If one phase of supply goes off in the case of 3-phase induction motor, the motor

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Option D: None of the above

Explanation & Concepts:
When one phase of a 3-phase supply to an induction motor is lost, the motor continues to run as a single-phase motor, a condition known as 'single phasing'. The motor does not stop automatically; instead, it continues to operate while drawing unbalanced, high-magnitude currents, which typically leads to overheating and potential winding failure if protection is not provided.

Working Principle: Upon losing one phase, the rotating magnetic field becomes elliptical rather than circular. To maintain the same mechanical load, the motor draws increased current from the remaining two phases to compensate for the lost phase torque. This causes a significant rise in copper losses and stator current, which is often roughly 1.732 times the normal full-load current in the supply lines, causing motor damage.

Key Formulas: $I_{line} \approx \sqrt{3} 	imes I_{phase(normal)}$ — represents the approximate increase in line current during single phasing; $T_{singlephase} = 0$ — implies that the motor cannot start on its own if single phasing occurs at rest

Key Points: A 3-phase induction motor will continue to run if one phase is lost while it is already under load.; The motor produces a pulsating torque which causes excessive vibration and humming noise.; Single phasing results in severe unbalanced currents in the motor windings.; Thermal overload relays are typically used to protect motors against single phasing.

Advantages: No immediate shutdown ensures process continuity in non-critical scenarios; Detection is possible via negative sequence current monitoring

Disadvantages: Severe overheating of stator windings; Rapid degradation of insulation leading to motor burnout; Significant reduction in efficiency and output torque

Additional Info: Option B is technically incorrect as the current does not exactly double, but typically rises by factor of $\sqrt{3}$.. Option C is incorrect as the current increases rather than decreases.. Single phasing protection is a standard feature in modern motor protection relays (MPRs).

Answer: D is correct — The motor continues to run as a single-phase induction motor while drawing unbalanced, high currents that lead to overheating.

Exam Tip: Always remember that 3-phase motors do not have starting torque under single-phase conditions, but if already running, they will continue to rotate until thermal failure occurs.

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Option A: Comes to a stop

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Option B: Draws double the initial current and continues to run

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Option C: Draws $1/\sqrt{3}$ times the initial current and continues to run

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