Answer Posted / satya

For an induction motor we have
s= (Ns-Nr)/Ns
so slip can be negative in the region where the value of
rotor speed exceeds that of the synchronous speed. usually
while working as an induction motor the speed of the rotor
cant touch the synchronous speed. because as it does so
there will be no cutting of the flux lines by the rotor
conductors. so there will be no emf induced so no current ,
so no force and no torque.
but when the induction motor is run by an external
primemover over the synchronous speed now rotor speed
increases that of the Ns. so rotor emf E2> stator applied emf.
the induction motor now functions as an induction generator.

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