AC MOTOR THEORYAC MotorsSummaryThe important information covered in this chapter is summarized below.AC Motor Theory SummaryA magnetic field is produced in an AC motor through the action of the three-phase voltage that is applied. Each of the three phases is 120° from the otherphases. From one instant to the next, the magnetic fields combine to producea magnetic field whose position shifts through a certain angle. At the end ofone cycle of alternating current, the magnetic field will have shifted through360°, or one revolution.Torque in an AC motor is developed through interactions with the rotor andthe rotating magnetic field. The rotating magnetic field cuts the bars of therotor and induces a current in them due to generator action. This inducedcurrent will produce a magnetic field around the conductors of the rotor,which will try to line up with the magnetic field of the stator.Slip is the percentage difference between the speed of the rotor and the speedof the rotating magnetic field.In an AC induction motor, as slip increases from zero to ~10%, the torqueincreases linearly. As the load and slip are increased beyond full-load torque,the torque will reach a maximum value at about 25% slip. If load isincreased beyond this point, the motor will stall and come to a rapid stop.The typical induction motor breakdown torque varies from 200 to 300% offull-load torque. Starting torque is the value of torque at 100% slip and isnormally 150 to 200% of full-load torque.ES-12 Page 8 Rev. 0
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