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Temperature rise and load carrying capacity of the three-phase induction motor will _________ & _________ respectively with increase in Voltage unbalance.
Decrease & Increase
Increase & Increase
Decrease & Decrease
Increase & Decrease
Increase & Decrease
When unbalanced voltages are applied to a three-phase induction motor, it sets up both positive-sequence and negative-sequence magnetic fields ┬╖ The negative-sequence field creates a counter-rotating magnetic field that induces high-frequency rotor currents, significantly increasing I2R losses and leading to a temperature rise ┬╖ Consequently, to prevent thermal damage, the load-carrying capacity (derating factor) of the motor decreases.
When unbalanced voltages are applied to a three-phase induction motor, it sets up both positive-sequence and negative-sequence magnetic fields ┬╖ The negative-sequence field creates a counter-rotating magnetic field that induces high-frequency rotor currents, significantly increasing I2R losses and leading to a temperature rise ┬╖ Consequently, to prevent thermal damage, the load-carrying capacity (derating factor) of the motor decreases.
%┬аVoltage┬аUnbalance=Average┬аVoltageMax┬аDeviation┬аfrom┬аAverage┬аVoltageтАЛ├Ч100" тАФ NEMA definition of voltage unbalance
snтАЛ = 2 - s" тАФ Negative-sequence slip, which is close to 2 during normal operation ]
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D is correct тАФ Voltage unbalance induces negative-sequence currents that raise internal temperature through increased I2R losses, requiring the motor's load-carrying capacity to be derated.
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Remember that negative-sequence currents act like a reverse torque generator, acting as a dynamic brake and generating excessive rotor heat due to double-frequency (2f) rotor current effects.
Voltage unbalance causes a disproportionately high negative-sequence current due to the low negative-sequence impedance of the motor (roughly equal to the locked-rotor impedance) ┬╖ The negative-sequence field rotates opposite to the rotor, causing high relative slip (snтАЛтЙИ2тИТs), which severely increases rotor copper losses and local heating while producing a opposing braking torque that reduces net mechanical output capability.