Health Monitoring and Fault Detection - PowerPoint Presentation

Slide1

Health Monitoring and Fault Detection of Claw-pole Generators

Siwei ChengCEME Seminar, April 2, 2012

Advisor : Dr. Thomas G. HabetlerSlide2

Condition Monitoring of Claw-pole Generators – BackgroundThe heart of virtually all automotive electric power systemsPassenger vehicles increasingly rely on electric powerAutomotive electric power systemClaw-pole generator (CPG) – a special type of wound-rotor sync. generatorBelt-driven by the engineField winding connected to a PWM regulator

Built-in three-phase diode rectifierDC output voltage regulated to approx. 13.8 VAlways connected to a lead-acid battery [1]Slide3

Condition Monitoring of Claw-pole Generators – BackgroundExisting data on failure modes of CPGs are extremely limitedWarranty data from a major US auto manufacturer

A breakdown of failure modes of CPGs replaced under warrantySlide4

Condition Monitoring of Claw-pole Generators – BackgroundInterview with industry expertsWorn bearingSerpentine belt slipStator insulation

Wire-wound stator

Bar-wound statorSlide5

Condition Monitoring of Claw-pole Generators – BackgroundInternal faultsWorn bearingStator winding faultsShort/open rectifier diodeVoltage regulator faultRotor winding faultsRotor eccentricityBrush and slip ring faultExternal faults (System-related faults)Serpentine belt slipSerpentine belt defectsLoose connection

Improper mountingSlide6

Fundamental Aspects on Condition Monitoring of CPGsReverse-engineering of the claw-pole generatorFinite-element simulation

DC B-H curve of rotor materialSlide7

Fundamental Aspects on Condition Monitoring of CPGsTest bench setupClaw-pole generator – 2 kW, 145 AmpSlide8

Fundamental Aspects on Condition Monitoring of CPGsField voltage and currentAC voltage and currentDC output voltage and currentSlide9

Fundamental Aspects on Condition Monitoring of CPGs

Spectrum of field voltageFundamental PWM frequency located at 398.3 HzHarmonics at the multiples of 398.3 Hz are also presentThe spectrum resembles that of a rectangular waveSpectrum of output voltageHarmonics induced by the switching field voltage (marked by red circles)Rectifier ripple harmonic at 2666 HzHarmonics at integral factors of the rectifier ripple frequency – 1/6th, 2/6th, 3/6th, 4/6th, 5/6th Spectrum of output currentSame harmonic contents but with larger magnitude (capacitive load)Slide10

Fundamental Aspects on Condition Monitoring of CPGs

The frequency of rectifier ripple harmonic is directly related to generator speed, may be used for speed estimation or belt-slip detection

The current harmonic at 1/3

rd

of the rectifier ripple frequency is related to three-phase asymmetry, may be used to detect stator turn fault

The side-band harmonics is related to belt cycle of the drive system, may be used to detect belt defect

Potentially useful harmonic components for fault detection purposes

The low-frequency harmonic is related to the mechanical shaft frequency of the generator, may be used to detect rotor eccentricitySlide11

Detection of Serpentine Belt SlipSerpentine belt slipReduce energy conversion efficiencyAccelerate belt wearRisk of premature belt breakageSudden loss of electric powerLoss of all the important accessories driven by the same belt (water pump, power steering pump, A/C compressor, air pump, etc)Slide12

Detection of Serpentine Belt SlipPrinciple of the belt-slip detectorSerpentine belt slips when,Engine speed > generator speed * pulley ratioEngine speed – measured and knowngenerator speed – inferred from the rectifier ripple frequency

Voltage waveforms of a 3-Φ

rectifier

n

gen

–

generator’s mechanical speed

f

ripple

–

rectifier ripple frequency

P

–

number of pole pairs of the generator

n

diode

–

number of diodes in the rectifier. Slide13

Detection of Serpentine Belt SlipBelt-slip detector – Interpolated FFT for real-time frequency trackingEstimated generator speed tracks the variation of engine speed wellAbrupt load change at 37 sec – generator speed dips, engine speed constantSlide14

Detection of Serpentine Belt DefectSerpentine belt defectTorn rubber in the groovesEarly sign of failing serpentine beltPrinciple of belt-defect detectorNonuniform thickness or fit of the belt along along its lengthAppears most obviously as sideband of rectifier ripple harmonic because of harmonic interaction

f

ripple

–

rectifier ripple frequency

p

1

–

generator pole number

k

belt

–

number of generator shaft revolutions

per BELT CYCLESlide15

Detection of Serpentine Belt DefectBelt defect detector – current spectra at 4491rpm

Good belt

Zoom-in spectrum

Defective Belt

A six-groove serpentine belt with a 2-cm defect on one grooveSlide16

Detection of Serpentine Belt DefectBelt defect detector – experimental results at different speedsSlide17

Detection and Protection of Stator Turn Faults Stator turn faults in claw-pole generatorsMay develop into phase-to-ground failure – sudden loss of power, increased risk of electric fireModeling of claw-pole generators with stator turn faults (delta-connected)how the fault interacts with the full-bridge rectifier and the connected batteryhow it would affect the generator’s output voltage and current. Slide18

Detection and Protection of Stator Turn Faults

v

ab

–

Phase-A voltage

e

0A

–

field-induced back-EMF

L

ar

–

armature reaction induct.

L

ls

–

leakage inductance

R

s

–

winding resistance

i

A

–

Phase-A current

m –

number of shorted turns

N

– total number of serial turns

i

f

–

circulating current in the shorted turns (fault current)

L

Af

,

L

Bf

,

L

Cf

–

mutual inductance between the shorted path and phase-A, B, C winding

L

m

+L

ls

–

self inductance of phase-A winding

k

–

mutual inductance coefficientSlide19

Detection and Protection of Stator Turn Faults Comparison of current waveforms from simulation model and real CPGSlide20

Detection and Protection of Stator Turn Faults Fault signature

Repetitive Pattern appears at 1/3 of the ripple frequency

Final choice of fault signature:

generator output current harmonic at 1/3 of rectifier ripple frequencySlide21

Detection and Protection of Stator Turn Faults Stator turn-fault detection – experimental resultsArtificially induced stator turn fault with fault-current limiting resistor (1-turn fault, 0.1 Ω short-circuit resistor)Significantly increased fault signature with fault (Ripple frequency – 1898 Hz)

Without short circuit

With short circuitSlide22

Detection and Protection of Stator Turn Faults Stator turn-fault detection – experimental resultsFault signatures at various operating conditions and fault signatures

with fault (0.1 Ω current-limiting resistor)without fault

with fault (0.2

Ω

current-limiting resistor)Slide23

Detection and Protection of Stator Turn Faults Post-fault protection should meet two objectives

Prevent or slow down further propagation of the stator turn faultsPreserve as much as possible the limp-home capability of the generatorFault severity evaluation – Fault current <–> fault signatureModel validationMeasured fault signatures w/ inherent asymmetry taken out

Simulated fault signatures

0.2

Ω

current-limiting resistor

0.1

Ω

current-limiting resistorSlide24

Detection and Protection of Stator Turn Faults Fault severity evaluation – mapping relationship between the short-circuit fault current and the fault signature0.066-Ω, 0.033-Ω, and 0.016-Ω current limiting resistor with 1-turn faultShort-circuit current can be looked up once a certain fault signature is observed

Fault signatures

Short-circuit currentSlide25

Detection and Protection of Stator Turn Faults Limit the overall current in the shorted turn below certain a limit to avoid further thermal degradationOptionsReduce the normal winding current – shedding loadReduce the short-circuit current – reduce the field – generator voltage drops below battery voltage – shut off the fieldStrategyShut off the field if

fault current > current limitShedding non-critical electric loads when fault current < current limit < (fault current + winding load current)Not necessary to shed electric loads when (fault current + winding load current) < current limitSlide26

Detection and Protection of Stator Turn Faults Illustration of the post-fault protectionFault signatures classified into three decision alternatives Slide27

Detection of Rotor Eccentricity Rotor eccentricityProduces unbalanced magnetic pull (UMP)Increase bearing wearMay eventually result in bearing failure or stator-to-rotor rubMixed rotor eccentricity – fault signaturesTorque oscillation produced by mixed eccentricity will produce a low-frequency harmonic at the mechanical frequency of the generator

f

ripple –

rectifier ripple frequency

P

–

generator pole-pair number

n

diode

–

number of diodes in the rectifierSlide28

Detection of Rotor EccentricityMixed rotor eccentricity – finite-element simulationgenerator with stator turn fault simulated in Maxwell 3DSimulation resultstorque

currentSlide29

Detection of Rotor EccentricityMixed rotor eccentricity – experimental results at 3222rpmWithout eccentricity

With eccentricitySlide30

Detection of Generalized Bearing Roughness FaultWorn bearingIncrease vibration and noise, eventually lead to bearing failureDetection without using vibration sensors is challengingAccelerated bearing aging testsControl tests without injecting bearing currentSlide31

Detection of Generalized Bearing Roughness FaultBearing pictures after aging testsWear of the outer race is only on one side

NSK B17-102DG46

SKF BB1-3065DDSlide32

Detection of Generalized Bearing Roughness FaultEvaluation of noise-cancellation (NC) methods for bearing fault detectionShaft vibrations caused by generalized bearing roughness faults will modulate the airgap of the electric machine irregularly [103, 104]Adaptive notching filtering method based on Wiener filteringSmaller airgap variations in CPGs because of tight bearing tolerance, large per-unit

airgap, and large pole number

Before aging test

After aging testSlide33

Detection of Generalized Bearing Roughness FaultA worn bearing will result in slight frequency shift of the rectifier ripple harmonic in the generator output current.Fault mechanismsChange of Contact Force Direction due to Skewed Rotor ShaftUneven Contact Force Distribution due to Skewed Rotor ShaftChange of Belt TensionIncreased VibrationTemperature Rise due to Increased Bearing Friction

One or more of above mechanisms lead to slight change of effective generator pulley diameterThe change of pulley ratio is going to further shift the frequency of rectifier ripple harmonic in the generator output current. Slide34

Detection of Generalized Bearing Roughness FaultAccelerated bearing tests (performed on three CPGs)

Control tests (performed on two CPGs)Slide35

ConclusionsUsing only the existing DC voltage/current sensors in the vehicle electric power system, a variety of insulation and mechanical faults in claw-pole generators can be detectedSerpentine belt slipSerpentine belt defectStator turn-to-turn faultRotor eccentricityGeneralized bearing roughness

The proposed methods for detecting stator turn faults and rotor eccentricity may be extended to other AC electric machines with a rectified DC linkThe proposed bearing fault detector may be extended to other electric machines driven byV-beltsSlide36

Q&AThank you!Questions?