Editing Synchronous motor (section)

==== 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.