Inductive Imbalanc e High and LowResistanc e High and LowInductanc eGr ound capacitanc e change 47,547,046,546,045,545,044,544,00 50 100 150 200 250 300 350Inductanc e [mH]DegreesLow MegOhm on Hipot Non linear Hipot indication10 0009 0008 0007 0006 0005 0004 0003 000-I [μA]-I [μA]-60504030201000 1 2 3 4 5 6 7 8Time [minutes]V [volts]High Pot ential Tes t Over cu rr ent TripHigh Line to Line Surge Comparison RatioUnstable Pulse to Pulse 1200 10 20 30 40 50 60 70Number of pulsesPP EAR [%] High Partial Discharge 3 0002 5002 0001 5001 0005000-500-1 000-1 500-2 000-2 500-3 0000 1 2 3 4 5 6 7 8Surge voltage [V]Time [μs]Static Electric Motor Tes ting Diagnostic Chart8 0002002Tes t v oltage20122018TimeVoltageDielectric strengthof insulation2 000460 V motorOperationalvoltage= spikes found during normal operation,e.g. caused by startup, lightning VFDs, etc. The word “Megger” is a register ed trademark © 2019 Megger- January 2019IndicationsProbable CauseRec ommended ActionsAdditional InformationIndicationsProbable CauseRec ommended ActionsAdditional InformationIndicationsProbable CauseRec ommended ActionsAdditional InformationLow MegOhm reading Low PIHigh PIPI Gr aph ShapesTimeNormal curveContamination causes a flat curveMoisture causes an erratic curveCurr entTimeNormal curveCurr entTimeInitial curveCurr entInsulation aging causes the curve to flatten over timeAgingContaminationMoistureAgingHigh resistance connec-tions, ground insulation failureFind the specific loca-tion of high resistance connection, repair and re-test. Ensure motor is not spinning.Shorted turns or coils, ground insulationfailureVerify motor leads are properly connected. Verify ground repair insulation integrity. Repair, re-test.Moisture or contamina-tion at connectionsImproper referencevalueImproper windingconnectionImproper temperature correctionHigh resistance contactsCheck all connection points for corrosion, contamination and proper torque.Verify temperature correctionVerify comparisonreference valueVerify the same motor is being testedPerform groundwall insulation testing for low ground resistanceImproper winding,connectionVerify comparisonreference valueImproper referencevalueVerify the same motor is being testedPerform groundwall insulation testing for low ground resistanceContamination / delami-nation, insulation failure, capacitor degradation if presentPerform groundwall insulation resistance test and PI/DA to aid in evaluting motors. All power factor correc-tion capacitors should be removed from the circuit in order to properly evaluate the motor winding.Rotor influence testingResistance, inductance, impedance, phase anglevs. rotor positionBroken rotar bar, eccen-tricity, rotor porosity, low magnetic signatureImbalance values should be <5% for lap wound motors. Low voltage concentric windings are likely to have a higher imbalance (<20%). Induced voltage and spin-ning rotors can also cause variation in measurement leading to high indicated imbalance. Line to line surge comparison variation will increase if an imbalance exists.Inductive imbalance is often caused by shorted turns or missed turns on a parallel connected circuit. If the short is to ground, a groundwall insulation test will fail at very low voltages. Inductive imbalance can occur even when the resistive unbalance is low.Typically faults associated with resistance will present as an imbalance in the circuit. Refer to the “Resistive Im-balance” section above for these indications.A reference value and winding correction temperature must be employed to validate high or low resistance values. If the motor was recently rebuilt, verify proper wire used.When comparing values to references/baselines, ensure that the reference applies to the motor under test. If the motor has been changed out variations in resistance will likely be present.As motor contamination builds up the effective capaci-tance of the circuit will increase. Insulation degradation in the of the circuit will increase. Insulation degradation in the capacitance readings with no corresponding reduction in insulation resistance. Delamination of older asphaltic insulation systems (1970 or older) can exhibit an increase in insulation capacitance resulting in increased PI/DA ratios > 6.0.Changes in inductance are caused by a change in the number of turns/loops/coils in the circuit under test. Hard shorts or internal connection issues can indicate both high and low value changes.Typically faults associated with inductance will present as an imbalance in the circuit. Refer to the “Inductive Imbalance” section above for these indications.When comparing values to references/baselines ensure that the reference applies to the motor under test. If the motor has been changed out variations will likely be pres-ent. Verify comparisons are made at the same test fre-quency.Rotor influence testing should be treated with caution. The data can vary based on motor design. The effect of rotor position on the circuit characteristics is evaluated for balance and symmetry. Resistance values are com-monly temperature corrected to 25° or 40°C. Tempera-ture correction is used to compare the measured value to a standard and should have no effect on the balance of the circuit.If motor is in service perform dynamic test-ing to validate the static rotor test find-ings. Trend data for evidence of further degradation.Low insulation resistance readings are most commonly caused by contamination and/or moisture. PI evaluation will aid in determining the environmental factors pres-ent. Driving off moisture through heating may be effective in increasing low readings. In some cases heat-ing the junction box may be sufficient to reduce leak-age current paths. Minimum thresholds could be set to reflect the nature of the operating environment. It is common to set a minimum threshold of 100 MegOhm @40°C for most industrial motors.Motors with IR values < (1 MegOhm per 1000V of nameplate + 1Megohm) @40°C should not be started until the IR value has been improved.Test voltage for MegOhm insulation resistance testing is typically at or below the motor’s nameplate rating.Evaluate PI/DA for evi-dence of moisture and contamination. Clean and inspect motor or send out for overhaul if efforts prove ineffec-tive.Contamination, deteriora-tion of groundwall insula-tionInsulation failure resulting in an overcurrent during a low voltage (at or below line voltage) insulation resis-tance test means the motor should not be started and will likely require rewind and repair.Isolate the location of the ground wall fail-ure. Repair/Replace motor as required.Failed groundwall insula-tionMegOhm Failure(over current)A clean dry insulation system should have a PI > 2.0. This means that the measured current has been reduced by half from the 1 minute to the 10 minute measurement. Contamination and moisture can signifi-cantly reduce this ratio due to parallel leakage paths. It is not uncommon for example to have PI readings of 1.5 in high humidity environments.For small motors (< 100 Hp), and motors with very little detectable polarization, like VFD insulation systems, a Dielectric Absorption test (DA) may be more represen-tative of the insulation’s ability to polarize. A DA is performed for only 3 minutes. The final reading is com-pared to the 30 second reading.Motors with modern insulation system can have very high PI values. This is not an indication of a problem with the system. Older asphaltic insulation systems from before 1970 could delaminate causing increased air spaces in the insulation build. This increased the capacitance of the circuit and therefore increased the PI values > 6.0.Clean / dry insulation system / overhaulContamination, insulation deteriorationIf motor winding is older than 1970, rewing; otherwise, no action is required.Delamination of asphaltic insulationWhen performing insulation testing at higher voltages it is possible that surface leakage paths will increase causing lower insulation resistance values during high potential testing. Contamination effects of this kind typically show up as a single linear step change in cur-rent and not a continual exponential increase.As voltage is increased during a DC test the current will increase proportionally until the insulation resistance begins to decrease. As the voltage approaches the insu-lation breakdown voltage the resistance will begin to decrease and the current will begin to increase in a greater than linear fashion. This departure from lineari-ty or “knee in the curve” indicates that the breakdown voltage is being approached. If this “knee” is present at or below test voltage the insulation is in the later stages of its life and has reduced reliability.Step voltage testing should include at least five (5) volt-age steps to ensure a linear change can be definitively observed.Motors with breakdown voltages at or near bus voltage should be replaced immediately. If the insulation breakdown voltage is well above nominal bus voltage the motor may start. It should be noted that attempt-ing to start a motor with a weak insulation system could result in additional damage to the stator if the motor were to fail on start.Line to line comparison: L - L EAR% (Error Area Ratio) is used to evaluate each phase circuit’s response to the surge impulse while the motor is disassembled and the rotor is removed from the stator. This is due to the sec-ondary rotor impedance influencing the stator circuit. A balanced winding will have identical surge ringing patterns that should overlay on top of each other. Be-cause the rotor has to be removed for this test, line to line comparison is rarely used as a field test.Pulse to pulse comparison (PP EAR%) is used to evalu-ate the winding insulation when the motor is fully assembled. Variation in PP EAR indicates an instability which is usually caused by weak turn-to-turn insulation. Up to 80% of winding failures begin as turn to turn weakness, progressing to phase to phase or groundwall failure. PP EAR values should be <5% and the curves should have no spikes.Partial discharge inception voltage (PDIV) is the voltage at which partial discharge events begin to occur within the insulation due to voids or surface effects. While performing surge testing, the gases in the voids are energized and will discharge internally within the insu-lation adding high frequency signals on the surge waveform. As the insulation degrades and internal de-fects such as cracks form, PDIV will begin to decrease. As PDIV and RPDIV begin to approach bus voltage, the number of discharges increases significantly, degrada-tion accelerates, and then the PDIV and RPDIV will increase, signalling imminent breakdown; action should be taken to repair or replace the motor.Evaluate PI/DA for evi-dence of moisture and contamination. Clean and inspect motor or send out for overhaul if efforts prove ineffec-tive.Groundwall contamina-tion, insulation deteriora-tionTest motor more fre-quentlyGroundwall insulation breakdown voltage reducedIsolate source of ground. Repair/replace motor as required.Insulation breakdown at indicated voltageVerify motor leads are connected properly. If not rotor influence repair/replace motor.Rotor influence; copper to copper insulation failureTest motor more fre-quentlyWeak insulation between coil turns, ground insula-tion weaknessVoids in the insulationContinue to monitor PD activity. Test the motor more frequent-ly. Repair or replace as necessary.Low Voltage Tes tingHigh Voltage Tes tingHigh Voltage Tes tingResistive ImbalanceElectric motor testing chart - static - Megger.pdf
