1. Cement Mill 1 (Jan 2006) Current & Voltage

2. Cement Mill 1 (Apr 2008) Current & Voltage

3. Cement Mill 2 (Jan 2006) Current & Voltage

4. Cement Mill 2 (April 2008) Current & Voltage

5. Cement Mill 1 Vibration data (2006 & 2008)

6. Cement Mill 2 Vibration data (2006 & 2008)

7. History Motors are 20 years old, not rewound till date Cement Mill 1 started showing significant hunting in the last few months Hunting was high enough to affect the captive power plant’s operation Cement Mill 1 motor was tested in the first week of April, failed the next week Stator winding has failed, flashover mark can be seen at 2 o’clock position

8. Questions? Data shows that Cement Mill 1 had current modulation two years ago, increased over time. In hindsight, the problem has developed over time. The modulation has a frequency of 6.2 Hz. I could not relate this to any mechanical components of the motor or the gearbox (approx. half the motor speed). Could a shorted rotor field coil (40 V DC) have led to this modulation? Could any mechanical fault in the gearbox have led to this modulation? Is the modulation the cause of the failure (I think so, due to heat cycling) or is it something else?