Carbon Brush Performance on Slip Rings

Carbon Brush Performance Carbon Brush Performance on Slip Ringson Slip Rings

Richard D. Hall Richard D. Hall –– Morgan AM&TMorgan AM&TRoland P. Roberge Roland P. Roberge –– Morgan AM&TMorgan AM&T

Gary Lozowski Gary Lozowski –– Morgan AM&TMorgan AM&T

Mining Electrical Maintenance and Mining Electrical Maintenance and Safety AssociationSafety Association

September 9, 2010 Clearwater, FLSeptember 9, 2010 Clearwater, FL

Machines that useMachines that use

Carbon BrushesCarbon Brushes

on Slip Ringson Slip Rings

Synchronous Motor or GeneratorSynchronous Motor or Generator

Slip RingsSlip Rings

Carbon BrushesCarbon Brushes

Synchronous MotorSynchronous Motor

•• Constant speedConstant speed•• Very efficientVery efficient•• Three phase windings in stator Three phase windings in stator

(rotating magnetic field)(rotating magnetic field)•• DC fields on rotorDC fields on rotor•• DC excitation supplied through carbon DC excitation supplied through carbon

brushes on two slip rings (collector brushes on two slip rings (collector rings)rings)

Synchronous MotorSynchronous Motor

•• Typical brush current density Typical brush current density –– 75 75 Amps/inAmps/in22 (11.6 Amps/cm(11.6 Amps/cm22) or less) or less

•• Collector rings are generally carbon Collector rings are generally carbon steel or bronzesteel or bronze

Sync Motor Sync Motor Collector RingsCollector Rings

Wound Rotor Induction MotorWound Rotor Induction Motor•• Variable speedVariable speed•• Three phase windings in stator (rotating Three phase windings in stator (rotating

magnetic field)magnetic field)•• Three phase windings in rotorThree phase windings in rotor•• Three phase (variable frequency) current Three phase (variable frequency) current

induced in rotorinduced in rotor•• Rotor current brought out to variable Rotor current brought out to variable

resistance/electronic controls through resistance/electronic controls through brushes on three slip rings to control speedbrushes on three slip rings to control speed

Wound Rotor Induction MotorWound Rotor Induction Motor

•• Typical brush current density varies widely Typical brush current density varies widely depending on motor design and application depending on motor design and application from about 75 Amps/infrom about 75 Amps/in22 (11.6 Amps/cm(11.6 Amps/cm22) to ) to 125 Amps/in125 Amps/in22 (19.4 Amps/cm(19.4 Amps/cm22))

•• Collector rings are most often bronze or Collector rings are most often bronze or stainless steelstainless steel

Wound Rotor Motor Wound Rotor Motor –– Three Slip RingsThree Slip Rings

Turbine Generator

Turbine Turbine generator generator

brush rigging brush rigging with with ““change change

on the flyon the fly””removable removable

brush holdersbrush holders

Turbine GeneratorTurbine Generator•• Synchronous generatorSynchronous generator•• Extremely efficientExtremely efficient•• Converts mechanical energy to AC electrical Converts mechanical energy to AC electrical

energy with steam or gas turbine as prime energy with steam or gas turbine as prime movermover

•• Three phase windings in stator (rotating Three phase windings in stator (rotating magnetic field)magnetic field)

•• DC Fields on rotorDC Fields on rotor•• DC excitation supplied through carbon DC excitation supplied through carbon

brushes on two slip rings (collector rings)brushes on two slip rings (collector rings)

Steam Turbine GeneratorSteam Turbine Generator

•• Typical brush current density 40 Typical brush current density 40 Amps/inAmps/in22 (6.2 Amps/cm(6.2 Amps/cm22) to 65 ) to 65 Amps/inAmps/in22 (10 Amps/cm(10 Amps/cm22))

•• Collectors rings are almost always Collectors rings are almost always steel or sometimes stainless steelsteel or sometimes stainless steel

•• Surface speeds up to 16,000 Feet per Surface speeds up to 16,000 Feet per minute ( 182 MPH, 81 mps )minute ( 182 MPH, 81 mps )

Brush Selection and Brush Selection and Collector Ring Collector Ring

FilmingFilming

Brush Material SelectionBrush Material Selection

•• Brushes and collectors form a sliding Brushes and collectors form a sliding electrical contactelectrical contact

•• Brushes are selected based on the current Brushes are selected based on the current density, ring material and surface speeddensity, ring material and surface speed

•• The original equipment supplier (OEM) will The original equipment supplier (OEM) will have selected grades, but grade changes have selected grades, but grade changes may be made if the machine operates at may be made if the machine operates at conditions different than ratedconditions different than rated

Typical Brush Materials UsedBrush Materials Used

Brush MaterialBrush Material Rated CurrentRated Current Metal ContentMetal Content

DensityDensity %%

Amp/inAmp/in2 2 Amp/cmAmp/cm22

GraphiteGraphite 6565 10.010.0 00

ElectrographiteElectrographite 8080 12.512.5 00

Copper GraphiteCopper Graphite 100100 15.515.5 5050

Copper GraphiteCopper Graphite 125125 19.419.4 7575

Brush WearBrush Wear

•• Mechanical Mechanical –– Frictional wearFrictional wear

•• Electrical Electrical –– Material removed with the Material removed with the flow of currentflow of current

•• Thermal Thermal –– With heat some carbon With heat some carbon oxidizes into COoxidizes into CO22

•• Film provides a low friction surfaceFilm provides a low friction surface

Common Collector Ring Materials

•• Bronze Bronze –– Resistance to chemical Resistance to chemical reactions. Limited speed capability. reactions. Limited speed capability. Good friction characteristics.Good friction characteristics.

•• Steel Steel –– Good mechanical strength. Can Good mechanical strength. Can oxidize under brush at standstill or rustoxidize under brush at standstill or rust

•• Stainless steel Stainless steel –– Contamination Contamination resistance. May have higher brush resistance. May have higher brush contact drop and selectivitycontact drop and selectivity

Brush HoldersBrush Holders

•• Brushes must be held in contact with the Brushes must be held in contact with the ring for good performancering for good performance

•• The holders must be supported so they do The holders must be supported so they do not vibratenot vibrate

•• Clock spring holders are a poor selection Clock spring holders are a poor selection where there is vibration and must be where there is vibration and must be adjusted as brushes wearadjusted as brushes wear

•• Constant pressure holders are common on Constant pressure holders are common on newer machines or retrofitsnewer machines or retrofits

Clock Spring HoldersClock Spring Holders

Adjustable Clock SpringAdjustable Clock Spring

A Different A Different Style Adjustable Style Adjustable

SpringSpring

Ladder BackLadder Back

HolderHolder

Short Style Brush Holder Short Style Brush Holder –– Not PreferredNot Preferred

Full Length Full Length

Brush HolderBrush Holder

PreferredPreferred

Brush HolderBrush Holder

Alignment andAlignment and

Design ConcernsDesign Concerns

““Constant Constant PressurePressure””

(Non(Non--Adjustable) Adjustable) SpringSpring

Constant Pressure SpringsConstant Pressure Springs

Brush holdersBrush holders

set too highset too high

Brush holder clearance to Brush holder clearance to

the rings should be set to the rings should be set to

manufacturermanufacturer’’s s

recommendation. recommendation.

This is typically about This is typically about

0.1250.125”” ( 1/8( 1/8”” ) maximum) maximum

A longer holder A longer holder would be better would be better

for brush for brush stabilitystability

BrushBrush

HolderHolder

RotationRotation

Brush Holder alignment OK,exactly perpendicularto the slip ring surface orcommutator surface

Slip ring orCommutator

RotationRotation

Brush Holder alignment ,not the best, but 1 or 2degrees trailing is OK


RotationRotation

Brush Holder alignmentneeds adjustment. This is called leading or stubbingand can cause fast wear orfriction chatter. Just 1 or 2degrees makes a bigdifference.


Not Good Not Good –– One brush trailing, one leadingOne brush trailing, one leading

Normal Performance

Uniform film appearance Uniform film appearance with no burn markswith no burn marks

No signs of excessive No signs of excessive ring wearring wear

No visible sparkingNo visible sparking

Brush faces uniform with Brush faces uniform with no sign of burningno sign of burning

Minimal Minimal ““side polishside polish””from brush movementfrom brush movement

Good brush lifeGood brush life

Two excellent tools for accessing ringsTwo excellent tools for accessing rings

Feeler StickFeeler Stick

Strobe LightStrobe Light

Collector condition for good performanceCollector condition for good performance

Total Indicator Runout Total Indicator Runout Inches (micron)Inches (micron)

PreferredPreferred 0.0010.001 (25)(25)

AllowableAllowable 0.0030.003 (76)(76)

Surface speed and type of TIR affects Surface speed and type of TIR affects brush performance.brush performance.

.00044” TIR3600 RPMSlip Ring

1.0

1.0

0.0


0.5

0.5

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Small BUMP on slip ring OD maycause problems

Brush

p g

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3 RingSlip Ring Assembly

Each ring hasa different

Profile

See next2 slides

Profiler MAS - Slip ring - Radial Plot

2 0 0 0 2 0 2 0 0 0 2 0

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Profiler MAS - Slip ring - Radial Plot

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Collector RingCollector Ring

ConditionsConditions

Foot Printing (photo Foot Printing (photo Imaging/ghosting)Imaging/ghosting)

•• Images appear on the ring that initially Images appear on the ring that initially reflect the size of the brush face and reflect the size of the brush face and brush spacingbrush spacing

•• Erosion from arcing makes Erosion from arcing makes ““low spotslow spots””

•• The marks may spread out over time The marks may spread out over time and make the ring surface not roundand make the ring surface not round

•• Sometimes the ring looks like the brush Sometimes the ring looks like the brush is bouncing or is bouncing or ““skipping skipping

Foot Printing Foot Printing –– Turbine GeneratorTurbine Generator

Photo Imaging - Advanced

Foot PrintingFoot Printing• Simultaneous loss of contact

• The initial trigger is a mechanical disturbance

• The damage to the ring is from subsequent electrical erosion

• Eventually, the initial marks may spread out and not be visible, but the ring will go out of round

• Damage is progressive and gets worse at an increasing rate

Foot PrintingFoot Printing•• Mechanical triggersMechanical triggers

–– Out of balance rotorOut of balance rotor–– High friction area on ringHigh friction area on ring–– Brush instabilityBrush instability–– ContaminationContamination–– High friction in brush holders, slow to High friction in brush holders, slow to

respond springsrespond springs–– Corrosion under brushes from moisture Corrosion under brushes from moisture

when brushes left in contact with the ring when brushes left in contact with the ring at standstill at standstill

Foot PrintingFoot Printing

•• The strobe light is an excellent tool for The strobe light is an excellent tool for spotting foot printing early so spotting foot printing early so corrective action can be taken before corrective action can be taken before damage progresses too fardamage progresses too far

Foot Printing Foot Printing –– Corrective ActionCorrective Action

•• Increase brush pressure to shorten periods Increase brush pressure to shorten periods of bounceof bounce

•• Eliminate contamination/high friction areas Eliminate contamination/high friction areas on ringson rings

•• Do not allow machines to remain idle for long Do not allow machines to remain idle for long periods with the brushes downperiods with the brushes down

•• Increase the number of brushes per ring to Increase the number of brushes per ring to increase the chance at least one brush will increase the chance at least one brush will be in contactbe in contact

•• Reposition brushes to increase the arc of Reposition brushes to increase the arc of contactcontact

Foot Printing _ Corrective ActionFoot Printing _ Corrective Action

• Make sure rotors are properly balanced

• Have brush holders properly aligned for brush stability

• Do not sandblast brush holders and eliminate burrs so brushes can move freely

Foot Printing Removed – Turbine Generator Ring After Machining

ThreadingThreading

•• Under normal conditions some metal is Under normal conditions some metal is removed from the rings and is vaporizedremoved from the rings and is vaporized

•• Sometimes the metal does not completely Sometimes the metal does not completely vaporize and transfers into the brush facevaporize and transfers into the brush face

•• These small particles work harden, becoming These small particles work harden, becoming harder than the ring surfaceharder than the ring surface

•• They then cut into the ring making what look They then cut into the ring making what look like fine pitch like fine pitch ““threadsthreads””

Threading

Brush Face Brush Face ––

ThreadingThreading

Microscopic View of Copper in Brush FaceMicroscopic View of Copper in Brush Face

ThreadingThreading

•• Low brush current decreases metal Low brush current decreases metal vaporization and can increase vaporization and can increase threadingthreading

•• Sulfur containing gasses (SOSulfur containing gasses (SO22, H, H22S) S) increase metal transfer and threadingincrease metal transfer and threading

•• Low brush pressure increases metal Low brush pressure increases metal transfer and threading transfer and threading

Typical Proper Brush PressuresTypical Proper Brush Pressures

ApplicationApplication Brush Pressure Brush Pressure PSIPSI (Kg/cm(Kg/cm22) )

IndustrialIndustrial 2 2 –– 4 (0.14 4 (0.14 –– 0.28)0.28)

Turbine Gen.Turbine Gen. 2 2 –– 2.75 (0.14 2.75 (0.14 –– 0.19)0.19)

Low speed orLow speed or 3.5 3.5 –– 44 (0.25 (0.25 –– 0.28)0.28)

Higher VibrationHigher Vibration

Threading Threading –– Corrective ActionCorrective Action

•• Reduce atmospheric contamination (not Reduce atmospheric contamination (not easy)easy)

•• Make sure brush pressure is adequateMake sure brush pressure is adequate

•• Check brush current density and use a Check brush current density and use a graphite grade for low current densitygraphite grade for low current density

•• Use caution in removing brushes to increase Use caution in removing brushes to increase brush current density as foot printing may brush current density as foot printing may become a problem become a problem

GroovingGrooving

•• Grooving is uniform wear around the Grooving is uniform wear around the circumference of the ringscircumference of the rings

•• There will always be some mechanical and There will always be some mechanical and electrical wear of the ringselectrical wear of the rings

•• However, the rate should not be high enough However, the rate should not be high enough to require frequent ring maintenanceto require frequent ring maintenance

•• Excessive mechanical wear can be caused Excessive mechanical wear can be caused by abrasive brush gradesby abrasive brush grades

•• Low brush pressure can increase electrical Low brush pressure can increase electrical wearwear

GroovingGrooving

•• Steel and stainless steel rings are more Steel and stainless steel rings are more resistant to mechanical wear than resistant to mechanical wear than bronze ringsbronze rings

Ring Ring GroovingGrooving

Ring Grooving

Ring Ring

GroovingGrooving

p gR

adia

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le (

+/- 1

0 M

ils)

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Recording Plot Rec# Recording start Size TIR Max STS Mean STS Std dev Suspect Remark

ABB Path # 2 16 18 Nov 2009 ( 4:02:34 am) 106 Segments 26.34 mil : 24-10 3.58 : 19-20 1.059 0.862 0

Polarity WearPolarity Wear•• One ring may wear more than the other, One ring may wear more than the other,

especially on synchronous motors or especially on synchronous motors or generators where the polarities are generators where the polarities are separateseparate

•• Proper brush pressure reduces the Proper brush pressure reduces the wearwear

•• Reversing polarities periodically can Reversing polarities periodically can make the wear more even. This is like make the wear more even. This is like rotating the tires on a car rotating the tires on a car

Grooving Grooving –– Corrective ActionCorrective Action

•• Make sure brush pressure is adequateMake sure brush pressure is adequate

•• Do not use brush grades that have more Do not use brush grades that have more ““polishing actionpolishing action”” (abrasiveness) than is (abrasiveness) than is requiredrequired

•• Brushes with some polishing action may be Brushes with some polishing action may be required on steel rings or in contaminated required on steel rings or in contaminated environmentsenvironments

•• To reduce polarity wear reverse leads to To reduce polarity wear reverse leads to brush holders periodicallybrush holders periodically

SelectivitySelectivity•• The brushes on collector rings operate in The brushes on collector rings operate in

electrical parallel with each otherelectrical parallel with each other

•• The brushes never wear at the same rateThe brushes never wear at the same rate

•• Uneven currents in the parallel paths will Uneven currents in the parallel paths will affect brush wear ratesaffect brush wear rates

•• The brushes with higher current and The brushes with higher current and temperature generally wear fastertemperature generally wear faster

•• Mechanical friction restricting movement of Mechanical friction restricting movement of the brush in the brush holder.the brush in the brush holder.

Selectivity Selectivity -- Typical Voltage Drops Typical Voltage Drops at 100 Amperes in the Brush at 100 Amperes in the Brush

•• Brush rigging to cableBrush rigging to cable 1 mV1 mV

•• Brush box to supportBrush box to support 1 mV1 mV

•• Brush terminal to holderBrush terminal to holder 1 mV1 mV

•• Terminal to shunt wireTerminal to shunt wire 1 mV1 mV

•• Shunt to carbon (2 shunts)Shunt to carbon (2 shunts) 150 mV150 mV

•• Carbon materialCarbon material 120 mV120 mV

•• Contact drop (brush to ring)Contact drop (brush to ring) 1000 mV1000 mV

SelectivitySelectivity

•• The largest resistance in the parallel The largest resistance in the parallel paths is the contact drop between the paths is the contact drop between the brush and the ringbrush and the ring

•• The The contact dropcontact drop has the largest has the largest influence on selectivityinfluence on selectivity

Selectivity Selectivity –– Corrective ActionCorrective Action

•• To minimize selectivity, the resistances To minimize selectivity, the resistances in the parallel paths must be as in the parallel paths must be as consistent as possibleconsistent as possible

•• Brush manufacturerBrush manufacturer’’s responsibilitys responsibility–– Resistivity of the carbon tightly controlledResistivity of the carbon tightly controlled–– Terminals applied correctlyTerminals applied correctly–– Tamped or riveted connections to the Tamped or riveted connections to the

carbon should have equal and low carbon should have equal and low resistanceresistance

Dirty brush holders and springs willDirty brush holders and springs willaffect performanceaffect performance


•• Maintenance personnel responsibilityMaintenance personnel responsibility–– Electrical connections clean and tightElectrical connections clean and tight–– Brush box to rigging clean and tightBrush box to rigging clean and tight–– Ensure brush moves freely in the brush Ensure brush moves freely in the brush

holderholder


•• Adjustable brush springs must be Adjustable brush springs must be adjusted for equal brush pressureadjusted for equal brush pressure

•• Constant pressure springs Constant pressure springs ““ageage”” from from heat and vibration and should be heat and vibration and should be replaced periodically (e.g. 5 years)replaced periodically (e.g. 5 years)

•• Do not mix brush grades in a machineDo not mix brush grades in a machine

•• Minimize contamination on the ringsMinimize contamination on the rings

•• Minimize brush and machine vibrationMinimize brush and machine vibration


•• Use a collector ring material suitable Use a collector ring material suitable for the applicationfor the application

•• Use a brush with the proper current Use a brush with the proper current density ratingdensity rating

Selectivity Corrective ActionSelectivity Corrective Action -- Helical Helical grooves in rings force current to move grooves in rings force current to move

around in brush faces preventing localized around in brush faces preventing localized hot spots hot spots

Selectivity Selectivity –– Corrective ActionCorrective Action•• A diagonal slot in brush can help reduce A diagonal slot in brush can help reduce selectivityselectivity

Loss of Brush ContactLoss of Brush Contact

•• Arcing between the brush and ring can Arcing between the brush and ring can deteriorate brush life and damage the deteriorate brush life and damage the ring surfacering surface

–– Weak brush springsWeak brush springs–– Brushes hung up in holdersBrushes hung up in holders–– VibrationVibration–– Brushes allowed to wear too shortBrushes allowed to wear too short

Brush Worn Too Short

Badly Pitted Stainless Steel Ring Badly Pitted Stainless Steel Ring –– Short BrushesShort Brushes

Metal Transfer to Brush – Brushes Run Too Short

Loss of Brush Contact Loss of Brush Contact ––Corrective ActionCorrective Action

•• Maintain brush springs to keep good Maintain brush springs to keep good brush pressurebrush pressure

•• Make sure brushes can move freely in Make sure brushes can move freely in the holdersthe holders

•• Minimize machine vibrationMinimize machine vibration•• Replace brushes in a timely fashion Replace brushes in a timely fashion ––

brushes are inexpensive compared to brushes are inexpensive compared to downtime and machine repair!downtime and machine repair!

Ring CorrosionRing Corrosion

• Steel rings can rust if exposed to moisture in the air or liquid water

• Copper bearing metals like bronze can react with sulfur compounds in the air (SO22, H, H22S) or acidsS) or acids

Rusty Steel Collector Rings –Emergency Generator

Rusty Steel Slip Rings –Temporarily Out of Service

Ring Corrosion Ring Corrosion –– Corrective ActionCorrective Action

•• Choose collector rings of the proper material Choose collector rings of the proper material for the environmentfor the environment

•• Minimize corrosives in the atmosphereMinimize corrosives in the atmosphere

•• Use space heaters to minimize moisture Use space heaters to minimize moisture condensation in idled equipmentcondensation in idled equipment

•• Use rust preventative on steel rings of Use rust preventative on steel rings of equipment that is out of service or sparesequipment that is out of service or spares

ConclusionsConclusions

•• There are a variety of issues that can There are a variety of issues that can adversely affect collector ring and carbon adversely affect collector ring and carbon brush performancebrush performance

•• Understanding the root causes can help the Understanding the root causes can help the operator apply carbon brushes and collector operator apply carbon brushes and collector ring materialsring materials

•• Auxiliary components such as brush holders Auxiliary components such as brush holders and springs must be of good design and and springs must be of good design and properly maintainedproperly maintained

ConclusionsConclusions

•• Good maintenance practices and Good maintenance practices and troubleshooting will result in good troubleshooting will result in good brush life and reduce downtime and brush life and reduce downtime and collector ring expensescollector ring expenses

Questions ?Questions ?

Carbon Brush Performance on Slip Rings

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