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    In three-phase systems, the harmonics of given orders have a particular Sequence.

    Positive Sequence, Zero Sequence and Negative Sequence.  Positive sequence harmonics are in the normal direction of the phase rotation. The harmonics of the 2nd, 5th, 8th and 11th order are the negative sequence harmonics, meaning that they generate the torque which counteracts normal motor direction of rotation, which can cause heating, unnecessary energy losses, and reduced efficiency. The last group are the zero sequence components, such as the 3rd, 6th and 9th, which do not generate torque but instead flow through the motor winding to cause additional heating.

    in Harmonics