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Effective motor torque of the three-phase induction motor will _________ with an increase in Voltage unbalance.
Decrease
Increase
Remains the same
Cant say anything using only this data
Decrease
Voltage unbalance in a three-phase supply generates negative sequence components along with positive sequence components · The negative sequence voltage produces a magnetic field rotating in the opposite direction to the motor's rotor, creating a counter-rotating torque (braking torque) · Consequently, the net or effective motor torque decreases.
Voltage unbalance in a three-phase supply generates negative sequence components along with positive sequence components · The negative sequence voltage produces a magnetic field rotating in the opposite direction to the motor's rotor, creating a counter-rotating torque (braking torque) · Consequently, the net or effective motor torque decreases.
Tnet=Tp−Tn — Net developed torque under unbalanced voltage condition
Vn=31(Va+a2Vb+aVc) — Negative sequence voltage component
sn=2−s — Slip for negative sequence component (sn≈2 at normal speed)
When three-phase voltages are unbalanced, symmetrical component analysis resolves the system into balanced positive, negative, and zero sequence voltages · Positive sequence voltage produces forward rotating torque (Tp), whereas negative sequence voltage produces reverse rotating torque (Tn) · The net motor torque is Tnet=Tp−Tn, which is significantly less than the rated torque under balanced conditions.
Negative sequence voltage creates a magnetic field rotating at synchronous speed in the reverse direction.
The rotor presents a very low impedance to negative sequence voltage, drawing high negative-sequence currents.
Counter-rotating torque acts as a braking force, reducing total useful mechanical torque output.
Voltage unbalance causes severe rotor heating, increased vibrations, noise, and reduced efficiency.
A small voltage unbalance (e.g., 2%) can cause a disproportionately large current unbalance (up to 15-20%).
Option B (Increase) is incorrect because negative sequence components oppose motion rather than aiding it.
Option C (Remains the same) is incorrect as torque is directly affected by the opposing magnetic component.
Option D is incorrect because symmetrical component theory provides a clear, quantitative prediction of torque reduction.
A is correct — Voltage unbalance creates negative sequence voltages that produce a reverse-rotating torque, decreasing the net effective motor torque.
In competitive exams, remember that negative sequence components always cause braking effects (sn=2−s) and significant rotor heating due to high relative slip frequency (~100 Hz).