AC MotorsAC MOTOR THEORYAC MOTOR THEORYAC motors are widely used to drive machinery for a wide variety of applications.To understand how these motors operate, a knowledge of the basic theory ofoperation of AC motors is necessary.EO 1.1 DESCRIBE how a rotating magnetic field is producedin an AC motor.EO 1.2 DESCRIBE how torque is produced in an AC motor.EO 1.3 Given field speed and rotor speed, CALCULATEpercent slip in an AC motor.EO 1.4 EXPLAIN the relationship between slip and torque inan AC induction motor.PrinciplesofOperationThe principle of operation for all AC motors relies on the interaction of a revolving magneticfield created in the stator by AC current, with an opposing magnetic field either induced on therotor or provided by a separate DC current source. The resulting interaction produces usabletorque, which can be coupled to desired loads throughout the facility in a convenient manner.Prior to the discussion of specific types of AC motors, some common terms and principles mustbe introduced.RotatingFieldBefore discussing how a rotating magnetic field will cause a motor rotor to turn, we must firstfind out how a rotating magnetic field is produced. Figure 1 illustrates a three-phase stator towhich a three-phase AC current is supplied.The windings are connected in wye. The two windings in each phase are wound in the samedirection. At any instant in time, the magnetic field generated by one particular phase willdepend on the current through that phase. If the current through that phase is zero, the resultingmagnetic field is zero. If the current is at a maximum value, the resulting field is at a maximumvalue. Since the currents in the three windings are 120° out of phase, the magnetic fieldsproduced will also be 120° out of phase. The three magnetic fields will combine to produce onefield, which will act upon the rotor. In an AC induction motor, a magnetic field is induced inthe rotor opposite in polarity of the magnetic field in the stator. Therefore, as the magnetic fieldrotates in the stator, the rotor also rotates to maintain its alignment with the stator’s magneticfield. The remainder of this chapter’s discussion deals with AC induction motors.Rev. 0 Page 1 ES-12
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