Negative Phase sequence in Induction motors

          Negative phase sequence in induction motors is caused due to unbalanced voltages in the supply voltage applied on the stator terminals or unbalanced windings.
        
          Negative phase sequence components create a rotating magnetic field in the stator which moves in the opposite direction.  This causes a decrease in the torque developed by the motor.  The motor will thus have to draw a higher current for the same mechanical load.

          The rotating magnetic field which rotates in the opposite direction induces voltages in the rotor.  These voltages have a frequency that is double the system frequency.  Since the frequency of this rotor voltage is higher, it flows on the surface of the rotor due to the skin effect and causes surface heating which can lead to motor damage.

          Negative phase sequence relays can identify negative phase sequence condition and trip the machine.  Negative phase sequence relays work by using a special filter which filters out the positive sequence and the zero sequence components.  The filtered negative phase sequence voltage alone is measured.  When the measured negative phase sequence voltage exceeds the set value, the relay trips the motor.

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