Cracked rotor bar research project (2300HP 8 pole motor) Revision: November 2005 Herman vd Merwe (Engineering Manager)

Cause of failure (generic sequence of events): Rationale: Catastrophic failure on electric motors, e.g. ID fan motor (Eskom, 11kV, 7250kW) ‘Inter bar’ currents cause electrical arcing between core laminations and the broken bar Localized heating could be so severe that the bridge portion of the laminations melts away Close to break location, current flows through core laminations to adjacent unaffected bars Bar of copper type rotor cage breaks __ current continues to flow in the bar Lifted bar invariably starts to rub against the overhang of the stator winding Broken bar then lifts out of the core slot under influence of centrifugal forces This rub leads to an inter turn fault on the stator winding and thereby an earth fault Cracked rotor bar research project (2300HP 8 pole motor)

Cracked rotor bar lifted out of the core slot (close-up Findings: & general view)

Cracked rotor bar research project (2300HP 8 pole motor) Damage to stator winding overhang (close-up of failed coil Findings (continued – 1): & other damaged coils)

Cracked rotor bar research project (2300HP 8 pole motor) Bridge of rotor laminations destroyedOverheated bridge of rotor laminations Findings (continued – 2):

Cracked rotor bar research project (2300HP 8 pole motor) Electrical arcing damage on broken rotor bar (bottom of slot side) Findings (continued – 3):

Research project, facilitated by the load test plant of M&C and involving Eskom and various industry and university based specialists Cracked rotor bar research project (2300HP 8 pole motor)

Basic research methodology: Use a wide range of test techniques (existing technologies & experimental / new) and measuring instruments, operated by a number of test personnel from different companies All tests are conducted for a number of progressive test conditions ( 1. “as is”, 2. half a broken bar & 3. one broken bar; tests with two broken bars required?…) Tests under load are taken at a number of different load points Tests under load – Monitoring of shaft earthing brush current Assembled off-line tests (inductance profile) Disassembled off-line tests (bar continuity and ‘growler’) Other tests: Stator current spectral analyses Stray flux measurement Vibration spectral and time waveform analyses

Cracked rotor bar research project (2300HP 8 pole motor) One rotor bar half broken Artificially introduced progressive test conditions: One rotor bar fully broken

Cracked rotor bar research project (2300HP 8 pole motor) Test results, analyses and comparison of techniques *** TO FOLLOW… ***