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

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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

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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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3 Ph. 50HzSine WaveSupply

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 CAUSESAIR GAP ECCENTRICITYSUPPLY UNBALANCEWINDING FAULTSMAGNETIC CENTERMISALIGNMENTBALANCE

CUREINSULATE 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 EXCENTRICITIESAND 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 SHAFTCOUPLER – ABB RARIC – WHAT CAN BE DONE INSIDE BEARING HOUSING – VOLTAGE MAY BETHE 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 FITSHAFT 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, excessivebearing noise, eventual bearing failure, and subsequent motor failure.

NORMAL APPEARANCEOF THE BALL IN AHEALTHY BEARING

DULL APPEARANCE OF A BALLSUBJECTED TO SHAFT CURRENTCAUSED BY VSD AND RUNNINGAT VARIABLE SPEED

FLUTING AS RESULT OFRUNNING AT CONSTANTSPEED FOR RELATIVELONG 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

CABLE

ROTOR

STATOR

INV

ER

TE

R

COUPLING

LOAD

STATOR TO ROTOR COUPLING CURRENT

ROTOR TO SHAFT CURRENT

STATOR WINDING TO FRAME SHAFT CURRENTSTATOR WINDING TO GROUND CURRENT

EARTH PATHS WILL HAVE DIFFERENT IMPEDANCES TO DIFFERENT CURRENTS

COMMON MODE CURRENT= C . dV/dtWhere C is theCircuit elementCapacitance to earth

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

IN ADDITION TO THIS, YOU MAY HAVE ALLTHE PROBLEMS 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

400

Vo

lt

SHAFT VOLTAGE DUE TO SNUBBERCAPACITOR DEVELOPING HIGHINTERNAL RESISTANCE OVER TIME

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SOME SOLUTIONS TO THE PROBLEM

Improve the high-frequency grounding connection from themotor 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 filmceramic coat on bearing race does not work due to capacitanceaction 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 COATEDJOURNALS ANDCERAMIC BALL- ORROLLER BEARINGS.

(But ensure that coating is thickenough to make a bad capacitor)

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THANK YOU

Thys Botha