shaft protectionaz817975.vo.msecnd.net/wm-418498-cmsimages/shaft... · origin of shaft voltage •...
TRANSCRIPT
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Shaft Protection
Origin and management of stray voltage and current
Mario Kuisis Oct 2016
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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
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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
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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
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Protect a shaft from WHAT?
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• 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
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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
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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
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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
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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
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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
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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
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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
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Shaft Protection Techniques
• Two basic approaches:
➢ Prevent current flow
➢ Manage current flow
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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
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Shaft Protection Techniques
• Manage current flow by deliberate, safe contact and current path
• Shaft earthing
• Very effective when applied correctly
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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
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Monitoring System Example
Courtesy Magnetic Products and Services, Inc.
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Plant Installation
Compressor Drive Train
Low speed gearboxLP compressor
High speed gearbox
HP compressor
Motor drive end
brushesPlacement
Of brushes
Outboard
Compressor
Bearing
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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
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Which brush?Various technologies utilised
• Carbon Block
• Silver Graphite
• SGR
• MFB
• Wire bristle brush
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Carbon brush contact surface comparison
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Silver/gold bristle brush
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Copper braid
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Copper braid
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Shaft Grounding Ring (SGR)
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Metal Fibre Brush (MFB)
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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
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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
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Thank youwww.pragmaworld.net
www.martec.co.za
Mario Kuisis