محرك تيار متردد

An industrial type of AC motor with electrical terminal box at the top and output rotating shaft on the left. Such motors are widely used for pumps, blowers, conveyors and other industrial machinery.

محرك تيار متردد AC motor هو محرك كهربي يحول الطاقة الكهربية القادمة من خلال تيار متردد إلى طاقة حركية (ميكانيكية) و يتركب شأنه شأن محركات الحركة التقليدية من عضو ساكن و عضو دوار و يعمل المحرك المتردد بواسطة مجال مغناطيسي ناتج من التيار المار في ملفات دائرة الساكن يقابله مجال مغناطيسي ناتج عن تيار مستحث و جار في ملفات دائرة الدوار و الجدير ذكره أن الجسم الدوار لا يمكنه قط الدوارن تحت تأثير مجال مغناطيسي وحيد

تنقسم المحركات المتردد إلى قسمين أولاهما المحرك الحثي أو التحريضي و مخترعه هو نيقولا تسلا و هو أكثر المحركات الكهربية انتشارا على الإطلاق و اختراعه شكل ما يعرف بالثورة الصناعية الثانية و ينعدم في هذا النوع منها وجود أي توصيل كهربي بين دائرتي المحرك و يتم توليد تيار في دائرة الدوار عن طريق مبدأ حث فرداي.

أما ثانيهما فهو المحرك المتزامن و فيه ينتقل مباشرة إلى دائرة الدوار.

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مبدأ العمل

يرتكز مبدأ عمل المحرك المتردد على مبدأ تحريض فرداي الذي ينص على أن مرور التيار المتردد ينتج مجال مغناطيسيا متردد و أن المجال المغناطيسي المتردد ينشئ تيار كهربي متردد و بسربلة هذا المبدأ على المحرك المتردد فإنه و نتيجة لإن مجوة التيار الكهربي هي موجة جيبية فإن المجال المغناطيسي المستحث حسبما ذكر سيكون كذلك على هيئة موجة جيبية .

سريان التيار الكهربي في الدائرتين المختلفتين ( دائرة الدوار و دائرة الساكن) يستحث و يحرض وجود مجالين مغناطيسين مختلفين كذلك و عند يعترك الجزء الدوار هذان المجالان فإنه يتأثر بالقوتين الناشئتين عنهما فيستهل الدوارن بسبب اختلاف القوى و بالتالي العزوم مقدارا و اتجاها .


التاريخ

The first AC motor in the world of Italian physicist Galileo Ferraris
Drawing from U.S. Patent 381968, illustrating principle of Tesla's alternating current motor

Alternating current technology was rooted in Michael Faraday's and Joseph Henry's 1830–31 discovery that a changing magnetic field can induce an electric current in a circuit. Faraday is usually given credit for this discovery since he published his findings first.[1]

In 1832, French instrument maker Hippolyte Pixii generated a crude form of alternating current when he designed and built the first alternator. It consisted of a revolving horseshoe magnet passing over two wound-wire coils.[2]

Because of AC's advantages in long-distance high voltage transmission, there were many inventors in the United States and Europe during the late 19th century trying to develop workable AC motors.[3] The first person to conceive of a rotating magnetic field was Walter Baily, who gave a workable demonstration of his battery-operated polyphase motor aided by a commutator on 28 June 1879, to the Physical Society of London.[4] Describing an apparatus nearly identical to Baily's, French electrical engineer Marcel Deprez published a paper in 1880 that identified the rotating magnetic field principle and that of a two-phase AC system of currents to produce it.[5] Never practically demonstrated, the design was flawed, as one of the two currents was “furnished by the machine itself.”[4] In 1886, English engineer Elihu Thomson built an AC motor by expanding upon the induction-repulsion principle and his wattmeter.[6] In 1887, American inventor Charles Schenk Bradley was the first to patent a two-phase AC power transmission with four wires.

"Commutatorless" alternating current induction motors seem to have been independently invented by Galileo Ferraris and Nikola Tesla. Ferraris demonstrated a working model of his single-phase induction motor in 1885, and Tesla built his working two-phase induction motor in 1887 and demonstrated it at the American Institute of Electrical Engineers in 1888[7][8][9] (although Tesla claimed that he conceived the rotating magnetic field in 1882).[10] In 1888, Ferraris published his research to the Royal Academy of Sciences in Turin, where he detailed the foundations of motor operation;[11] Tesla, in the same year, was granted a United States patent for his own motor.[12] Working from Ferraris's experiments, Mikhail Dolivo-Dobrovolsky introduced the first three-phase induction motor in 1890, a much more capable design that became the prototype used in Europe and the U.S.[13][14][15] He also invented the first three-phase generator and transformer and combined them into the first complete AC three-phase system in 1891.[16] The three-phase motor design was also worked on by the Swiss engineer Charles Eugene Lancelot Brown,[13] and other three-phase AC systems were developed by German technician Friedrich August Haselwander and Swedish engineer Jonas Wenström.[17]

محرك حثي

المقالة الرئيسية: محرك حثي
محرك حثي ذو قفص سنجابي

Slip

If the rotor of a squirrel cage motor were to run at the true synchronous speed, the flux in the rotor at any given place on the rotor would not change, and no current would be created in the squirrel cage. For this reason, ordinary squirrel-cage motors run at some tens of RPM slower than synchronous speed. Because the rotating field (or equivalent pulsating field) effectively rotates faster than the rotor, it could be said to slip past the surface of the rotor. The difference between synchronous speed and actual speed is called slip, and loading the motor increases the amount of slip as the motor slows down slightly. Even with no load, internal mechanical losses prevent the slip from being zero.

The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:

حيث

Ns = Synchronous speed, in revolutions per minute
F = AC power frequency, in cycles per second
p = Number of poles per phase winding

The constant 120 results from combining the conversions of 60 seconds per minute and that each phase requires 2 poles.

Actual RPM for an induction motor will be less than this calculated synchronous speed by an amount known as slip, that increases with the torque produced. With no load, the speed will be very close to synchronous. When loaded, standard motors have between 2–3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall).

The slip of the AC motor is calculated by:

حيث

Nr = Rotational speed, in revolutions per minute.
S = Normalised Slip, 0 to 1.

As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800 RPM. The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.

This kind of rotor is the basic hardware for induction regulators, which is an exception of the use of rotating magnetic field as pure electrical (not electromechanical) application.

Polyphase cage rotor

Most common AC motors use the squirrel-cage rotor, which will be found in virtually all domestic and light industrial alternating current motors. The squirrel-cage refers to the rotating exercise cage for pet animals. The motor takes its name from the shape of its rotor "windings"- a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often use cast copper to reduce the resistance in the rotor.


المراجع

  1. ^ Ari Ben-Menahem (2009). Historical Encyclopedia of Natural and Mathematical Sciences. Springer Science & Business Media. p. 2640. ISBN 978-3-540-68831-0. Archived from the original on 3 December 2016.
  2. ^ Matthew M. Radmanesh Ph.D. (2005). The Gateway to Understanding: Electrons to Waves and Beyond. AuthorHouse. p. 296. ISBN 978-1-4184-8740-9.
  3. ^ Jill Jonnes (2003). Empires of Light: Edison, Tesla, Westinghouse, and the Race to Electrify the World. Random House Publishing Group. p. 162. ISBN 978-1-58836-000-7.
  4. ^ أ ب Marc J. Seifer (1996). Wizard: The Life and Times of Nikola Tesla : Biography of a Genius. Citadel Press. p. 24. ISBN 978-0-8065-1960-9.
  5. ^ Silvanus Phillips Thompson (1895). Polyphase Electric Currents and Alternate-current Motors. Spon. p. 87.
  6. ^ W. Bernard Carlson (2003). Innovation as a Social Process: Elihu Thomson and the Rise of General Electric. Cambridge University Press. p. 258. ISBN 978-0-521-53312-6.
  7. ^ Fritz E. Froehlich; Allen Kent (1998). The Froehlich/Kent Encyclopedia of Telecommunications: Volume 17 – Television Technology. CRC Press. p. 36. ISBN 978-0-8247-2915-8.
  8. ^ The Electrical Engineer. (1888). London: Biggs & Co. Pg., 239. [cf., "[...] new application of the alternating current in the production of rotary motion was made known almost simultaneously by two experimenters, Nikola Tesla and Galileo Ferraris, and the subject has attracted general attention from the fact that no commutator or connection of any kind with the armature was required."]
  9. ^ Galileo Ferraris, "Electromagnetic rotation with an alternating current," Electrican, Vol 36 [1885]. pg 360-75.
  10. ^ Prodigal Genius: The Life of Nikola Tesla. Pg 115
  11. ^ "Two-Phase Induction Motor" Archived 18 نوفمبر 2012 at the Wayback Machine (2011), The Case Files: Nikola Tesla, The Franklin Institute.
  12. ^ Lance Day; Ian McNeil (2003). Biographical Dictionary of the History of Technology. Taylor & Francis. p. 1204. ISBN 978-0-203-02829-2.
  13. ^ أ ب Arnold Heertje, Mark Perlman Arnold Heertje; Mark Perlman (1990). Evolving Technology and Market Structure: Studies in Schumpeterian Economics. University of Michigan Press. p. 138. ISBN 0-472-10192-7. Archived from the original on 5 May 2018.
  14. ^ Victor Giurgiutiu; Sergey Edward Lyshevski (2003). Micromechatronics: Modeling, Analysis, and Design with MATLAB (Second ed.). Taylor & Francis. p. 141. ISBN 978-0-203-50371-3. Archived from the original on 5 May 2018.
  15. ^ M. W. Hubbell (2011). The Fundamentals of Nuclear Power Generation: Questions & Answers. Author House. p. 27. ISBN 978-1-4634-2658-3.
  16. ^ Center, Copyright 2014 Edison Tech. "History of Transformers". edisontechcenter.org. Archived from the original on 14 October 2017. Retrieved 5 May 2018.{{cite web}}: CS1 maint: numeric names: authors list (link)
  17. ^ Neidhöfer, Gerhard (2007). "Early Three-Phase Power (History)". IEEE Power and Energy Magazine. IEEE Power & Energy Society. 5 (5): 88–100. doi:10.1109/MPE.2007.904752. ISSN 1540-7977. S2CID 32896607.

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