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Shaft Protection
Origin and management of stray voltage and current
Mario Kuisis Oct 2016
Agenda▌ Why shafts need protection
▌ Examples of failed protection
▌ Effects of uncontrolled stray current
▌ Origin of shaft voltages and currents
▌ Shaft protection techniques
▌ The need for monitoring
▌ Protection solutions
Protect a shaft from WHAT?
• Typical shafts are electrically isolated
• Potential difference is inevitable
• Will cause current to flow
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Protect a shaft from WHAT?
• Lubricant is an insulator
• Low electrical withstand
• Electrical breakdown occurs between shaft and earth at low voltage
• Produces arc discharge current
• High impedance source = Spark
• Low impedance source = Weld
• Affects bearings, couplings, gears, seals, etc
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Protect a shaft from WHAT?
• Arc discharge associated with very high temperatures (> 6000 °C)
• High impedance source:
• Spark produces microscopic craters
• Electrical Discharge Machining (EDM)
• Slow degradation process
Protect a shaft from WHAT?
• Low impedance source:
• Weld type discharge
• Results in gross damage
• Can be very fast degradation process
• Often catastrophic
• Evidence usually destroyed
Protect a shaft from WHAT?
• Electrical machines
• Mechanical equipment with
electrical machines in the drive
train
• All-mechanical equipment
• In general, risk increases with:
• Size
• Rotational speed
• Close tolerances between moving
parts
• Large generators and motors
• Turbines
• High speed compressors
• All sizes of VSD driven ac motors
and inverter fed dc motors
• Web processing equipment
(printers, laminators, etc.)
• Fans
• Wind turbines
Machines at Risk
Origin ofShaft Voltage
• We are concerned about current, but this results
from the potential difference (voltage) between the
shaft and other parts
• Common causes of shaft voltage
• Static
• Residual magnetism
• Asymmetric magnetic circuits
• Faults in windings
• Faults in laminated cores
• VSD’s and inverters
Static• Commonly found in
• Gas systems
• Steam systems
• Web pay-offs / wind-ups
• Fans and wind turbines
• High impedance source
• Can occur in any machine
• Causes EDM
• Can expect frosting but not fluting
Residual Magnetism
• Localised residual magnetism causes electromagnetic voltage generation
• The magnetic material itself forms the winding conductor > eddy currents
• Can occur in any machine, but
• Requires high speeds in close proximity
• Low impedance source
• Result:
Weld type discharge
Asymmetric Magnetic Flux
• Asymmetric magnetic field causes electromagnetic
voltage generation
• The magnetic material itself forms the winding
conductor
• Only occurs in electrical machines
• Low impedance source
• Result:
Weld type discharge
Faults in Windings
• Winding faults (usually inter-turn short circuits)
disturb the magnetic field
• Result is asymmetric magnetic field causing
electromagnetic voltage generation
• Only occurs in electrical machines
• Low impedance source
• Result:
Weld type discharge
Faults in Laminated Cores
• Inter-laminar faults cause circulating currents
> disturb the magnetic field
• Asymmetric magnetic field
> electromagnetic voltage generation
• Only occurs in electrical machines
• Low impedance source
• Result:
Weld type discharge
VSD’s and Inverters
• Fast rise times of solid state switching causes
voltage build-up on shaft due to capacitive coupling
• Only occurs in electrical machines
• High impedance source
• Usually continuous with high repetition rate
• Highest incidence, possibility always exists
• Result:
Frosting and fluting
Shaft Protection Techniques
• Two basic approaches:
➢ Prevent current flow
➢ Manage current flow
Shaft Protection Techniques
• Prevent current flow by enhancing the insulation between shaft and other
components
• Insulation barriers, ceramic bearings, etc.
• Cannot provide full protection!
• Problem only shifted elsewhere
Shaft Protection Techniques
• Manage current flow by deliberate, safe contact and current path
• Shaft earthing
• Very effective when applied correctly
Monitoring• The integrity of the shaft protection system is
essential to reliable plant operation
• On critical plant it is customary to monitor the shaft
protection system
• The monitor continuously measures and trends
shaft voltage and earthing current
• System alarms on:
• Overvoltage
• Over and under current
• The data is also useful for diagnostics
Monitoring System Example
Courtesy Magnetic Products and Services, Inc.
Plant Installation
Compressor Drive Train
Low speed gearboxLP compressor
High speed gearbox
HP compressor
Motor drive end
brushesPlacement
Of brushes
Outboard
Compressor
Bearing
Shaft Grounding Brushes
Requirements of shaft earthing brushes
• Ability to maintain reliable contact whilst carrying low and high currents
• Low maintenance
• Low contact resistance
• High current carrying capabilities
• Ability to function in “dirty” conditions
• Low tension on the shaft
• Non-damaging
Which brush?Various technologies utilised
• Carbon Block
• Silver Graphite
• SGR
• MFB
• Wire bristle brush
Carbon brush contact surface comparison
Silver/gold bristle brush
Copper braid
Copper braid
Shaft Grounding Ring (SGR)
Metal Fibre Brush (MFB)
Summary• Stray electrical discharge current is very damaging
• It is only a problem on certain machines
• There are two discharge forms: EDM or weld type
• Protection is by either preventing or managing the
current
• Current prevention alone carries inherent risks
• Managing the current is usually preferred
• Brush selection is important
• Critical protection systems require monitoring
Martec shaft protection solutions
Detect
Measure
Prevent
Monitor
▌ Shaft voltage probes
▌ Shaft voltage oscilloscopes
▌ Gaussometers
▌ Degaussing equipment
▌ Shaft Grounding Rings
▌ Metal Fibre Brushes
▌ Shaft grounding ropes
▌ Shaft voltage current monitors
Thank youwww.pragmaworld.net
www.martec.co.za
Mario Kuisis
