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SHAFT CURRENT

SHAFT CURRENT. BY M.M.(Thys) Botha. mathysb@absamail.co.za Tel: +27 11 849 8878 Mobile: +27 76 956 9063 Fax: +27 86 502 8023. MMB EMC. TYPES OF SHAFT CURRENT. ELECTROMAGNETIC – CAUSED BY MAGNETISATION OF MACHINE PARTS, AIR GAP ECCENTRICITIES AND SUPPLY IMBALANCES.

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SHAFT CURRENT

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  1. SHAFT CURRENT BY M.M.(Thys) Botha mathysb@absamail.co.za Tel: +27 11 849 8878 Mobile: +27 76 956 9063 Fax: +27 86 502 8023 MMB EMC

  2. TYPES OF SHAFT CURRENT • ELECTROMAGNETIC – CAUSED BY MAGNETISATION OF MACHINE PARTS, AIR GAP ECCENTRICITIES AND SUPPLY IMBALANCES. 2. ELECTROSTATIC – CAUSED BY FRICTION, AIR FLOW, BELT DRIVES, ETC. 3. VARIABLE SPEED DRIVE INDUCED CURRENT 4. THYRISTOR DRIVE INDUCED CURRENT ON DC MACHINES AND CYCLO-CONVERTERS MMB EMC

  3. ELECTROMAGNETIC SHAFT CURRENT • MAIN CAUSES • STRAY MAGNETIC FIELDS – AIR GAP, FAULTY COUPLINGS, VIBRATION • EARTH CURRENT – ELECTROLYSIS, LEAKAGE, RAILWAY LINES, WELDING • MAGNETISED MACHINE PARTS- FAULTY WINDINGS, MAGNETIC BASE TOOLS • 4. SELF-EXCITED MAGNETIC FIELDS- ROTOR WINDING EXCITATION, SUPPLY UNBALANCE MMB EMC

  4. ELECTROMAGNETIC SHAFT CURRENT PROBLEM TO MEASURE ACCURATELY – CAN BE GENERATED INSIDE A BEARING HOUSING, SO NO EXTERNAL CURRENTCAN HAVE VERY HIGH FREQUENCY – CAUSED BY ELECTROMAGNETIC COMMUNICATION CAN BE AC OR DC CAN BE SPEED DEPENDANT

  5. SHAFT CURRENT What gauss levels are acceptable?

  6. SOME CAUSES AIR GAP ECCENTRICITY SUPPLY UNBALANCE WINDING FAULTS MAGNETIC CENTER MISALIGNMENT BALANCE CURE INSULATE ONE BEARING SHAFT CURRENT DUE TO NORMAL SUPPLY

  7. Incorrect stator / rotor slot combination effect on shaft current Low freq = slip frequency F = 1/T = 106 / 620 = 1612.9 Hz = 96 774 .2 CPM = 32.25 x Run Speed. F = (S2 – ((S1-S2)xPoles))x Run Spd. S1 and S2 = slots SHAFT VOLTAGE CAUSED BY AIR GAP EXCENTRICITIES AND STATOR / ROTOR SLOT COMBINATION OF 2 X POLES.

  8. PREVENTION OF ELECTROMAGNETIC SHAFT CURRENT MEASURE CURRENT USING THE BEST PRACTICE FOR PURPOSE APPROACH DO NOT USE BEARING HOUSING AS EARTH CONNECTION FOR SHAFT BRUSH OR SEAL FOR INTERNAL CURRENT MEASUREMENT, IT MAY BE NECESSARY TO FIT SPECIAL SHAFTCOUPLER – ABB RARIC – WHAT CAN BE DONE INSIDE BEARING HOUSING – VOLTAGE MAY BE THE ONLY WAY HERE DE-GAUSS ALL MAGNETIC MACHINE PARTS USING THE CORRECT PROCEDURE CYCLE THOUGH UNTIL REMANENT MAGNETISM IS REDUCED TO LESS THAN 5 GAUSS. CORRECT AIR GAP AND COUPLING PROBLEMS INSULATE BEARING AND PROVIDE SHUNT PATH FOR STRAY EARTH CURRENT OBSERVE CORRECT WELDING PROCEDURES AROUND MACHINES

  9. ELECTROSTATIC SHAFT CURRENT NORMALLY CAUSED BY AERODYNAMICS – MATIMBA 9m FANS, CONVEYOR BELTS, V-BELTS CAN BE PREVENTED BY CONTROLLING HUMIDITY AND TO FIT SHAFT BRUSH OR SEAL BETWEEN COUPLING AND BEARING.

  10. VSD INDUCED SHAFT CURRENT Shaft currents induced by VFDs can lead to motor failures. Without some form of mitigation, shaft currents travel to ground through bearings, causing pitting, fusion craters, fluting, excessive bearing noise, eventual bearing failure, and subsequent motor failure. NORMAL APPEARANCE OF THE BALL IN A HEALTHY BEARING DULL APPEARANCE OF A BALL SUBJECTED TO SHAFT CURRENT CAUSED BY VSD AND RUNNING AT VARIABLE SPEED FLUTING AS RESULT OF RUNNING AT CONSTANT SPEED FOR RELATIVE LONG PERIODS

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

  12. COMMON MODE CURRENT = C . dV/dt Where C is the Circuit element Capacitance to earth IN ADDITION TO THIS, YOU MAY HAVE ALL THE PROBLEMS ASSOCIATED WITH A NORMAL 3 PH. SUPPLY! EARTH PATHS WILL HAVE DIFFERENT IMPEDANCES TO DIFFERENT CURRENTS FOUR TYPES OF CURRENT CAN FLOW IN THIS INVERTER FED MOTOR CURE:- INSULATE BOTH BEARINGS – FIT LOW VOLT DROP BRUSH OR SHAFT SEALS

  13. THYRISTOR DRIVES DC FOUR QUADRANT DRIVE CYCLO-CONVERTER

  14. SNUBBER CIRCUIT

  15. DEFECTIVE SNUBBER CIRCUIT EFFECT (IN EXCESS OF 400Volt PULSES WERE MEASURED ON THE SHAFT) SHAFT VOLTAGE DUE TO SNUBBER CAPACITOR DEVELOPING HIGH INTERNAL RESISTANCE OVER TIME

  16. SOME SOLUTIONS TO THE PROBLEM Improve the high-frequency grounding connection from the motor to the drive and from the motor to the driven equipment. Install one insulated bearing on the non-drive end of the motor. Install insulated bearings on both sides of the motor. Thin filmceramic coat on bearing race does not work due to capacitance action with high frequency pulses from VSD’s.

  17. SOME SOLUTIONS TO THE PROBLEM Install a shaft grounding brush/seal across one motor bearing. Install a Faraday shielded motor. Install an insulated coupling between the motor and the driven equipment. Use special shaft brush or micro-fibre seal to shunt current

  18. MORE SOLUTIONS USE CERAMIC COATED JOURNALS AND CERAMIC BALL- OR ROLLER BEARINGS. (But ensure that coating is thick enough to make a bad capacitor)

  19. THANK YOU Thys Botha

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