tailieunhanh - Chapter 2: Construction and steady-state operation of induction motors

Construction and operating principles of induction motors are presented in this chapter. The generation of a revolving magnetic field in the stator and torque production in the rotor are described. The per-phase equivalent circuit is introduced for determination of steady-state characteristics of the motor. Operation of the induction machine as a generator is explained. | 2 CONSTRUCTION AND STEADY-STATE OPERATION OF INDUCTION MOTORS Construction and operating principles of induction motors are presented in this chapter. The generation of a revolving magnetic field in the stator and torque production in the rotor are described. The per-phase equivalent circuit is introduced for determination of steady-state characteristics of the motor. Operation of the induction machine as a generator is explained. CONSTRUCTION An induction motor consists of many parts the stator and rotor being the basic subsystems of the machine. An exploded view of a squirrel-cage motor is shown in Figure . The motor case frame ribbed outside for better cooling houses the stator core with a three-phase winding placed in slots on the periphery of the core. The stator core is made of thin mm to mm soft-iron laminations which are stacked and screwed together. Individual laminations are covered on both sides with insulating lacquer to reduce eddy-current losses. On the front side the stator housing is closed by a cover which also serves as a support for the front bearing 15 16 CONTROL OF INDUCTION MOTORS FIGURE Exploded view of an induction motor 1 motor case frame 2 ball bearings 3 bearing holders 4 cooling fan 5 fan housing 6 connection box 7 stator core 8 stator winding not visible 9 rotor 10 rotor shaft. Courtesy ofDanfoss A S. of the rotor. Usually the cover has drip-proof air intakes to improve cooling. The rotor whose core is also made of laminations is built around a shaft which transmits the mechanical power to the load. The rotor is equipped with cooling fins. At the back there is another bearing and a cooling fan affixed to the rotor. The fan is enclosed by a fan cover. Access to the stator winding is provided by stator terminals located in the connection box that covers an opening in the stator housing. Open-frame partly enclosed and totally enclosed motors are distinguished by how well the inside of stator is sealed from the ambient .