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SHAFT CURRENTSHAFT CURRENT

BYBY

M.M.(Thys) BothaM.M.(Thys) Bothamathysb@absamail.co.za

Tel: +27 11 849 8878Mobile: +27 76 956 9063Fax: +27 86 502 8023

MMBEMC

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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 MACHINESAND 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, SUPPLYUNBALANCE

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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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3 Ph. 50Hz

Sine Wave

Supply

LOAD

CONDUCTIVE

COUPLING

ROTOR

STATOR

STATOR

TIME-VARYING FLUX ENCIRCLING THE SHAFT

TIME-VARYING FLUX ENCIRCLING THE SHAFT

SHAFT CURRENT

SHAFT CURRENT DUE TO NORMAL SUPPLY

SOME CAUSES

AIR GAP ECCENTRICITY

SUPPLY UNBALANCEWINDING FAULTSMAGNETIC CENTERMISALIGNMENTBALANCE

CURE

INSULATE ONE BEARING

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3.1 x 200 = 620

F = 1/T = 106 / 620

= 1612.9 Hz

= 96 774 .2 CPM

= 32.25 x Run Speed.

SHAFT VOLTAGE CAUSED BY AIR GAP EXCENTRICITIES

AND STATOR / ROTOR SLOT COMBINATION OF 2 X POLES.

Incorrect stator / rotor slot combination effect on shaft current

F = (S2 ((S1-S2)xPoles))x Run Spd.

Low freq = slip frequency

S1 and S2 = slots

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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 PROCEDURECYCLE 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 APPEARANCEOF 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!!

MMB

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FOUR TYPES OF CURRENT CAN FLOW IN THIS INVERTER FED MOTOR

CA LE

ROTOR

STATOR

INVERTER

COUPLIN

LOAD

STATOR TO ROTOR COUPLIN CURRENT

ROTOR TO SHAFT CURRENT

STATOR WINDIN TO FRAME SHAFT CURRENT

STATOR WINDIN TO ROUND CURRENT

EARTH PATHS WILL HAVE DIFFERENT IMPEDANCES TO DIFFERENT CURRENTS

COMMON MODE

CURRENT

= C . dV/dt

Where C is the

Circuit element

Capacitance to earth

CURE:- INSULATE BOTH BEARINGS FIT LOW VOLT DROP BRUSH OR SHAFT SEALS

IN ADDITION TO THIS, YOU MAY HAVE ALLTHE PRO LEMS ASSOCIATED WITH A NORMAL3 PH. SUPPLY!

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THYRISTOR DRIVES

HSCB

ARM.

DC FOUR QUADRANT DRIVE

MOTOR

CYCLO-CONVERTER

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R

C

R

C

SNUBBER CIRCUIT

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DEFECTIVE SNUBBER CIRCUIT EFFECT(IN EXCESS OF 400Volt PULSES WERE MEASURED ON THE SHAFT)

Sin wave RMS value = 6600 Volt.

Spectrum of voltage

Real Time Wave Healthy Snubber

400Volt

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 themotor.

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 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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THANK YOU

Thys Botha