STATOR MAGNETIC CORE

DESIGN, DETERIORATION

AND FAILURE

CLYDE MAUGHAN, PRESIDENT

MAUGHAN GENERATOR CONSULTANTS

SCHENECTADY, NEW YORK

Phone: 518 377 5351

Email: cmaughan@nycap,rr,com Web: clydemaughan.com

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CORE HISTORICAL

PERFORMANCE

COMMON MAINTENANCE

– MINOR MECHANICAL DAMAGE

– MINOR LOCAL OR GENERAL LOOSENESS

OCCASIONAL MAJOR FAILURES

– LARGE MAGNETIC OBJECT IN THE AIRGAP

– FRACTURE OF A STATOR BAR

– GROSS CLAMPING FORCE LOOSENESS

– OVER-FLUXING

RARE CATASTROPHIC SPONTANEOUS

(WEAR-OUT) MELTDOWN

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STATOR CORE DESIGN &

CONSTRUCTION

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STATOR CORE DESIGN &

CONSTRUCTION

THE CORE PROVIDES THREE BASIC

FUNCTIONS

– 1) PATH FOR THE MAGNETIC FLUX

– 2) SUPPORT FOR THE STATOR BARS

– 3) SHIELDS THE STATOR BAR

CONDUCTORS FROM AIR-GAP

FLUX

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3) MAGNETIC SHIELD FROM

THE AIR-GAP FLUX

A VITAL FUNCTION NOT OFTEN DISCUSSED

TYPICAL HIGH-SPEED GENERATOR

– TOP OF THE CONDUCTOR (STRANDS) IS AT A DEPTH

WHERE VIRTUALLY ALL OF THE AIR-GAP FLUX HAS

ALREADY TRANSFERRED INTO THE LOW-RELUCTANCE

MAGNETIC TOOTH

– THUS THERE IS LITTLE IF ANY SIDE THRUST ON ANY

PORTION OF THE CONDUCTORS

– WITHOUT THIS MAGNETIC SHIELDING OF THE BAR

CONDUCTORS, THERE WOULD BE A VERY HIGH SIDE-

THRUST ON THESE CONDUCTORS

» DOUBTFUL THAT ANY ELECTRICAL INSULATION SYSTEM

KNOWN COULD RESTRAIN THESE FORCES WITHOUT

FAILING

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FLUX PATTERN – HIGH-SPEED GENERATORS

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CORE AND FRAME DESIGN

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CORE AND FRAME DESIGN

INHERENTLY COMPLEX STRUCTURE

ADDITIONAL 2-POLE GENERATOR

COMPLICATION

– THE HUGE MAGNETIC PULL OF THE ROTOR AIRGAP

FLUX ON THE CORE CAUSES THE CORE TO DEFORM

INTO AN OVAL

» THIS DEFORMATION IS SMALL, ABOUT 2 MILS ON A WELL-

DESIGNED UNIT

– BUT MUST PROVIDE FOR ISOLATION OF THE FRAME

FROM THIS CORE MAGNETIC DEFORMATION

» NUMEROUS APPROACHES TAKEN, MOSTLY SUCCESSFUL

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EXAMPLES OF GOOD

ISOLATION

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UNSUCCESSFUL ISOLATION

CAUSE: COST REDUCTION

– LOW-PROPERTY STEEL

» CRACKS

– INEFFECTIVE ISOLATION

» MAYBE VIBRATION TRANSMITTAL

» MAYBE FATIGUE CRACKS

– NO ISOLATION

» SEVERE FRAME VIBRATIONS

» HIGH NOISE LEVELS, 116DB

» EXPOSURE TO FRAME AND FOUNDATION CRACKING

» NO FIX, REPLACE STATOR, OR GENERATOR

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LOW PROPERTY STEEL

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DETERIORATION AND

FAILURES

PHOTOS OF MANY EXAMPLES

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COMMON MINOR

PROBLEMS

USUALLY REPAIRABLE WITH

LOW COST WITHOUT OUTAGE

EXTENSION

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MAJOR PROBLEMS

LESS COMMON AND OFTEN

EXPENSIVE IN REPAIR COSTS

AND OUTAGE TIME

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MASSIVE PROBLEMS

INFREQUENT, BUT REQUIRES

FULL RESTACK OF CORE –

LONG EXPENSIVE REPAIRS

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AN UNRESOLVED CONCERN

(UNRESOLVED IN MY MIND, AT

LEAST)

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INSPECTION OF CORE OUTSIDE

DIAMETER

ON MOST GENERATORS THESE BURN LOCATIONS CAN BE

SEEN DURING A CAREFUL INSPECTION OF THE CORE OD

AT THE ENDS OF THE CORE

THESE LOCAL HOT SPOTS PROBABLY NOT A CONCERN, IN

AND OF THEMSELVES

– THE LAMINATIONS ARE ALREADY SHORTED TOGETHER BY

CONTACT WITH THE KEYBARS

– SO UNLESS THE BURN (WHICH IS DRIVEN BY KEYBAR

VOLTAGE, NOT AIRGAP FLUX) BEGINS TO GROW IN DEPTH, IT

SHOULD DO NO HARM

BUT THE BURNS LOOK SUSPICIOUSLY LIKE THE

CORES THAT FAILED FROM OVER-FLUX . . .

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STILL, FOR ALL MY WORRYING . . .

THERE ARE MANY HUNDREDS OF GENERATORS OF THIS

GENERAL DESIGN IN SERVICE

– APPARENTLY NONE HAS FAILED DUE TO THIS CURRENT

FLOW

– THUS IT APPEARS LIKELY THAT THESE DESIGNS ARE SAFE

SO THERE IS A HIGH PROBABILITY THAT I HAVE BEEN

WORRYING ABOUT HIGH BACK-IRON FLUX DENSITIES

OVER NOTHING

BUT JUST A MINUTE . . .

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RECENT GENERATOR FAILURE

ONE OF 9 LARGE, NEW GENERATORS IN THE USA

– 850-1100 MVA

– JAPANESE OEM

CORE FAILURE

– FAILED WITH CORE DAMAGE APPEARANCE SIMILAR TO THE

CORE FAILURES JUST DESCRIBED

– UNIT HAD NOT EXPERIENCED OVER-FLUX

– THUS FAR THE OEM HAS NOT MADE AVAILABLE TO THE

INDUSTRY THE ROOT CAUSE OF FAILURE

BUT OTHER UNITS OF DUPLICATE DESIGN HAVE HOT

SPOTS ON BACK OF CORE

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DUPLICATE GENERATOR BACK-

OF-CORE BURNING

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THE FAILED CORE

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TEST & INSPECTION DILEMMA

TEST

– DAMAGE AT THE BOTTOM-OF-SLOT LOCATION

» DETECTABLE BY LOW FLUX TEST

» MAY BE DETECTABLE BY HIGH FLUX (IF SIGNIFICANT

AND TEST IS WELL PERFORMED)

– DAMAGE AT CORE OD

» NEITHER TEST WILL DETECT

INSPECTION

– BOTTOM OF SLOT

» MAYBE BY BORESCOPE

– OD

» POSSIBLE ON SOME DESIGNS, WITH DIFFICULTY

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ROOT CAUSE

OPINION OF THE EXPERTS (IF I UNDERSTAND CORRECTLY FROM

THE LIMITED INFORMATION PUBLICLY AVAILABLE)

– THE CURRENT IS FLOWING LOCALLY BETWEEN PAIRS OF KEYBARS, A

PARTIAL AMORTISSEUR WINDING

– (BUT THIS DOES NOT EXPLAIN BURNING AT THE BOTTOM OF SLOTS)

MY OPINION:

– THE CURRENT THAT HAS TRANSFERRED TO THE LAMINATIONS WILL

TRAVERSE THE 180O PATH PASSING FROM LAMINATION TO

LAMINATION THROUGH THE LAMINATION INSULATION

» WITH THE KEYBAR SYSTEM ACTING AS THE AMORTISSEUR

WINDING OF AN INDUCTION MOTOR

– THUS FOCUSING BURN DAMAGE AT THE BOTTOM OF THE SLOTS, THE

LOCATION OF ADJACENT LAMINATIONS

– ALSO THE LOCATION OF THE OVER-FLUXED CORE FAILURES

WHOSE OPINION ARE YOU GOING TO BELIEVE???

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SUMMARY

THE FOCUS, THEN, OF THIS PAPER (AND MY 5-YEAR CAMPAIGN)

– ENCOURAGING DISCUSSION WHICH MAY ASSIST IN BETTER

INDUSTRY UNDERSTANDING OF CORE DETERIORATION

» AUGUST 2015 EPRI MINNEAPOLIS MEETING

– AND THEREBY HOPEFULLY LEAD TO REDUCED EXPOSURE TO

THE FORTUNATELY UNCOMMON BUT UNFORTUNATELY

POTENTIALLY ENORMOUSLY EXPENSIVE EXPOSURE TO CORE

FAILURE

CERTAINLY THERE ARE ERRORS (PROBABLY BIG ERRORS) AND

MISCONCEPTIONS IN WHAT I HAVE WRITTEN AND AM

SAYING

– HOPEFULLY, OTHERS WITH MORE INFORMATION AND

DIFFERENT KNOWLEDGE BASES (AND SMARTER THAN ME) WILL

OFFER CLARIFICATION AND CORRECTION

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CONCLUSION

HOPEFULLY WITH TIME, AND BROADER DIALOGUE

AMONG CORE EXPERTS, CORES WILL BECOME BETTER

UNDERSTOOD IN THE INDUSTRY

WITH THIS IMPROVED UNDERSTANDING SHOULD COME

INCREASED GENERATOR RELIABILITY AND LOWER COST

OF POWER PLANT OPERATION

QUESTIONS ON THIS TOPIC, OR ON SERVICE PROBLEMS

– EMAIL ME OR PHONE ME (MY MIND STILL SEEMS TO WORK, WELL

I THINK IT DOES)

OR TALK WITH ONE OF THE FOLKS WHO KNOWS MORE

THAN I ABOUT GENERATORS

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IGTC . . .

WEB SITE: IGTC (International Generator Technical Community)

www.generatortechnicalforum.org INITIATED BY NATIONAL ELECTRIC COIL 5 YEARS AGO

IF YOU AREN’T ON THIS REMARKABLE SITE, YOU SHOULD BE, WITH 2800+ OTHERS

BONUS (DOWNLOAD, NO CHARGE) ON THIS SITE AT “INNER WATER-COOLED STATOR WINDINGS”

UNDER “DOCUMENTS”

MY RECENT “LIBRARY” OF 28 TECHNICAL PAPERS AND 240-PAGE BOOK ON GENERATOR MAINTENANCE

SITE HAS BEEN BROADENED TO SUPPORT THIS GENERATOR USERS GROUP

ANNOUNCEMENTS

PRODUCT OF THE GUG MEETINGS

STATOR MAGNETIC CORE

DESIGN, DETERIORATION

AND FAILURE

CLYDE MAUGHAN, PRESIDENT

MAUGHAN GENERATOR CONSULTANTS

SCHENECTADY, NEW YORK

Phone: 518 377 5351

Email: cmaughan@nycap,rr,com Web: clydemaughan.com

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