Negative-Sequence Current — Motor Overheating from Voltage Unbalance

A phenomenon in which a relatively small voltage unbalance on the supply of a three-phase motor causes a disproportionately large negative-sequence current (counter-rotating field) in the stator winding, because the motor's negative-sequence impedance is much lower than its normal (positive-sequence) operating impedance — as a rule of thumb, 1% voltage unbalance typically already produces 6 to 10% negative-sequence current. This current creates a magnetic field rotating opposite to the rotor, which induces additional eddy-current losses in the rotor bars at a frequency close to twice the grid frequency, resulting in concentrated, localised overheating of the rotor that is not detected by a conventional thermal (I²t) stator protection, which only measures total stator current. Protection relays with a negative-sequence current function (ANSI code 46) therefore separately measure the negative-sequence component of the motor current and trip the motor before the rotor is damaged, even if total stator current still remains within normal limits.