why induction motor torque oscillating?
The torque delivery of an induction motor oscillates if;


  If the incoming supply is unbalanced.The unbalance voltage cause unbalance current. The unbalance current cause the negative torque in the motor.
  Also unbalance winding resistance produce the oscillating torque


The induction motor gives the best performance if the supply voltage is perfectly sinusoidal and the impedance of the stator and rotor winding is balanced.

Read More:Motor torque under unbalance voltage and/or unbalance impedance