A 400kW 660V squirrel cage induction motor used in a rubber compound homogenizing process was failing.  The motor was started using a softstart in order to limit the initial current. The original motor had aluminum bars in the rotor and copper windings in the stator.  The original failure was with the aluminum rotor bars.  These were replaced with copper bars but subsequently the copper stator windings failed.  Always the failure was during motor start.

A PowerPro was brought in to analyze the starting current characteristics.  Two different starting characteristics were observed.  One had a very large ½ cycle inrush followed by oscillations in the RMS current.  During this start attempt the rotor did not start turning.  The other characteristic was indicative of a normal start.

It was determined that the motor failure was due to the stress on the machine caused by the unsuccessful starts.  It was further determined that misfiring of the softstart was responsible for the unsuccessful starts.

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Figure 1:Inrush RMS current during unsuccessful start.

Figure 2: Current waveforms during unsuccessful start.

Figure 3: Inrush RMS current during successful start.

Figure 4: Current waveforms during successful start.