Definition: In a three-phase power system, the order in which each phase voltages attain a positive peak value is called phase sequence. The phase sequence is also known as phase rotation. If the order of phase sequence in the three-phase system is denoted as RYB, then the R phase attains its positive peak value first, after 120 degrees the Y phase attains its positive peak value, and after 120 degrees B phase attains its positive peak value.
Look at the winding of a three-phase alternator in the figure below.
In a three-phase electrical system, the phase difference is 120 degrees between each phase. Why 120 degrees? This is because the windings are placed 120 degrees apart. If you want a phase difference of 90 degrees, position the windings 90 degrees apart. The phase difference depends on how the windings are arranged.
In the three-phase waveform shown above, the phase sequence or phase rotation is RYB. Here, R, Y, and B phases attain positive peak values after 90, 210, and 330 degrees respectively.
In a three-phase system, the voltages have the same magnitude but are 120° apart electrically.
There are two possible phase sequences:
- Positive Sequence (Clockwise): RYB, YBR, BRY
- Negative Sequence (Anti-clockwise): RBY, BYR, YRB
Positive Phase Sequence( Clock-wise Sequence)
The possible positive phase sequences are RYB, YBR, and BRY. In the waveform shown above, the possible orders of attaining maximum voltage are RYB, YBR, or BRY. These represent positive phase sequences that rotate clockwise. The figure below shows the concept of positive phase rotation.
Negative Phase Sequence( Anti Clock-wise Sequence)
The negative phase sequence follows the order RBY, BYR, or YRB, where each phase reaches its maximum voltage 120° apart in a counterclockwise direction. In the waveform shown above, this phase sequence represents negative phase rotation, which moves in the anti-clockwise direction. The figure below shows the concept of negative phase rotation.”
How to Reverse Phase Sequence?
To reverse the phase sequence, there are two options: one is by changing the rotation direction of the generator’s magnet (though this isn’t practical for end users), and the other is by interchanging any two phase conductors at the load side.
For example, if the phase sequence is initially RYB, swapping any two phases—such as R and Y—will change it to BYR or YBR, which reverses the sequence. The phase sequence plays a crucial role in determining the direction of rotation in three-phase motors. If the motor is running in the opposite direction, the phase sequence could be the cause, and simply swapping two phase connections will reverse the motor’s rotation.
However, the effect of changing the phase sequence differs depending on the type of load. For purely resistive loads, such as heaters or light bulbs, the phase sequence has no effect, as the current direction does not matter. But for inductive or capacitive loads, especially when the load is unbalanced, changing the phase sequence can impact the power factor and system stability, potentially leading to operational issues.
Importance of Phase Sequence
The phase sequence is essential in three-phase electrical systems because of the following reasons.
- The positive phase sequence RYB, YBR, and BRY rotates in the clockwise direction. If the stator of the motor receives a three-phase supply in the above-said phase sequence, then the motor rotates in the same direction for the three sets of order RYB, YBR, and BRY. The RBY, BYR, and YRB rotate clockwise and the motor will rotate in the opposite direction if feeds the supply of anti-clockwise sequence voltage.
- The parallel operation of the transformers if both transformers have the same vector group. It means the phase rotation of both transformers is the same. Both the transformer’s phase rotations should be RYB YBR or BRY.
- The parallel operation of the generators is possible when the generated voltages of both generators have the same phase rotation. Both the generators’ phase rotations should be RYB, YBR, or BRY.
- When two power sources need to operate in parallel, the phase rotation of both sources should be the same. The synchronizing check relay permits if the phase rotation, frequency, and voltage of the two power sources are the same. Here phase rotation of both sources should be RYB, YBR, or BRY. One source phase rotation is RYB and another source phase rotation is YBR means both the phase rotation is clockwise. But, we should not synchronize the power sources in this case.
The phase sequence indicator is very useful for knowing the phase rotation, whether it has positive phase rotation or negative phase rotation.
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