Phase sequence, also known as phase rotation, is the order in which the voltage waveforms of a polyphase AC source reach their respective peaks. In a three-phase system, the phase sequence is the order in which the voltages attain their maximum positive value. The phase sequence is important for several reasons, including:

• Parallel operation of three-phase transformers or alternators: The phase sequence must be known to parallel these devices.

• Direction of rotation of three-phase induction motors: The phase sequence determines the direction of rotation of these motors.

• Connecting one three-phase system parallel with another: The phase sequence determines whether this is possible or not.

To determine the phase sequence, one can use a phase sequence detector that compares the brightness of two lamps. In a three-phase system, if we make winding 1 our "reference" voltage source for phase angle (0°), then winding 2 will have a phase angle of -120° (120° lagging, or 240° leading) and winding 3 an angle of -240° (or 120° leading). This sequence of phase shifts has a definite order. For clockwise rotation of the shaft, the order is 1-2-3 (winding 1 peak first, then winding 2, then winding 3) . If the phases of any coil are named as R, Y, B, then the positive phase sequence will be RYB, YBR, BRY (clockwise sequence), and the negative phase sequence will be RBY, BYR, YRB (anti-clockwise sequence) .