SUMMARY (Induction Motor)

2.7
Operation of the induction motor is based on the ingenious principle of induction of EMFs and currents in the rotor that is not directly connected to any supply source. Three-phase currents in stator windings produce a
revolving magnetic field, whose angular velocity, called a synchronous velocity of the motor, is proportional to the supply frequency and inversely proportional to the number of pole pairs. The latter parameter, an integer, depends on the configuration of the windings, and it determines the field pattern. The rotor rotates with a speed different than that of the field. Consequently, lines of magnetic flux intersect rotor conductors, inducing the EMFs and currents. Slip, s, which is the relative difference of speeds of the field and rotor, is one of the most important quantities defining operating conditions of an induction machine.
Analysis of the steady-state operation of the induction motor is based on the per-phase equivalent circuit. The mechanical load of the motor is modeled by the equivalent load resistance. The developed torque resulting from interaction between the field and rotor currents strongly depends on the slip. It can be calculated as a ratio of power dissipated in equivalent load resistances of all three phases of the motor to the angular velocity of the rotor. The torque reaches a maximum value, the pull-out torque, at a speed lower than rated. The pull-out torque and the starting torque are higher than the rated torque. Other steady-state characteristics, such as the stator current versus speed, can also be determined from the equivalent circuit.
An induction machine running with a supersynchronous speed operates in the generating mode. Usually, the generating is performed by motors connected to the power system, which provides the reactive power needed for the magnetic field. Stand-alone induction generators are feasible, with capacitors connected across the stator terminals and acting as sources of reactive power.


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