For the same load, if the phase sequence of a three-phase supply changes, then:

Explanation:

Phase Sequence in Three-Phase Systems

Definition: The phase sequence (or phase rotation) in a three-phase system refers to the order in which the three phases (commonly labeled as A, B, and C) reach their respective maximum positive values. This sequence can be either ABC or ACB, and it is crucial for the correct operation of three-phase equipment, especially motors and other rotating machinery.

Working Principle: In a three-phase system, three sinusoidal voltages of equal magnitude and frequency are generated, with each voltage phase-shifted by 120 degrees from the others. The standard phase sequence ensures that the voltages reach their peak values in a specific order (e.g., A first, then B, then C). If the phase sequence is changed (e.g., from ABC to ACB), the direction of rotation of the magnetic field in motors will reverse, which can cause the motor to run in the opposite direction.

Correct Option Analysis:

The correct option is:

Option 3: Phase current changes by angle but not by magnitude.

This option correctly describes the effect of changing the phase sequence on the phase currents in a three-phase system. When the phase sequence is altered, the phase currents will shift in phase angle by 120 degrees, but their magnitudes will remain unchanged. This is because the phase sequence change does not affect the amplitude of the sinusoidal currents, only their relative timing.

Detailed Explanation:

In a three-phase system, the voltage and current waveforms are typically represented as:

Original Phase Sequence (ABC):

• Phase A: V_A(t) = V_msin(ωt)
• Phase B: V_B(t) = V_msin(ωt - 120°)
• Phase C: V_C(t) = V_msin(ωt - 240°)

Here, V_m is the peak voltage, ω is the angular frequency, and t is time.

When the phase sequence changes from ABC to ACB, the voltage waveforms become:

• Phase A: V_A(t) = V_msin(ωt)
• Phase C: V_C(t) = V_msin(ωt - 120°)
• Phase B: V_B(t) = V_msin(ωt - 240°)

Consequently, the phase currents will also shift in phase angle by 120 degrees, but their magnitudes will remain the same. This shift in phase angle is crucial for devices that rely on the direction of the rotating magnetic field, such as induction motors, as it will cause them to rotate in the opposite direction.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Magnitude of phase power is changed.

This option is incorrect because the magnitude of the phase power in a balanced three-phase system does not depend on the phase sequence. Phase power is primarily determined by the voltage, current, and power factor. Changing the phase sequence only affects the direction of rotation of the magnetic field, not the magnitude of the power.

Option 2: Magnitude of phase current is changed.

This option is also incorrect because changing the phase sequence does not affect the magnitude of the phase currents. The phase currents will have the same amplitude but will be phase-shifted by 120 degrees.

Option 4: Total power consumed will change.

This option is incorrect because the total power consumed in a balanced three-phase system is the sum of the power consumed in each phase. Since the power in each phase remains unchanged regardless of the phase sequence, the total power consumed will also remain unchanged.

Conclusion:

Understanding the impact of phase sequence on three-phase systems is crucial for the correct operation of equipment, especially motors. Changing the phase sequence results in a phase shift of the currents by 120 degrees, but their magnitudes remain unchanged. This phase shift can reverse the direction of rotation of motors, which is essential information for ensuring the proper functioning of three-phase machinery.