AC MotorsAC MOTOR THEORYTorqueThe torque of an AC induction motor is dependent upon the strength of the interacting rotor andstator fields and the phase relationship between them. Torque can be calculated by usingEquation (12-3).T = K F IR cos qR(12-3)whereT= torque (lb-ft)K = constantF= stator magnetic fluxIR= rotor current (A)cos qR= power factor of rotorDuring normal operation, K, F, and cos qRFigure 4 Torque vs Slipare, for all intents and purposes, constant,so that torque is directly proportional tothe rotor current. Rotor current increasesin almost direct proportion to slip. Thechange in torque with respect to slip(Figure 4) shows that, as slip increasesfrom zero to ~10%, the torque increaseslinearly. As the load and slip areincreased beyond full-load torque, thetorque will reach a maximum value atabout 25% slip. The maximum value oftorque is called the breakdown torque ofthe motor. If load is increased beyondthis point, the motor will stall and cometo a rapid stop. The typical inductionmotor breakdown torque varies from 200to 300% of full load torque. Startingtorque is the value of torque at 100% slipand is normally 150 to 200% of full-load torque. As the rotor accelerates, torque will increaseto breakdown torque and then decrease to the value required to carry the load on the motor ata constant speed, usually between 0-10%.Rev. 0 Page 7 ES-12
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