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Cracked rotor bar research project (2300HP 8 pole motor)

Revision: November 2005Herman vd Merwe(Engineering Manager)

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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)

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Cracked rotor bar research project (2300HP 8 pole motor)

Cracked rotor bar lifted out of the core slot (close-up

Findings:

& general view)

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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)

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Cracked rotor bar research project (2300HP 8 pole motor)

Bridge of rotor laminations destroyed Overheated bridge of rotor laminations

Findings (continued 2):

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Cracked rotor bar research project (2300HP 8 pole motor)

Electrical arcing damage on broken rotor bar (bottom of slot side)

Findings (continued 3):

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Research project, facilitated by the

load test plant of M&C and involvingEskom and various industry anduniversity based specialists

Cracked rotor bar research project (2300HP 8 pole motor)

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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 abroken 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

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Cracked rotor bar research project (2300HP 8 pole motor)

One rotor bar half broken

Artificially introduced progressive test conditions:

One rotor bar fully broken

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Cracked rotor bar research project (2300HP 8 pole motor)

Test results, analyses and comparison of techniques

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