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BROKEN ROTOR BARS. &. SHAFT CURRENTS. BY. M.M. (Thys) BOTHA. ELECTRICAL MACHINES CONSULTANT Tel.:- 076 956 9063 or 011 849 8878 email:- mathysb@absamail.co.za. MMB. MMB. WHAT ARE THE CAUSES?. MISALIGNMENT DUE TO:- 1. Bad installation, soft foot etc. 2. No dowel pins

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  1. BROKEN ROTOR BARS & SHAFT CURRENTS BY M.M. (Thys) BOTHA ELECTRICAL MACHINES CONSULTANT Tel.:- 076 956 9063 or 011 849 8878email:- mathysb@absamail.co.za. MMB

  2. MMB

  3. WHAT ARE THE CAUSES? MISALIGNMENT DUE TO:- 1. Bad installation, soft foot etc.2. No dowel pins 3. Coupling wear4. Foundation problems5. Poor design6. Structure vibrationetc. MMB

  4. AIR GAP EXCENTRICITY MMB

  5. Winding faults MMB

  6. SHAFT CURRENT What gauss levels are acceptable? MMB

  7. HOW MUCH MISALIGNMENT CAN A ROTOR TAKE? MMB

  8. HOW MANY VOLTS? – THIS WAS 150mV!! MMB

  9. THIS IS WHAT YOU SEE ON SLEEVE BEARINGS – IF YOU ARE VERY LUCKY !!! MMB

  10. HOW MANY BARS CAN BE OFF ? MMB

  11. BEFORE THE SMOKE SEAL BREAKS !!! MMB

  12. WHAT DO WE MEASURE ?HOW DO WE MEASURE IT ? WHERE DO WE MEASURE ? WHAT THEORY DO WE APPLY ? WHO TOLD YOU SO ? WHEN DO WE MEASURE WHAT AND WHERE ? WHAT IS ACCEPTABLE ? MMB

  13. SO YOU LOOK AT MAINS FREQUENCY AND SIDE BANDS ! HERE COMES RED HERRING No. 1 DID YOU CHECK THE“CLEAN” SUPPLY? MMB

  14. OH! YOU BELIEF IN CHECKING VIBRATION ? RED HERRING No.2 BEST EQUIPMENT ON THE MARKET, BUT WITH INCORRECT SETTING ! MMB

  15. LESSON 2. CURRENT DOES NOT HAVE TO FLOW IN THE BARS AT ALL TIMES!!SOMETIMES THE CORE AND SHAFT WORKS BETTER MMB

  16. 74 ROTOR BARS, 84 STATOR SLOTS, 4 POLE. NDE AXIAL PEAK AT 78 X RUN SPEEDOR IS IT 39.03 X LINE FREQ.? = 117095 CPM OR 1951.5833 Hz WITH SIDE BANDS AT 115595 AND 111095 BELOW AND 123095 ABOVE. SO FIRST PEAK BELOW MAX. IS RUN SPEED AWAY AND SECOND PEAK BELOW AND FIRST PEAK ABOVE MAX. IS 2 X LINE FREQUENCY AWAY FROM PEAK VALUE. MMB

  17. NDE HOR. NOW THERE ARE LOTS MORE TO SEE. AGAIN THE 78 X RUN SPEED BUT ALSO NOW THE 74 AND 84 X RUN SPEEDS. TWO PEAKS AT 78 X AND AT 85 TIMES RUN SPEED. NDE HOR. CAUSE OF PROBLEM ? RATTLING CAGE !! MMB

  18. WELCOME TO REAL WORLD !! YOU WILL NEVER HAVE JUST ONE PROBLEM ON THE MACHINE !! WHATEVER YOU MEASURE, MAY BE MASKED BY SOMETHING ELSE! INDICATIONS ARE:-THAT ROTOR PROBLEMS SHOW UP AT VIBRATION FREQUENCIES OF RUN SPEED TIMES STATOR SLOTS WITH RUN SPEED SIDE BANDS AND THATSTATOR PROBLEMS SHOW UP AT VIBRATION FREQUENCIES OF RUN SPEED TIMES ROTOR SLOTS WITH MAINS OR 2 X MAINS FREQUENCY SIDE BANDS. IN MY OPINION VIBRATION ANALYSIS GIVES THE MOST ACCURATE RESULT, PROVIDING YOU TAKE CARE OF MISALIGNMENT AND MECHANICAL LOOSENESS. DO NOT FORGET DIRTY SUPPLIES AND PHASE UNBALANCE!!! MMB

  19. SHAFT CURRENT DOES NOT ALWAYS ORIGINATE IN THEELECTRICAL MACHINE. 4 VOLTS IS ENOUGH TO BREAK THROUGH THE OIL FILM OF A SLEEVE BEARING – SO A BRUSH ON THE SHAFT CAN HELP TO SUPPLY A CURRENT PATH OUTSIDE OF THE BEARING. 100 mV IS ENOUGH TO DAMAGE AN ANTI-FRICTION BEARING SO YOU WILL NEED A GOLD BRUSH !! BEST PRACTICE IS TO DEMAGNETISE COMPONENTS AND TO INSULATE BEARING SHELLS OR RACES. KEEP MAGNETIC BASES AWAY FROM SHAFTS AND HOUSINGS, HAVE GOOD EARTHING PRACTICES DO NOT WELD ON MACHINE FRAMES KEEP AIR GAPS AND MAGNETIC CENTERS IN PLACE CHECK FOR CRACKED OR BROKEN ROTOR BARS. MMB

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