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SHAFT CURRENT BY M.M.(Thys) Botha [email protected] Tel: +27 11 849 8878 Mobile: +27 76 956 9063 Fax: +27 86 502 8023 MMB EMC TYPES OF SHAFT CURRENT 1. 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 ELECTROMAGNETIC SHAFT CURRENT MAIN CAUSES 1. STRAY MAGNETIC FIELDS – AIR GAP, FAULTY COUPLINGS, VIBRATION 2. EARTH CURRENT – ELECTROLYSIS, LEAKAGE, RAILWAY LINES, WELDING 3. MAGNETISED MACHINE PARTS- FAULTY WINDINGS, MAGNETIC BASE TOOLS 4. SELF-EXCITED MAGNETIC FIELDS- ROTOR WINDING EXCITATION, SUPPLY UNBALANCE MMB EMC ELECTROMAGNETIC SHAFT CURRENT PROBLEM TO MEASURE ACCURATELY – CAN BE GENERATED INSIDE A BEARING HOUSING, SO NO EXTERNAL CURRENT CAN HAVE VERY HIGH FREQUENCY – CAUSED BY ELECTROMAGNETIC COMMUNICATION CAN BE AC OR DC CAN BE SPEED DEPENDANT MMB EMC SHAFT CURRENT What gauss levels are acceptable? MMB EMC MMB EMC TIME-VARYING FLUX ENCIRCLING THE SHAFT SHAFT CURRENT STATOR SOME CAUSES AIR GAP ECCENTRICITY SUPPLY UNBALANCE WINDING FAULTS MAGNETIC CENTER MISALIGNMENT BALANCE CONDUCTIVE COUPLING ROTOR 3 Ph. 50Hz Sine Wave Supply STATOR TIME-VARYING FLUX ENCIRCLING THE SHAFT LOAD CURE INSULATE ONE BEARING SHAFT CURRENT DUE TO NORMAL SUPPLY MMB EMC Incorrect stator / rotor slot combination effect on shaft current Low freq = slip frequency 3.1 x 200 = 620 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. MMB EMC 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 SHAFT COUPLER – 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 MMB EMC 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. MMB EMC 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 MMB EMC HOW MANY VOLTS? – THIS WAS 150mV!! MMB MMB EMC 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! STATOR LOAD INVERTER ROTOR COUPLING CABLE EARTH PATHS WILL HAVE DIFFERENT IMPEDANCES TO DIFFERENT CURRENTS STATOR TO ROTOR COUPLING CURRENT ROTOR TO SHAFT CURRENT STATOR WINDING TO FRAME SHAFT CURRENT STATOR WINDING TO GROUND CURRENT FOUR TYPES OF CURRENT CAN FLOW IN THIS INVERTER FED MOTOR CURE:- INSULATE BOTH BEARINGS – FIT LOW VOLT DROP BRUSH OR SHAFT SEALS MMB EMC THYRISTOR DRIVES ARM. HSCB DC FOUR QUADRANT DRIVE MOTOR CYCLO-CONVERTER MMB EMC SNUBBER CIRCUIT R R C C MMB EMC DEFECTIVE SNUBBER CIRCUIT EFFECT (IN EXCESS OF 400Volt PULSES WERE MEASURED ON THE SHAFT) Sin wave RMS value = 6600 Volt. Healthy Snubber 400 Volt Real Time Wave Spectrum of voltage SHAFT VOLTAGE DUE TO SNUBBER CAPACITOR DEVELOPING HIGH INTERNAL RESISTANCE OVER TIME MMB EMC 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 film ceramic coat on bearing race does not work due to capacitance action with high frequency pulses from VSD’s. MMB EMC 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 MMB EMC MORE SOLUTIONS USE CERAMIC COATED JOURNALS AND CERAMIC BALL- OR ROLLER BEARINGS. (But ensure that coating is thick enough to make a bad capacitor) MMB EMC THANK YOU Thys Botha MMB EMC