Transcript SHAFT CURRENT - Rotating Machines Working Group

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
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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
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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
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SHAFT CURRENT
What gauss levels are acceptable?
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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
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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.
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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
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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.
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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
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HOW MANY VOLTS? – THIS WAS 150mV!!
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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
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THYRISTOR DRIVES
ARM.
HSCB
DC FOUR QUADRANT DRIVE
MOTOR
CYCLO-CONVERTER
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SNUBBER CIRCUIT
R
R
C
C
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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
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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.
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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
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MORE SOLUTIONS
USE CERAMIC COATED
JOURNALS AND
CERAMIC BALL- OR
ROLLER BEARINGS.
(But ensure that coating is thick
enough to make a bad capacitor)
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THANK YOU
Thys Botha
MMB EMC