Phase Sequence Indicator Rotation Explained

A phase sequence indicator is a device used to determine the phase sequence of a three-phase power supply. The standard phase sequence is often denoted as RYB (Red, Yellow, Blue) or ABC. The direction in which the indicator rotates depends directly on the order of the phases applied to its terminals.

Understanding Phase Sequence

The phase sequence refers to the time order in which the voltage waveforms of the three phases reach their peak positive values. In a three-phase system, the voltages are typically 120 degrees apart. For a sequence RYB, the voltage R peaks first, then Y 120 degrees later, and then B another 120 degrees later.

Effect of Phase Sequence on Rotation

Phase sequence indicators are designed such that a specific phase sequence causes rotation in one direction (e.g., clockwise), while the reverse phase sequence causes rotation in the opposite direction (e.g., anticlockwise).

• Given: Phase sequence RYB causes clockwise rotation.

Analyzing the New Phase Sequence BRY

The new phase sequence is given as BRY. Let's compare this sequence to the original RYB sequence in terms of cyclic order:

• Standard cyclic order is often represented as R → Y → B → R → Y → B ...
• In the RYB sequence, we follow this order: R → Y → B.
• Now consider the BRY sequence. Let's see the order: B → R → Y.

Starting from B and following the standard cycle B → R → Y → B → R → Y ..., we see that B is followed by R, and R is followed by Y. This is the same cyclic order as R → Y → B, just starting from a different point in the cycle (B). Both sequences RYB and BRY follow the same progression through the phases in the cycle.

For example, if we list the sequences cyclically:

• RYB → R, Y, B, R, Y, B ...
• BRY → B, R, Y, B, R, Y ...

Both sequences represent the same cyclical order of phases, only the starting point is different.

Conclusion on Rotation

Since the phase sequence BRY maintains the same cyclic order as RYB, the phase sequence indicator will rotate in the same direction as it did for RYB.

• Given: RYB causes clockwise rotation.
• Analyzed: BRY has the same cyclic order as RYB.
• Therefore: BRY will also cause clockwise rotation.

A sequence that would cause anticlockwise rotation would be the reverse sequence, such as RBY or BYR or YRB.