tailieunhanh - Induction Machines

Consider an electric machine with six windings. Stator and rotor are of cylindrical cross-section and three windings are situated on stator while the remaining three windings are on the rotor, as shown in Fig. 1. Both stator and rotor windings are displaced in space for 120 degrees electrical. In this electromechanical converter a continual electromechanical energy conversion may take place provided that, if angular frequency of stator currents is ωs and angular frequency of rotor currents is ωr, rotor speed is ω = ω s − ω r . Note that, according to the condition of average torque existence, this. | ENGNG2024 Electrical Engineering INDUCTION MACHINES 1. PRELIMINARY CONSIDERATIONS Consider an electric machine with six windings. Stator and rotor are of cylindrical cross-section and three windings are situated on stator while the remaining three windings are on the rotor as shown in Fig. 1. Both stator and rotor windings are displaced in space for 120 degrees electrical. In this electromechanical converter a continual electromechanical energy conversion may take place provided that if angular frequency of stator currents is as and angular frequency of rotor currents is ar rotor speed is a as -ar. Note that according to the condition of average torque existence this is the only possible correlation between stator and rotor frequency and frequency of rotation when both stator and rotor carry AC currents. Then the developed torque becomes time independent and equal to the average torque value. This type of the electromechanical energy converter is called asynchronous machine or induction machine the origin of this name will become clearer later because the rotor rotates with speed as while the stator revolving field rotates with synchronous speed as. Rotor currents form a revolving field as well which rotates with angular velocity ar with respect to rotor while with respect to stator its angular velocity is a ar as . synchronous speed. Note that creation of the rotating field in stator is enabled by displacing in space the three windings of the stator by 120 degrees and by feeding these three windings with a system of three phase currents with mutual phase displacement of 120 degrees. Fig. 1 - Cross-section of a three-phase induction machine. Before proceeding further into discussion of operating principles and analytical theory of induction machines let us here briefly review the main constructional features of induction machines. Stator of induction machines together with its three-phase winding completely corresponds to the stator of synchronous machines. This