Editing Synchronous motor (section)

==Types==
The two major types of synchronous motors are distinguished by how the rotor is magnetized: non-excited and direct-current excited.<ref>James G Stallcup, ''Stallcup's Generator, Transformer, Motor and Compressor'', page 15-13, Jones & Bartlett, 2012 {{ISBN|1-4496-9519-1}}.</ref>

===Non-excited===
[[File:Synchronous motor from Teletype machine.jpg|thumb|Single-phase 60&nbsp;Hz 1800&nbsp;[[revolutions per minute|RPM]] synchronous motor for [[Teleprinter|Teletype]] machine, non-excited rotor type, manufactured from 1930 to 1955]]

In non-excited motors, the rotor is made of steel. It rotates in step with the stator's rotating magnetic field, so it has an almost-constant magnetic field through it. The external stator field magnetizes the rotor, inducing the magnetic poles needed to turn it. The rotor is made of a high-[[retentivity]] steel such as [[cobalt]] steel. These are manufactured in permanent [[magnet]], [[Reluctance motor|reluctance]] and [[hysteresis]] designs:<ref name=HSEM>William Yeadon (ed.), ''Handbook of Small Electric Motors'', McGraw-Hill 2001 {{ISBN|0-07-072332-X}},  Chapter 12 "Synchronous Machines"</ref>

==== Permanent-magnet ====
A permanent-magnet synchronous motor (PMSM) uses [[permanent magnet]]s embedded in the rotor to create a constant magnetic field. The stator carries windings connected to an AC electricity supply to produce a rotating magnetic field (as in an [[asynchronous motor]]). At synchronous speed the rotor poles lock to the rotating magnetic field. PMSMs are similar to [[brushless DC motors]]. [[Neodymium magnets]] are the most common, although rapid fluctuation of neodymium magnet prices triggered research in [[ferrite magnets]].<ref>{{Cite journal |last1=Eriksson |first1=S |last2=Eklund |first2=P |date=2020-11-26 |title=Effect of magnetic properties on performance of electrical machines with ferrite magnets |url=http://dx.doi.org/10.1088/1361-6463/abbfc5 |journal=Journal of Physics D: Applied Physics |volume=54 |issue=5 |pages=054001 |doi=10.1088/1361-6463/abbfc5 |issn=0022-3727 |s2cid=225152358|url-access=subscription }}</ref> Due to inherent characteristics of [[Ferrite (magnet)|ferrite magnets]], the [[magnetic circuit]] of these machines needs to be able to concentrate the magnetic flux, typically leading to the use of spoke type rotors.<ref name=":0">{{Cite journal |last1=Luk |first1=Patrick Chi-Kwong |last2=Abdulrahem |first2=Hayder A. |last3=Xia |first3=Bing |date=November 2020 |title=Low-cost high-performance ferrite permanent magnet machines in EV applications: A Comprehensive Review |url=http://dx.doi.org/10.1016/j.etran.2020.100080 |journal=ETransportation |volume=6 |pages=100080 |doi=10.1016/j.etran.2020.100080 |issn=2590-1168 |s2cid=224968436|url-access=subscription }}</ref> Machines that use ferrite magnets have lower power density and torque density when compared with neodymium machines.<ref name=":0" />

PMSMs have been used as gearless elevator motors since 2000.<ref>{{cite web |last=Mehri |first=Darius |date=18 September 2000 |title=Belts Lift Performance |url=https://www.designnews.com/document.asp?doc_id=226553 |archive-url=https://web.archive.org/web/20130629134620/https://www.designnews.com/document.asp?doc_id=226553 |archive-date=29 June 2013 |access-date=10 May 2016 |website=DesignNews.com}}</ref>

Most PMSMs require a [[variable-frequency drive]] to start them.<ref>
R. Islam; I. Husain; A. Fardoun; K. McLaughlin.
[https://ieeexplore.ieee.org/document/4757411 "Permanent-Magnet Synchronous Motor Magnet Designs With Skewing for Torque Ripple and Cogging Torque Reduction"].
Industry Applications, IEEE Transactions on.
2009.
{{doi|10.1109/TIA.2008.2009653}}
</ref><ref>
Ki-Chan Kim; Seung-Bin Lim; Dae-Hyun Koo; Ju Lee.
[https://ieeexplore.ieee.org/document/1704668 The Shape Design of Permanent Magnet for Permanent Magnet Synchronous Motor Considering Partial Demagnetization"].
Magnetics, IEEE Transactions on.
2006.
{{doi|10.1109/TMAG.2006.879077}}
</ref><ref>
P. Pillay; R. Krishnan.
[https://ieeexplore.ieee.org/document/90357 "Application characteristics of permanent magnet synchronous and brushless DC motors for servo drives"].
Industry Applications, IEEE Transactions on.
1991.
{{doi|10.1109/28.90357}}
quote:
"The permanent magnet synchronous motor (PMSM) and the brushless DC motor (BDCM) have many similarities; they both have permanent magnets on the rotor and require alternating stator currents to produce constant torque."
</ref><ref>
Y. Honda; T. Nakamura; T. Higaki; Y. Takeda.
[https://ieeexplore.ieee.org/document/643011 "Motor design considerations and test results of an interior permanent magnet synchronous motor for electric vehicles"].
Industry Applications Conference, 1997. Thirty-Second IAS Annual Meeting, IAS '97., Conference Record of the 1997 IEEE.
1997.
{{doi|10.1109/IAS.1997.643011}}
</ref><ref>
M.A. Rahman; Ping Zhou.
[https://ieeexplore.ieee.org/document/491349 "Analysis of brushless permanent magnet synchronous motors"].
Industrial Electronics, IEEE Transactions on.
1996.
{{doi|10.1109/41.491349}}
</ref> However, some incorporate a squirrel cage in the rotor for starting—these are known as line-start or self-starting.<ref>{{cite journal |last1=Hassanpour Isfahani |first1=Arash |last2=Vaez-Zadeh |first2=Sadegh |date=Nov 2009 |title=Line Start Permanent Magnet Synchronous Motors: Challenges and Opportunities |journal=Energy |volume=34 |issue=11 |pages=1755–1763 |doi=10.1016/j.energy.2009.04.022|bibcode=2009Ene....34.1755H }}</ref> These are typically used as higher-efficiency replacements for induction motors (owing to the lack of slip), but must ensure that synchronous speed is reached and that the system can withstand [[torque ripple]] during starting.

PMSMs are typically controlled using [[direct torque control]]<ref name="IEEE Conference Publication 2020-1">{{cite book |last1=Suman |first1=K. |url=https://ieeexplore.ieee.org/document/6484405 |last2=Suneeta |first2=K. |last3=Sasikala |first3=M. |title=2012 IEEE International Conference on Power Electronics, Drives and Energy Systems (PEDES) |chapter=Direct Torque Controlled induction motor drive with space vector modulation fed with three-level inverter |date=2020-09-09 |isbn=978-1-4673-4508-8 |pages=1–6 |doi=10.1109/PEDES.2012.6484405 |access-date=2020-09-23 |s2cid=25556839}}</ref> and [[field oriented control]].<ref name="IEEE Journals & Magazine 2020">{{cite journal |last1=Wang |first1=Zheng |last2=Chen |first2=Jian |last3=Cheng |first3=Ming |last4=Chau |first4=K. T. |date=2020-09-09 |title=Field-Oriented Control and Direct Torque Control for Paralleled VSIs Fed PMSM Drives With Variable Switching Frequencies |url=https://ieeexplore.ieee.org/document/7113904 |journal=[[IEEE Transactions on Power Electronics]] |volume=31 |issue=3 |pages=2417–2428 |doi=10.1109/TPEL.2015.2437893 |s2cid=19377123 |access-date=2020-09-23|url-access=subscription }}</ref>

==== Reluctance ====
{{Main|Reluctance motor}}

Reluctance motors have a solid steel cast rotor with projecting (salient) toothed poles. Typically there are fewer rotor than stator poles to minimize [[torque ripple]] and to prevent the poles from all aligning simultaneously—a position that cannot generate torque.<ref name=Fitzgerald1971a>
{{cite book
  | last = Fitzgerald
  | first = A. E.
  | author2 = Charles Kingsley Jr.
  | author3 = Alexander Kusko
  | title = Electric Machinery, 3rd Ed.
  | publisher = McGraw-Hill
  | year = 1971
  | location = USA
  | pages = 536–538
  | chapter = Chapter 11, section 11.2 Starting and Running Performance of Single-phase Induction and Synchronous Motors, Self-starting Reluctance Motors
  | id = Library of Congress Catalog No. 70-137126
}}</ref><ref name="Gottlieb">
{{cite book
  | last =  Gottlieb
  | first = Irving M.
  | title = Practical electric motor handbook, 2nd Ed.
  | publisher = Newnes
  | year = 1997
  | location = USA
  | pages = 73–76
  | url = https://books.google.com/books?id=Irj9w5IE31AC&q=shaded-pole+synchronous+motor&pg=PA72
  | isbn = 978-0-7506-3638-4
}}</ref> The size of the air gap in the magnetic circuit and thus the [[reluctance]] is minimum when the poles align with the stator's (rotating) magnetic field, and increases with the angle between them. This creates torque that pulls the rotor into alignment with the nearest pole of the stator field. At synchronous speed the rotor is thus "locked" to the rotating stator field. This cannot start the motor, so the rotor poles usually have [[squirrel-cage rotor|squirrel-cage]] windings embedded in them, to provide torque below synchronous speed. The machine thus starts as an induction motor until it approaches synchronous speed, when the rotor "pulls in" and locks to the stator field.<ref>{{citation |page=19/8 |chapter=19.2.5 Reluctance motors | title=Electrical Engineer's Reference Book |author=Michael A. Laughton |publisher=Newnes |year=2003 |isbn=978-0-7506-4637-6}}</ref>

Reluctance motor designs have ratings that range from fractional horsepower (a few watts) to about {{nowrap|22 kW}}. Small reluctance motors have low [[torque]], and are generally used for instrumentation applications. Moderate torque, multi-horsepower motors use squirrel cage construction with toothed rotors. When used with an adjustable frequency power supply, all motors in a drive system can operate at exactly the same speed. The power supply frequency determines motor operating speed.

====Hysteresis====
[[Hysteresis]] motors have a solid, smooth, cylindrical rotor, cast of a high [[coercivity]] magnetically "hard" cobalt steel.<ref name="Gottlieb"/> This material has a wide [[hysteresis loop]] (high [[coercivity]]), meaning once it is magnetized in a given direction, it requires a high magnetic field to reverse the magnetization. The rotating stator field causes each small volume of the rotor to experience a reversing magnetic field. Because of hysteresis the phase of the magnetization lags behind the phase of the applied field. Thus the axis of the magnetic field induced in the rotor lags behind the axis of the stator field by a constant angle δ, producing torque as the rotor tries to "catch up" with the stator field. As long as the rotor is below synchronous speed, each particle of the rotor experiences a reversing magnetic field at the "slip" frequency that drives it around its hysteresis loop, causing the rotor field to lag and create torque. The rotor has a 2-pole low reluctance bar structure.<ref name="Gottlieb" /> As the rotor approaches synchronous speed and slip goes to zero, this magnetizes and aligns with the stator field, causing the rotor to "lock" to the rotating stator field.

A major advantage of the hysteresis motor is that since the lag angle δ is independent of speed, it develops constant torque from startup to synchronous speed. Therefore, it is self-starting and doesn't need an induction winding to start it, although many designs embed a squirrel-cage conductive winding structure in the rotor to provide extra torque at start-up.{{citation needed|date=January 2013}}   
   
[[AC motor#Hysteresis synchronous motor|Hysteresis motors]] are manufactured in sub-fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.{{citation needed|date=January 2013}}

=== Externally excited motors ===
[[File:Electrical Machinery 1917 - Westinghouse motor.jpg|thumb|Externally excited motor, 1917. The exciter is on the left.]]
Usually made in larger sizes (larger than about 1 horsepower or 1 kilowatt) these motors require [[direct current]] (DC) to excite (magnetize) the rotor. This is most straightforwardly supplied through [[slip ring]]s.

A [[Brushless DC electric motor|brushless]] AC induction and rectifier arrangement can also be used.<ref>H.E. Jordan, ''Energy-Efficient Electric Motors and Their Applications'', page 104, Springer, 1994 {{ISBN|0-306-44698-7}}</ref>

The power may be supplied from a separate source or from a generator directly connected to the motor shaft.