f901c92b44 By switching in different numbers or combinations of poles a limited number of fixed speeds could be obtained. 12: 149, 161. ^ a b NEMA MG-1, p. Starting Three phase induction motors and some synchronous motors are not self starting but design modifications such as auxiliary or "damper" windings on the rotor are incorporated to overcome this problem. Retrieved 2 December 2012. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. p.27. (See also Energy savings).
Polyphase Induction Motors One third of the world's electricity consumption is used for running induction motors driving pumps, fans, compressors, elevators and machinery of various types. .. Increasing the rotor resistance will reduce the current and increase the slip; hence a form of speed and torque control is possible with wound rotor motors. Santa Barbara: ABC-CLIO. Increased rotor resistance also has the added benefit of reducing the input surge current and increasing starting torque on switch on, but all of these benefits are at the expense of more complex rotor designs and unreliable slip rings to give access to the rotor windings. "Torque Slip Characteristics of Induction Motor". Single-phase motors require some mechanism to produce a rotating field on startup. Arago's Experiments on the Magnetism Manifested by Various Substances during the Act of Rotation". The rotor however needs salient poles to create a variable reluctance in the motor's magnetic circuit which depends on the angular position of the rotor.