Definition: The phase sequence in a three-phase system shows the order of voltages attaining their peak or maximum value. The three phases in the 3-phase system have the same magntidte and frequency but are displaced 120-degree electrically apart and attains the maximum value in a particular order.
In polyphase system, the order in which voltage waveform attains their respective maximum values is called phase sequence.
For instance, if the phases of a coil are labeled as R, Y, and B, then the positive phase sequence would be RYB, YBR, and BRY, also referred to as the clockwise sequence. In the RYB phase sequence, the voltage of the R phase reaches its maximum value first, followed by the Y phase, and then the B phase. The RYB sequence is shown in the image below.
Image Source: Wikipedia
Conversely, the Negative phase sequence would be RBY, BYR, YRB respectively, known as the anti-clockwise sequence.
Importance of Phase Sequence
The phase sequence is crucial in electrical systems for the following reasons.
- Motor Rotation Direction: In three-phase motors, the phase sequence determines the direction of rotation. Correct phase sequence ensures that motors run in the intended direction, preventing equipment damage and ensuring proper operation. If an induction motor rotates clockwise with the RYB phase sequence. If you change the sequence to the YRB, RBY, or BYR, the motor will rotate in an anti-clockwise direction.
- Parallel Operation of Transformers: To operate the transformers in parallel, the phase sequence of the voltages of all the transformers must be the same. This is one of the essential conditions for parallel operation of transformers.
- Parallel operation of alternators: To operate the alternators in parallel, the phase sequence of the voltages of all the alternators must be the same. This is one of the essential conditions for parallel operation of alternators.
The sequence of the three-phase voltage can be determined by a phase sequence meter.