prefer electronic boxes...
So, dear Nikolay Pavlov , more than 3 phases would be welcome...Why don´t they exist ?? Thank you for the interest!
Sérgio Henrique Lopes Cabral More phases - more wires. However, if it short distance (within a car ) the >3 phases can be used Conference Paper Control potentials and advantages of multiphase AC drives
I see it, Nikolay Pavlov , but I would like to see these calculations that reinforce your proposal. Would you indicate where to find it , please ?
Sérgio Henrique Lopes Cabral , it is not may proposal - I just see it is mentioned in the publications, e.g. the one above, Article Nontraditional control and advantages of multiphase AC inverter drives
Article Novel Types of AC Motors and Drives for Electrical and Hybrid Vehicles
Wow, dear Nikolay Pavlov !Thank you very much for this. Although it is not directly related to may question , it fits very well for VFDs .