Electric Motor Diagnostics Defect Frequencies and Data Colle

1

Electric Motor Diagnostics

Defect Frequencies

and

Data Collection Methods

DATA COLLECTION

*ObjectivesåIdentify different defect frequencies related to

motors.

åDescribe why different setups are required forelectrical defect detection.

åState how certain defects appear in both thevibration and current domains

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DATA COLLECTION

*Calculating Defect Frequencies* Number of Poles:

åWhere: 7200 /Actual speed (nameplate rpm) = Poles

åFor Hertz divide by 60).

Note: This calculation results in a fractional number. Round to thenearest whole number.

DATA COLLECTION

* Slip Frequency Sidebands:åThe difference in speed (frequency) between

synchronous speed and the motor's actualrunning speed

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Induction Motor Faults

* Static Eccentricity: Exists when the airgap between the stator and the rotor iseccentric (out of round). This defect is notcaused by a rotating component in themotor. Therefore, it is considered static orstationary in nature. The soft foot is themost common among many causes.


* Static eccentricity appears in the vibrationdomain at 2xFL and will not vary withrespect to speed. It can also show up at FSP

with sidebands of 2xFL.åSlot pass frequency (FSP) = number of stator

slots times the rotational frequency of the rotor

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* Dynamic Eccentricity: Defined as anuneven rotating air gap. In the vibrationdomain the defect appears at 2*Fl and at FSP

with sidebands spaced at 1*TS

* In the current domain dynamic eccentricityappears at Fsp with 1*TS sidebands.

*


* In the vibration domain, this defect appearsat 2xFL and FSP with sidebands of 1xRPM.

* Electrically, eccentricity appears in thecurrent domain at FSP with 1xRPMharmonics

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* Note: The vibration domain harmonics ofFSP are even (0, 2, 4, etc.).

* The current domain harmonics of FSP areodd numbers not divisible by 3 (1, 5, 7 etc.).


* Rotor Bar Pass: Defined as the frequencyat which the number of motor rotor bars anddefects directly related to them appear in thespectrum; (e.g., Open or Broken RotorBar(s), cracked end rings appear (No. ofRotor Bars * 1xRPM), etc.).

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* In the vibration domain, depending on theseverity of the defect, the 1xRPM peakexhibits sidebands at a frequency equal to(SL * P). You may possibly see the FRB, orharmonics, exhibit sidebands of 1xRPM.


* Electrically, when acquiring current data,this fault shows up as; (SL * P) sidebandsaround line frequency (60 Hz USA and 50Hz Europe).åDepending on the defect severity, you may induce

eccentricity. The eccentricity (dynamic) is the resultspot heating due to the excess current in the rotor barsadjacent to the broken or open bar.

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Transducers

*FunctionåConvert mechanical signal to

an analog electrical signal.

*Three basic typesåDisplacement transduceråVelocity transduceråAccelerometer

Displacement probes act much like a transformer

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Displacement Xducers

* Advantages +åMeasures relative movementågood for sleeve brg. machinesåvery good for heavy machines

* Disadvantages -åpermanently installedåfrequency response : DC to 1KHzåpower required

Velocity XducerConnection

CaseConductor Coil

Spring

Magnet

Damper

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Velocity Xducer* Advantages +

åbest signal to severityratio

ågood signal to noiseratio

åno power requiredåsingle dif. / int.

neededåusually pretty hardy

* Disadvantages -åusually bigåheavyåfreq. 10 to 2Kzåtemperature sensitiveåcomparatively expensiveåmagnetic field sensitiveåorientation sensitiveåwear and temp. changes

calibration

Accelerometers

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Accelerometers* Advantages

åbroad frequency rangeåsmall, light, ruggedåICP needs no signal conditioningåeasy to mount

* Disadvantagesåpoor as a hand heldålimited signal to noise ratioåreads accelerationåpower requiredådouble integration needed

Accelerometers

*Stud Mount : up to about 16KHzMounting Response

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Accelerometers

*Mounting ResponseQuick Lock Mount : up to about 10KHz

Accelerometers*Mounting Response

Rare Earth Magnet : linear to @ 7KHz

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Accelerometers

*Mounting ResponseSuper Mag. : linear to @ 3KHz

Accelerometers

*Mounting ResponseHand Held w/2” stinger :linear to @ 800 Hz

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Accelerometers*Mounting Response

Hand Held w/ 8.5 “ stinger : linear to @ 500 Hz.

Other Sensors

*Current TransformersåMust have a voltage output

*Flux coilsåPermanent mount or portable

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Current Transformers

From top to bottom is the 341D, 341B, and 341C.


* Technical Specifications (341b):ë 1.0 to 600 A AC (BNC(M) output), 5 mV/Aë Accuracy: (48 to 1000Hz, load .lt. 40 m ohmsë 1 to 25A: +/- (1% reading +0.3A)ë 25 to 100A: +/- (1% reading + 0.2A)ë 100 to 600A: +/- (1% reading + 0.1A)ë Div. Transformer: 1000:1ë Output Voltage Sensitivity: 5m V/Aë Working Voltage: 660V max.ë Max. Conductor Size: 1.18" (30 mm)

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* 341c: Typically used on 1Kv to 25Kv motorstarters on the installed CT’s secondary

* Technical Specifications:ë .50 mA to 150 A AC (BNC(M) output), 5 mV/Aë Current Range: 50 mA to 150 A ACë Accuracy: +/- (1% reading +0.02 A) with non-

inductive load .ge. 7 ohmsë Div. Transformer: 1000:1ë Output Voltage Sensitivity: 5mV/Aë Working Voltage: 600V maxë Max Conductor size: 0.47" (12mm)

Flux Coil

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Flux Coil

* Benefits:åReduces safety concerns by not having to

address live power leads.åOn-line non-intrusive motor diagnostic tool.åEasy to mount on the opposite drive end of

motor.åEliminates need for current clamp in most

cases.åDetection of electrical faults in ac induction

motors.

Flux Coil

* Technical Description:åThe CSI Model 343 flux coil is designed for use with

CSI machinery analyzers to detect flux generated byelectric motors. Except for the initial calibration andpossible verification, the use of the 343 flux coileliminates the need for current clamp measurements.The flux coil captures flux signals which provide anelectrical "quality" signature.

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Flux Coil

* Technical Description cont.:åThis electrical signature is sensitive to conditions which

alter the electrical characteristics of the motor, such asbroken rotor bars, eccentricity, voltage imbalancebetween phases, and stator faults.

åFlux readings are acquired by consistent placement ofthe flux coil on the axial outboard end of the motor andautomatically stored in the analyzer. Spectra of thesemeasurements may be permanently stored, trended, oranalyzed for alarms in CSI MotorView II software.

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Electric Motor Diagnostics

A.C. INDUCTION MOTORS

A.C. MOTOR THEORY

*Objectives:åDescribe the general configuration of an AC

Motor.

åState the process that allows AC inductionmotors to function.

åIdentify electrical defects in spectral andwaveform data.

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A.C. MOTOR THEORY

* First where does the power come from?å3 Phase power is produced at a power plant,

transmitted via the power grid to your plant.

å3 Phase power generators use a rotatingmagnetic field inside a 3 phase winding toproduce three sinewaves 120o degrees apart.

Rotating Magnetic Field

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4



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*Speed of the rotating field

RPM = 120 * Frequency Poles

120 * 60 = 3600 2

3 PHASE STATOR

*Stator Core:åStationary part of the induction motor

åCylindrical laminated slotted core

åWindings that are placed in the core

åFrame holds the core

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3 PHASE STATOR

* Magnetic material used for the core is by naturealso a conductor.

* Voltage induced in the core steel causes a currentto flow in the core. This eddy current flow in thecore raises stator temperature and lowers motorefficiency. A laminated core offers highresistance to current flow; hence, the eddy currentsand resulting losses are reduced.

3 PHASE STATOR

*Example of single stator lamination

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3 PHASE STATOR

*WindingsåThe stator windings are comprised of turns of

insulated copper wire that form a coil. Thesecoils are placed in the slots of the core. Thecoils are separated from each other and the coreby insulation.

åThe windings in the stator provide the ability todevelop the required magnetic field for motoraction to occur.

3 PHASE STATOR

*Example of motor winding

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Types of 3 Phase Motors

* Induction - Barred rotor, cast orfabricated

*Synchronous - Wound rotor, energizedwith DC, locks on to rotating field

*Wound Rotor - Variable speed motor

INDUCTION MOTORS

*Rotorï The rotor is the rotational element of the electric

motor. A squirrel cage rotor design is commonlyused in most induction motors. This design uses alaminated slotted core in which the conductivematerial for the rotor bars is placed in the slots. Therotor bars are then shorted together by the end rings.

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INDUCTION MOTORS

* Squirrel cage induction motors have very few maintenancerequirements, and they have a rugged and dependablereputation.

* You can double-wind the squirrel cage with a high and lowresistance winding. This configuration combines both highstarting torque and excellent constant speed control

INDUCTION MOTORS

*Rotor

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INDUCTION MOTORS

RotorBarShapes

INDUCTION MOTORS

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INDUCTION MOTORS

*SLIPåThe difference between synchronous

speed and the speed of the rotor.

åThe means by which an induction motordevelops torque.

INDUCTION MOTORS

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INDUCTION MOTORS

*Slip = Synchronous speed - Actual speed

*Sidebands appear at #Poles * Slip*Vibration Domain SDBDS of Turning speed

*Current Domain SDBDS of Line Frequency

INDUCTION MOTORS

*Example: 2 pole motor, turns at 3585 RPM

å Synchronous speed = 120 * 60 (Lf) / 2 = 3600 rpm

å Slip = 3600 - 3585 = 15 cpm / 60 = .25 Hz

å Poles * slip = 2 * .25 Hz = .5 Hz

åBin Width in spectrum Must be < .5 Hz to seesidebands

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INDUCTION MOTORS

*Rotor Defect detectionåFmax 3.5 * Turning SpeedåLOR 1600 LinesåAverages 4åAvg. Mode NormalåWindow HanningåSensor Accelerometer

* Fmax high enough to see 3 multiples of turning speed* Bin Width of ~ .12 Hz for 2 pole (sufficient to see poles * slip SDBDS)

INDUCTION MOTORS

*Stator Fault detectionåFmax 130 * Turning SpeedåLOR 3200 LinesåAverages 4åAvg. Mode NormalåWindow HanningåSensor Accelerometer

* Fmax high enough to cover # Rotor Bars and # Stator Slots* Bin Width of ~ 2.5 Hz (sufficient to see turning speed SDBDS and 2 *

LF SDBDS

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INDUCTION MOTORS

*Routine Current Data CollectionåLargest benefit realized by comparison to

baseline data, indicates rate of changeåManual manipulation of the data can be

done with any vibration analyzeråAutomated analysis with an expert system

such as MotorView

INDUCTION MOTORS

*Data Collection SetupåFmax 80 HzåLOR 3200 LinesåAverages 4åAvg. Mode NormalåWindow HanningåSensor Current Transformer

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INDUCTION MOTORS

*Data Collection Continued:åThe output of a current transformer is

current and must be converted to a voltageto be used by a Machinery Analyzer

åMedium Voltage (1KV - 25KV) must betaken from installed CT’s for safety

åSince readings are taken in db (decibels)actual values of current are not critical, butcare should be taken to properly scalereadings

INDUCTION MOTORS

*Typical Current SpectrumLCRA - BOILER CIRC WATER PUMPS 1CBCWP 1C -RB1 CURRENT (ROTOR BAR ANA.) PHASE A

Spectrum Display 10-OCT-00 07:12

AMPS = 92.17 LOAD = 74%

RPM = 1772. RPS = 29.54

50 52 54 56 58 60 62 64 66 68 70

-20

0

20

40

60

80

Frequency in Hz

dB A

mps

Freq: Ordr: Spec: Dfrq:

58.16 1.969 5.880 1.841

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MotorView

An EffectiveMotor Monitoring Program

n Route Based

n Similar to Vibration

n Routine Monitoring

n 3 Phase Motors only– Induction Motors

– Variable Speed Drive

– Synchronous Motors (in development)

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Critical Motors

u Process is altered or stopped due tounexpected down-time

u Important to:– predict faults

– proactively avoid shortening motor life

Causes of Electric FailuresIn Electric Motorsn Electrical insulation is the single most

important part influencing the reliability ofelectric motors

n Three insulation systems– Turn insulation

– Phase insulation

– Groundwall insulation

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Causes of Electric FailuresIn Electric Motors

0

10

20

30

40

50

60

Turn toTurn

Phase Ground

Percent ofFailures

Results of G.E. Study

Causes of Electric FailuresIn Electric Motorsn Thumb Rule

– For every 10 deg C above the rated temperatureof the motor the life of the insulation is cut inhalf

– Temperature Rating is a letter found onnameplate, eg: A, B, F, H or N

n Chemically a motor should lastapproximately 100,000 hours

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Causes of Electric FailuresIn Electric Motorsn Other Causes of Insulation Failure

– Line Disturbances

– Mechanical Faults

– Movement in the Windings

What Will MotorView Do?

n Monitors Condition of Rotor– Broken rotor bars

– Porosity in aluminum cages

n Detects Changes in Stator Condition– Voltage imbalances

– Rotating Air Gap

– Uneven Air Gap

– Magnetic Flux Asymmetry

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Rotor Condition

n Standard 2120

n Current Clamp

n Stroboscope or Tachometer

n Affected by Load– Must be Greater than 50%

– Load determined by Speed

Stator Condition

n Standard 2120

n Flux Coil

n Stroboscope or Tachometer

n Affected by Load– Must be Greater than 50%

– Load determined by Speed

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Data Collection

n Safety Issues– Working in the vicinity of hazardous voltages

– Rotating equipment

Current Data Collection

n Five minute test

n Collects data on all three phases

n One phase used for MCSA

n Others used for balance

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Current Data Collection

n Medium Voltage Motors– Readings taken from installed CT’s secondary

n Low Voltage Motors– Readings taken directly from motor ‘T’ leads

Flux Data Collection

n Taken on the Motor

n Repeatability Critical– Install permanent Flux Coils

n Five minute test

n Measures the magnetic flux of the Motor

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Case History

n 4160 Volt 200 Hp Compressor

n Rotor bar failure

Current Spectrum (baseline)LEHI - E-5B BLEND SILO COMP - 590

E-5B COMP -RB1 CURRENT (ROTOR BAR ANA.) PHASE A

Spectrum Display 03-JUN-98 13:02

AMPS = 28.28 LOAD = 72%

RPM = 593. RPS = 9.89

50 52 54 56 58 60 62 64 66 68 70

0

10

20

30

40

50

60

70

Frequency in Hz

dB

Am

ps


58.67 5.933 8.561 1.329

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Current Spectrum (baseline)

n RESULTS OF ROTOR BAR ANALYSn DATE TIME RPS %LOAD Amps LF - NPxSF SLIP CONFD

n ---- ---- ---- ----- ------ ---------- ---- -----

n 03-JUN-98 13:02 9.89 72% 28.3 58.67 Hz .111 48%

n

n LF - NPxSF LINE FREQ CALC. ADJUSTED ESTIMATED

n AMPLITUDE AMPLITUDE DELTA DELTA BROKEN BARS

n ---------- --------- ----- -------- -----------

n 8.56 60.00 51.44 49.80 1.0

n

n

Current Spectrum (baseline)

n *** ADVISORY RECOMMENDATION ***

n Rotor shows some indication of high resistance joints?

n Watch out for progressive degradation on future analyses.

n

n Note: For aluminum cast rotors, voids in the rotor castings are

n equivalent to high resistance joints in fabricated rotors.

n

n Note: CONFIDENCE FACTOR ( 48%) of the located NPxSFsideband is marginal!

n Current Analysis indicates 1 broken rotor bar

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Flux Trend

LEHI - E-5B BLEND SILO COMP - 590E-5B COMP -FC1 FLUX COIL - AXIAL - CENTER

Trend Display of Rotor Bar Cond

-- Baseline -- Value: 56.92 Date: 03-JUN-98

0 20 40 60 80 100 120 140

-10

0

10

20

30

40

50

60

70

Days: 03-JUN-98 To 12-OCT-98

dB

Flu

x-A

U

WARNING

ALERT

FAULT

LF-NPxSF Ampl

Flux after RepairLEHI - E-5B BLEND SILO COMP - 590

E-5B COMP -FC1 FLUX COIL - AXIAL - CENTER

Trend Display of Rotor Bar Cond

-- Baseline -- Value: 56.92 Date: 03-JUN-98

0 30 60 90 120 150 180 210 240

-10

0

10

20

30

40

50

60

70

Days: 03-JUN-98 To 06-JAN-99

dB

Flu

x-A

U

WARNING

ALERT

FAULT

LF-NPxSF Ampl

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Case History Rotor Bar Fault6000 GPM Pump

n Condition of Rotor trended using Flux only

n No current spectrum available

Flux SpectrumBLFN - 6K PUMP COOL H2O (SOUTH) {4

PMP SOUTH -FC1 FLUX COIL - AXIAL - CENTER

Label: Multiple Broken Rotor BarsPriority: 2

Spectrum Display 09-FEB-98 10:23

AMPL = 15.01 LOAD = 93%

RPM = 1187. RPS = 19.78

50 52 54 56 58 60 62 64 66 68 70

-10

0

10

20

30

40

50

60

70

Frequency in Hz

dB

Flu

x-A

U


58.63 2.965 .07213 1.401

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Analysis

n RESULTS OF ROTOR BAR ANALYSISn Station: BLFN --> BLAST FURNANCE

n Machine: PMP SOUTH --> 6K PUMP COOL H2O (SOUTH) {4

n Meas Point: FC1 --> FLUX COIL - AXIAL - CENTER

n Data Label: Multiple Broken Rotor Bars

n

n SPECTRUM DATE TIME RPS %LOAD FluxAU LF - NPxSF SLIP CONFD

n -------- ---- ---- ---- ----- ------ ---------- ---- -----

n Reference 09-FEB-98 10:23 19.78 93% 15.0 58.63 Hz .234 99%

n

n SPECTRUM LF - NPxSF LINE FREQ CALC. ADJUSTED ESTIMATED

n (dB) AMPLITUDE AMPLITUDE DELTA DELTA BROKEN BARS

n -------- ---------- --------- ----- -------- -----------

n Reference 0.07 23.53 23.46 23.11 15.8

n

Analysis continued


n Rotor Analysis indicates presence of multiple broken bars!!!

n Maintenance actions should be performed immediately.

n

n Note: Possibility exist that the end ring is cracked.

n

n Perform an electric current evaluation to verify results

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Trend of Rotor Bar conditionLEHI - F20A COOLER - RPM 1765 (new)

F20A COOL -FC1 FLUX COIL - AXIAL - CENTER

Trend Display of Line Freq Ampl

-- Baseline -- Value: -9.886 Date: 03-JUN-98

0 30 60 90 120 150 180 210 240

-10

0

10

20

30

40

50

60

70


dB

Flu

x-A

U

ALERT

FAULT

Date: Time: Ampl:

09-NOV-98 15:05:45 -3.370

Stator Fault

n Bad connection in motor connection box

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Stator FaultTrend of Line Frequency Amplitude

LEHI - F20A COOLER - RPM 1765 (new)F20A COOL -FC1 FLUX COIL - AXIAL - CENTER

Trend Display of Line Freq Ampl

-- Baseline -- Value: -9.886 Date: 03-JUN-98

0 30 60 90 120 150 180 210 240

-10

0

10

20

30

40

50

60

70


dB

Flu

x-A

U

ALERT

FAULT

Date: Time: Ampl:

09-NOV-98 15:05:45 -3.370

Stator Fault (turn to turn short)Baseline

n RESULTS OF ROTOR BAR ANALYSISn ****************************

n Data Label: Motor in good condition

n Motor ID: WES Stator Ex Rated RPS = 29.42

n Frame Size: 445S ( 4 Pole) Rated AMPS = 58.0

n Volts/Powr: 440 - 50 Hp Rotor Bars = 54

n

n SPECTRUM DATE TIME RPS %LOAD FluxAU LF - NPxSF SLIP CONFD

n -------- ---- ---- ---- ----- ------ ---------- ---- -----

n Reference 24-MAY-94 13:18 29.36 110% 30.0 57.49 Hz .618 100%

n

n SPECTRUM LF - NPxSF LINE FREQ CALC. ADJUSTED ESTIMATED

n (dB) AMPLITUDE AMPLITUDE DELTA DELTA BROKEN BARS

n -------- ---------- --------- ----- -------- -----------

n Reference -20.93 29.55 50.49 47.01 0.8

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Stator Fault (turn to turn short)Baseline


n Rotor Analysis indicates presence of high resistance joints.

n Watch out for progressive degradation on future analyses.

n

n Note: For aluminum cast rotors, voids in the rotor castings are

n equivalent to high resistance joints in fabricated rotors.

n

n Perform an electric current evaluation to verify results

n

n

Stator Fault (turn to turn short)

n RESULTS OF FLUX SPECTRA COMPARISONS

n ***********************************n FREQUENCY ORDER PEAK IDENTIFICATION LIMIT PERCENT

n (Hz ) (xRPM) VALUE LF/RPS/PxS/SLP VALUE LIMIT

n --------- ------ ----- -------------- ----- -------

n 28.084 .96 -13.19 1/ -1/ -1/ 0 -15.49 130.4

n -> 30.630 1.04 -3.35 1/ -1/ 0/ 0 -4.02 108.1

n *> 89.344 3.04 -8.20 1/ 1/ 0/ 0 -15.17 222.9

n ** 118.701 4.04 -13.98 1/ 2/ 0/ 0 -23.01 282.8

n -> 148.057 5.04 -17.67 1/ 3/ 0/ 0 -18.20 106.3

n

n Line Freq RPS Poles X Slip Slip Freq

n ----------- ------- ------------ -----------

n 59.987 Hz 29.357 Hz 2.547 Hz .637 Hz

n

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n Total Peak Amplitude (FLUX-AU ) = 17.5576 vs 1.0004 (Ref)

n Line Freq Amplitude (FLUX-AU ) = 17.5227 vs 30.0381 (Ref)

n Total Excess Amplitude = -1.43 / -11.68 ( 5 Peaks)

n Percent Load = 110.5 vs 110.1 (Ref)

n

n ** Research has shown growth in the magnitude of this peak to indicateincreased change in the voltage balance. Research has also shown thatstator faults can

– cause this peak to change.

n

n Research has correlated stator faults with significant change in peak(s)

n at running speed sidebands about line frequency and/or line frequency.


n The amplitude of the peak(s) marked '*> ' have significantly changed.

n Increase the frequency of data collection in order

n to watch for further changes in stator condition.

n Significant change in any peak marked '-> ' indicates a change

n in stator condition.

n

n WARNING: If the flux coil measurement was not acquired at thesame

n location or if the signal is noisy, amplitude values

n could change significantly.

n If this occurs, the diagnosis could be incorrect.

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Current Test Results

n Current test takes less than 5 minutes

n Monitors rotor condition

n Calibrates Flux data

Flux Results

n Flux Coil test takes less than 5 minutes

n Trends rotor condition

n Monitors stator condition

n May indicate faults in motor circuit

Electric Motor Diagnostics Defect Frequencies and Data Colle

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Transcript of Electric Motor Diagnostics Defect Frequencies and Data Colle