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The operation of a motor with _________ unbalance condition is not recommended, and will probably result in damage to the motor.
above a 10%
above a 7.5%
above a 5%
above a 15%
above a 5%
Operating a three-phase induction motor with a voltage unbalance above 5% is not recommended as it severely degrades performance and can cause permanent thermal damage · According to NEMA (National Electrical Manufacturers Association) MG-1 standards, voltage unbalance should generally not exceed 1%, and operation above 5% unbalance is strictly discouraged due to excessive heating.
Operating a three-phase induction motor with a voltage unbalance above 5% is not recommended as it severely degrades performance and can cause permanent thermal damage · According to NEMA (National Electrical Manufacturers Association) MG-1 standards, voltage unbalance should generally not exceed 1%, and operation above 5% unbalance is strictly discouraged due to excessive heating.
% Voltage Unbalance=Average Line VoltageMax Deviation from Average Line Voltage×100
In \approx$$\frac{V_n}{Z_2} — Negative sequence current is disproportionately high due to low Z_2
Voltage unbalance causes a disproportionately large unbalanced current due to the low negative-sequence impedance of the motor · This creates a strong negative-sequence magnetic field rotating in opposition to the rotor, inducing high-frequency rotor currents that cause severe local overheating, insulation degradation, and increased losses.
NEMAMG-1 limits maximum voltage unbalance to 1% without derating.
A voltage unbalance of 3.5% can cause a temperature rise of nearly 25% in the critical motor winding.
Unbalance leads to increased negative sequence currents, noise, vibration, and reduced motor efficiency.
Option A (above 10%), Option B (above 7.5%), and Option D (above 15%) exceed the maximum safe threshold for continuous or temporary motor operation.
Motor load must be heavily derated using a NEMA derating factor when operating between 1% and 5% unbalance.
C is correct — Motor operation above a 5% voltage unbalance is not recommended as it leads to excessive overheating and severe winding damage.
In competitive exams, remember that current unbalance can be 6 to 10 times the percentage of voltage unbalance; thus, even a 2% voltage unbalance can cause over 15% current unbalance.