Electric Power Utility Motor Case Study Diagnostics & Resolution Power utility contracted to perform off-line circuit and insulation testing of critical motors during major planned outage. Tests included MCA, IR & overvoltage hipot and impulse. New 4 kV motors are impulse tested to 11.4 kVpk (IEEE). Recommended maintenance test is 9 kVpk. Impulse test discovered a 4 kV pump motor with phase insulation breakdown voltage of 7.2 kVpk. No indication of problem during MCA, IR and hipot tests. Utility opted to start the motor repeatedly to study failure of weak motor insulation system. Insulation failed catastrophically on fifth startup. Motor was rewound during planned shutdown Cost of Diagnostics to Utility: <$10,000 including travel Savings to Utility No lost production: Potentially $100Ks No emergency repair costs: $10Ks

TESTDESCRIPTION EFFECTIV ENESS PRECAUTIONS/CONSIDERATION S STANDARD Winding Resistance DC winding resistance between phases, compute phase balance TrendableTemp correction required, under voltage IEEE 112 & 118 Winding Inductance & Impedance AC winding inductance between phases, compute phase balance Trendable Temp correction required, effected by rotor position, under voltage IEEE 388 & 389 Capacitance AC test measures capacitance winding to ground Trendable Must obtain a baseline prior to surface contamination or overheating IEEE 286 Insulation ResistanceMeasures IR winding to ground Pass/Fail & Trendable Temp correction required, under or over voltage IEEE 43 & others Polarization Index/ Dielectric Absorption Measures IR winding to ground to verify polarizing effect Pass/Fail & Trendable Temp correction required, under voltageIEEE 43 & 95 Dissipation Factor (Tan Δ) Power Factor AC winding to ground, measures ratio of power loss to total power Pass/Fail & Trendable V Line voltageIEEE 286 AC HIPOTLine frequency over voltage winding to groundPass/Fail Destructive, used primarily by manufacturers NEMA MG 1 IEEE 95 & 112 DC HIPOT DC over voltage winding to ground, measures leakage current & IR Pass/Fail & Trendable Undertake after passing IR & PI, use ramped or stepped voltage method IEEE 95 EASA VLF HIPOT Low frequency (0.1 Hz) over voltage winding to ground Pass/FailLess destructive than ACHNew Std Surge/Impulse Over voltage impulse applied to each phase, detects turn & phase insulation weaknesses Pass/FailUndertake after passing IR, PI and Hipot IEEE 522 EASA Partial Discharge PD Inception Voltage during ACH or surge tests Pass/Fail & Trendable Undertake after passing IR & PI IEEE 522 & 1434 Core Loss High current AC test for shorted stator core laminations Pass/FailTypically done only during rewindIEEE 62 Common Off Line Electrical PdM Motor Tests Over Voltage Under Voltage { {

Electric Power Utility Motor Case Study Diagnostics & Resolution Power utility contracted to perform off-line circuit and insulation testing of critical motors during major planned outage. Tests included MCA, IR & overvoltage hipot and impulse. New 4 kV motors are impulse tested to 11.4 kVpk (IEEE). Recommended maintenance test is 9 kVpk. Impulse test discovered a 4 kV pump motor with phase insulation breakdown voltage of 7.2 kVpk. No indication of problem during MCA, IR and hipot tests. Utility opted to start the motor repeatedly to study failure of weak motor insulation system. Insulation failed catastrophically on fifth startup. Motor was rewound during planned shutdown Cost of Diagnostics to Utility: <$10,000 including travel Savings to Utility No lost production: Potentially $100Ks No emergency repair costs: $10Ks

High Voltage Surge or Impulse IEEE 522 & EASA Simulates the transients on power lines which can cause insulation failure. Rise time = 0.1 µsec Impulse test of turn and phase insulation where test voltage is incrementally increased in a stepped mode from 0V voltage to target test voltage looking for change in response frequency Final impulse test voltage typically at 2 x line voltage V (peak) Only test capable of complete predictive testing of turn and phase insulation

Steps of Degrading Insulation Electrical Failure 1.New insulation has high break down voltage 2.Motor insulation experiences aging Thermal aging Chemical or moisture contamination Mechanical wear Corona/Partial Discharge 3.Turn, phase or ground insulation break down voltage degrades resulting in arcing due to transients, start up coil movement, etc

Steps of Degrading Insulation Electrical Failure 4.Induced arcs cause insulation to deteriorate much faster 5.Turn to turn or ground insulation breakdown voltage drops below operating voltage effectively causing a short between turns or a short to ground. Shorted turns transformer action causes a large induced current in the short which creates a hot spot 6.Heat causes rapid insulation failure (typically minutes to hours)

Breakdown Voltage Degradation v. Time

Electric Power Utility Motor Case Study Diagnostics & Resolution Power utility contracted to perform off-line circuit and insulation testing of critical motors during major planned outage. Tests included MCA, IR & overvoltage hipot and impulse. New 4 kV motors are impulse tested to 11.4 kVpk (IEEE). Recommended maintenance test is 9 kVpk. Impulse test discovered a 4 kV pump motor with phase insulation breakdown voltage of 7.2 kVpk. No indication of problem during MCA, IR and hipot tests. Utility opted to start the motor repeatedly to study failure of weak motor insulation system. Insulation failed catastrophically on fifth startup. Motor was rewound during planned shutdown Cost of Diagnostics to Utility: <$10,000 including travel Savings to Utility No lost production: Potentially $100Ks No emergency repair costs: $10Ks

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