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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

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www.martec.co.za

Mario Kuisis

mario@martec.co.za