A three-phase power system with three voltages E_A, E_B and E_C have an equal magnitude and are displaced at ____ from each other.

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ALP CBT 2 Electrician Official Paper (Held On: 23 Jan 2019 Shift 3)

Answer 1

In a three-phase system, the order in which the voltages attain their maximum positive value is called Phase Sequence.
There are three voltages or EMFs in the three-phase system with the same magnitude, but the frequency is displaced by an angle of 120° electrically.
Consider the R, Y, and B the three phases of the supply system.
Taking an example, if the phases of any coil are named as R, Y, B then the Positive phase sequence will be RYB, YBR, BRY also called clockwise sequence and similarly the Negative phase sequence will be RBY, BYR, YRB respectively and known as an anti-clockwise sequence.
For a phase sequence RBY,
VRN = V∠ϕ°, VYN = V∠(120° + ϕ), VBN = V∠(240° + ϕ) = V∠(-120° + ϕ)

F1 U.B Madhu 07.01.20 D6

For a three-phase system, there are only two possible phase sequences RYB and RBY corresponding to the two possible directions of alternator rotation.
Phase rotation has no impact on resistive loads, but it will have an impact on unbalanced reactive loads, as shown in the operation of a phase rotation detector circuit.
Phase rotation can be reversed by swapping any two of the three terminals supplying three-phase power to a three-phase load.