SHAFT CURRENT

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SHAFT CURRENT

• BY
• M.M.(Thys) Botha

mathysb_at_absamail.co.za Tel [phone]
8878 Mobile [phone] Fax [phone]
MMB EMC
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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
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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
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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
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SOME CAUSES AIR GAP ECCENTRICITY SUPPLY
UNBALANCE WINDING FAULTS MAGNETIC
CENTER MISALIGNMENT BALANCE
CURE INSULATE ONE BEARING
SHAFT CURRENT DUE TO NORMAL SUPPLY
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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 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
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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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SNUBBER CIRCUIT
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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 filmceramic coat on
bearing race does not work due to
capacitance action with high frequency pulses
from VSDs.
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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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