Motor switching inrush current

Motor switching inrush current with soft starter

Motor switching inrush current: The recorded Current Waveform view, as shown in the caption picture above for a few milliseconds, shows normal oscillations with the 3phae system voltage and motor currents of a motor starting with soft starter for a Tyre Industry's Banbury Mixture Motor of rating 400kw, 660v, 3ph, 50hz. The RMS snapshot view, as shown in the picture below lasting about 7seconds, shows that after the initial millisecond starting oscillations, both the system voltage and motor currents drooped back to their rated values, which depicts a normal motor starting characteristics.  

Motor starting inrush current graph
Motor: Inrush V&I with its normal switching

This Squirrel Case Induction Motor SQIM is always loaded with its charge of rubber mixtures from its Banbury.  Due to its high starting current, and even after providing with a soft starter; it's in-addition provided with a standalone 1250kva, 11kv/660v isolation transformer at the immediate upstream of the motor.

Even while switching it ON with a soft-starter and isolation transformer, the motor has had a history of switching on failures, like rotor bar twist, dislodge and Stator winding burning.

This starting record depicts a normal motor start event though.