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Understanding Induction Motor Performance Characteristics

Learn about the performance characteristics of induction motors, including synchronous speed, slip, equivalent circuit, power developed, torque, efficiency, and speed-torque characteristic.

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Understanding Induction Motor Performance Characteristics

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  1. The stator winding are supplied with balanced three-phase AC voltage, which produce induced voltage in the rotor windings. It is possible to arrange the distribution of stator winding so that there is an effect of multiple poles, producing several cycle of magnetomotive force (mmf) or field around the air gap. The speed of rotation of field is called the synchronous speedws , which is defined by : ωs is syncronous speed [rad/sec] Ns issyncronousspeed [rpm] p is numbers of poles ω is the supply frequency [rad/sec] f is the supply frequency [Hz] Nm is motor speed or

  2. The motor speed The rotor speed or motor speed is : Where S is slip, as defined as : Or

  3. Equivalent Circuit Of Induction Motor Where : Rs is resistance per-phase of stator winding Rr is resistance per-phase of rotor winding Xs is leakage reactance per-phase of the winding stator Xs is leakage reactance per-phase of the winding rotor Xm is magnetizing reactance Rm is Core losses as a reactance

  4. Performance Characteristic of Induction Motor Stator copper loss : Rotor copper loss : Core losses :

  5. Performance Characteristic of Induction Motor • Power developed on air gap (Power fropm stator to • rotor through air gap) : • Power developed by motor : or or • Torque of motor : or

  6. Performance Characteristic of Induction Motor Input power of motor : Output power of motor : Efficiency :

  7. Performance Characteristic of Induction Motor If and so, the efficiency can calculated as :

  8. Performance Characteristic of Induction Motor Generally, value of reactance magnetization Xm >> valueRm (core losses) and also So, the magnetizing voltage same with the input voltage : Therefore, the equivalent circuit is ; Xm

  9. Performance Characteristic of Induction Motor Total Impedance of this circuit is : Xm The rotor current is :

  10. Torque – speed Characteristic

  11. Three region operation : • Motoring : • Regenerating : • Plugging :

  12. Performance Characteristic of Induction Motor Starting speed of motor is wm = 0 or S = 1, Starting torque of motor is : Slip for the maximum torque Smax can be found by setting : So, the slip on maximum torque is :

  13. Performance Characteristic of Induction Motor Torque maximum is : And the maximum regenerative torque can be found as : Where the slip of motor s = - Sm

  14. Speed-Torque Characteristic : For the high Slip S. (starting) So, the torque of motor is : And starting torque (slip S=1) is :

  15. For low slip S region, the motor speed near unity or synchronous speed, in this region the impedance motor is : So, the motor torque is : And the slip at maximum torque is : The maximum motor torque is :

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