38649431 Carbon Brush

Carbon Brush & Brush Gear

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SI . No. TOPICS PAGE

01 INTRODUCTION 02

02 BRUSH GEAR 02

03 TYPES & GRADES OF CARBON BRUSHES 04

04 CONTACT PROPERTIES OF CARBON BRUSHES 06

05 FACTORS AFFECTING BRUSHES 10

06 WEARING OF CARBON BRUSHES 10

07 FACTORS FOR SELECTION OF CARBON BRUSHES

10

08 APPLICATIONS OF CARBON BRUSHES 11

09 BEDDING OR SEATING OF CARBON BRUSHES 12

10 FAULTS AND TROUBLE SHPOPTING 14


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INTRODUCTION

Carbon brush plays an important role in the operating of electrical machines and has

a direct on the efficiency and life of machine according to its quality and

appropriateness of its selection.

Carbon brushes belong to the most important constructional components of rotary

generator and motors. The flawless function and service life of these machines

depend on their properties.

BRUSH GEAR

Brush gear consists of the following:

1. Carbon Brush:

"A carbon brush, as used in the electrical manufacturing industry, is a conductor,

usually composed in part of some of the element carbon, serving to maintain an

electrical connection between stationary and moving parts of a machine or

apparatus."

For the production of carbon brushes different forms of Carbon are used: coke from

coal, charcoal from wood and lampblack from oil in addition to graphite and metal

powder. All raw materials are laboratory. The raw materials are pulverised, blended

together in the desired proportion and mixed with temporary binding agents in

heated mixers.

The most modern method of manufacture, the use of high grade raw materials and

strict inspection of the production process ensure high quality of the carbon brushes.

2. Brush Holder:

It is a device which holds the brush in position. There are several forms of holder,

the box type probably being the best.

3. Brush Holder Spring:

A brush holder spring is that part of the brush holder which provides pressure to hold

the brush against the collector ring or commentator.


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4. Brush Holder Stud:

A brush holder stud is an intermediate member between the brush holder and

supporting structures.

5. Brush Holder Stud Insulation:

A brush holder stud insulation is an assembly of insulating material which insulates

the brush holder or stud from supporting structure.

6. B. H. Yoke :

A brush holder yoke is a rocker arm, ring, quadrant or other support for maintaining

the brush holders or brush holder studs in their relative position.

BRUSH HOLDERS

Whatever be the design, the brush holders are mainly intended for carring out the

following functions :

1. Guide the brush at the correct angle for firm, intimate collector contact.

2. Provide a smooth channel for free movement of brush up and down to

compensate for slight collector eccentricity and vibrations.

TYPES OF BRUSH HOLDERS

1. GRIP TYPE;

In this, the brush is rigidly fixed in the holder-however these holders ae suitable only

for low speeds as the entire holders have to move with the brush while the following

irregularities of the collector surface.


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2. SLIDE TYPE : In this design, the brush moves through the guide channel in the holder. These are

made of brass sheet or brass die-casting and are available with several pressure

systems.

i) Helical compression spring pressing directly on the top of the brush.

ii) Clock or tension spring acting directly on top of brush. The pressure fall off in

this design with brush wear can be made as low as 15% and is much less

than the one with compression spring.

iii) Lever type pressure finger with torsion spring or helical tension spring.

iv) Self coiling or constant force spring.


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TYPES AND GRANDES OF CARBON BRUSHES

According to the type of material used, brushes are classified in the following

grades or types:

A. HARD CARBON –

The term hard carbon, when used in the industry to mean specific type of brush,

signifies a brush compound principally of amorphous carbon. This type is usually

very hard. It can carry only moderate currents and is adapted to low speeds.

Grades : A, AO, C4 (M.Co.); H14, L11 (SCHUNK & EBE Co.)

Current density = 6.5 A/sq.cm to 7 A/sq. cm.

Peripheral Speed = 15 m/sec. to 18 m/sec.

Brush pressure = 140 gm/sq. cm.

B. CARBON GRAPHITE –

The term carbon graphite brush signifies an amorphous carbon brush to which

graphite has been added. This type of brush can vary from medium hard and is

adapted to moderate speeds.

Grades : B2, (GE) ; WM44A (WH) ; E(NC)

Current density = 5 to 7 A/sq. cm.

Peripheral speed = 20 m/sec.

Brush perssure = 170 to 350 gm/sq. cm


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GRAPHITE –

Grades :

IM3, IM9101 (MC) ; 71 (STAR); R, H, R310 (GE) ; 9613 (NC) ; E3 (OHIO) ; F10,

F19(CN & E)

Current density = 6.5 to 10 A/sq.cm.

Peripheral speed = 20 to 30 m/sec.

Brush pressure = 210 gm/sq.cm

C. NATURAL GRAPHITE –

The term natural graphite brush signifies a brush composed principally of graphite.

this type of brush alter greatly in current carrying capacities and in operating speeds.

Grades: LFC4 (LEC); HM5 (MC);

Current density = 9.5 to 11.5 A/sq.cm


Brush pressure = 140 gm/sq.cm

D. ELECROGRAPHITE –

The term electrographite brush specifies a brush composed of selected amorphous

carbon, which in the process of manufacture, is carried to a temperature of the order

of 2500 deg. centigrade for a prolonged time so as to convert carbon to the

graphitised form. This type of brush is extremely versatile in that it can be made soft

or very hard and slightly or non-abrasive. Grades of this type have high current

capacities but greatly in their operating speeds.

Grades : EGO, EG14 (MC & LEC); D, E2, G, D3 B18, 377, 501T563(GE); W535,

WW, WP, WA (WH); 255, SA45, AX5, TA45AY (NC) ; E (CHE) ; G153 (REKOFA) ;

GH125 (HITACHI) ; RE11 (RINGS DROFF)

Current density = 8.5 to 12.3 A/sq.cm

Peripheral speed = 18 to 60 m/sec

Brush pressure = 140 to 240 gm/sq.cm


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E. METAL-IMPREGNATED GRAPHITE –

It consists of graphite or carbon which has been impregnated with molten metal

under pressure. The composition of grades in this class differ from those in the

metal graphite class in that the metal is present in a continuous phase instead of as

a discrete particles.

F. METAL GRAPHITE –

The term metal graphite brush signifies a brush composed of varying percentages of

metal and graphite, copper being the metal generally used. This type of brush is

soft. Grades of brushes of this type have extremely high current carrying capacities

but differ greatly in operating speeds from low to high.

Grades : CM, CM3 CM3H (MC); X, L4, L (GE); W100, W933(WH) AYK (NC); M13

(OHIO); M15E (AC)

Current density = 11 to 19 A/sq.cm


Brush pressure = 140 to 350 gm/sq.cm.

G. RESIN-BONDED GRAPHITE –

It consists of carbon or graphite bonded with synthetic resin. This type of bond

produces a high electrical resistance which endows these grades with good

commutating ability. Their current loading ability is however limited.

Grade : BG62(LEC);

Current density = 2 to 3 A/sq.cm

Peripheral speed = 25 m/sec.

Brush pressure = 180 to 250 gm/sq. cm

CONTACT PROPERTIES OF CARBON BRUSHES

The carbon brush is one application of carbon wherein contact properties of

materials play a more important role than the physical properties. Neverthless the

physical properties also have a bearing on operation of carbon brushes.

1. FRICTION


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The friction between the surface of the collector and the carbon brush play a very

important part in the performance of a brush. It is friction that is responsible for such

observed phenomena like rapid brush wear, brush chatter (and even destruction)

and bouncing, dusting (extremely rapid wear of the carbon brush) etc.

2. PATINA FORMATION

When a carbon brush, carrying current, slides over a metal surface, the original

colour of the metal surface changes and a colouration from light yellow (straw

colour) to deep, reddish brown of dark brown is visible. The colouration is due to an

oxide film formed on the collector surface. This film is called patina. The formation of

this film is mostly brought about the presence of moisture.

The movement of copper ions at elevated temperatures due to an electric field also

plays an important role in the so called fritting process. By fritting is meant the

process of bridging a direct contact through the oxide film.

3. CONTACT VOLTAGE DROP

When two surfaces, say like copper and carbon brush are pressed together with a

certain force, contact areas are formed between the two. This mechanical contact

area is only about 10% of the total area in touch with each other. If now a current is

made to pass between the surfaces through the contact area, a certain amount of

resistance is experienced which is not proportional to mechanical contact area

because of patina film which acts like a semicoductor allowing current to pass in one

direction only. However there are certain points of mechanical contact area where

the film breaks down and current is allowed to pass through. This electrical contact

area is hardly 10% of the mechanical contact area. The voltage drop across this


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contact area is know as contact voltage drop and this is the extra ordinary property

of the brush which makes it capable of commutation of current by obstructing the

path of the short circuiting of the eddy currents when the brush is passing over two

adjacent segments of a commentator.

FACTOR AFFECTING BRUSH PERFORMANCE

1. CURRENT DESITY

This is an important factor for the satisfactory performance of a brush. If there is to

be a good film (patina), the current density should be a minimum value. There is a

certain temperature at which a stable, dynamic film forms, given all other conditions

like humidity, brush pressure, surrounding temperature etc.

The current density indicates the optimum current which can pass through the brush

without overheating it and causing any other damage to the brush.

2. VIBRATIONS

There are two types of vibrations experienced - external and internal. External

vibration from the nature of the operation is entirely external to the machines. This

can be eliminated by employing damping devices to isolate the machines from the

source of vibration.

Internal vibrations are caused due to factors like rough commentator, eccentric

commutator, wrong brush pressure, defective brush holders, incorrect brush angle,

incorrect brush grade, etc.

3. SPLIT BRUSHES

A thick brush has high inertia and due to high speed of rotor, it rides the peaks of the

collector surface undulations. However maintaining the same surface area, if

brushes are split and allowed to move independently they will have more points on

the commutator surface.


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4. BRUSH SPACING

Accurate and equal spacing of the brushes around the commutator is major

importance in obtaining the satisfactory brush performance. If the spacing is not

equal, circulating currents flow between brush studs of like polarity due to difference

in potentials or in counter emfs. The spacing from toe to toe of the brushes on two

consecutive studs will be exactly of the holder from commutator surface should be

between 1/8" to 3/16". It can be easily adjustable by a suitable backelite piece.

5. STAGGERING OF BRUSHES

AXIAL STAGGERING :

The collector wear under brushes of opposite polarities can be very unequal. To

minimise the formation of ridges and to have a uniform wear on all the portions of

the commutator, it is beneficial to stagger the brushes axially where there are more

than 2 brushes per arm and more than 2 poles.

When all tracks of the commutator are swept by an equal number of positive and

negative brushes by shifting the brush holders in the same stud, it is known as axial

staggering.

CIRCUMFERENTIAL STAGGERING:

Instead of all brushes on one arm being in true alignment, they are set so that

approximately half line is in advance of the other half in direction of rotation. The

purpose of such staggering is to improve commutation on machines where the

operating condition make it difficult to prevent sparking.

6. LIFTING-OFF OF BRUSHES DURING SHUT DOWNS:

This is necessary as a prophylactic measure from deterioration of the surface film. If

the brushes rest on the collector surface, electrolytic action is likely to occur in the

presence of moisture in the surrounding air. Such an electrolytic action will result in

passage of current through the patina and the patina may be disturbed.

7. BRUSH POSITIONING:

There are three types of brush positioning :–


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a) Radial type – This typy is used for reversible dc motor as shwon in the fig.:

b) Trailing type – This type is used for non-reversible dc motors and dc generators

as shown in the fig.:

c) Reaction type – This type is used for high capacity non-reversible dc motors and

dc generators where the load is fluctuating as shown in the fig.

WEARING OF CARBON BRUSHES

Wear is a cutting and grinding action in the presence of debris generated by the

electrical, mechanical and chemical effects taking place on the surfaces. Fortunately

most of the wear takes place on the carbon brush. Wear is dependent of the

following factors:

1. SURFACE SMOOTHNESS OF THE COLLECTOR AND CARBON BRUSH.

2. BRUSH PRESSURE

3. CURRENT OVERLOAD

4. POLARITY OF THE BRUSH

5. SPEED OF ROTATION

6. ROTOR & AIR TEMPERATURE

7. HUMIDITY OF THE AIR

8. ECCENTRICITY OF THE ROTOR SURFACE

9. DUST CONTENT OF AIR


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FACTORS FOR SELECTION OF CARBON BRUSHES

1. MECHANICAL FACTORS

a) Friction co-efficient

b) Condition of ring and commutator surfaces

c) Vibration

d) Spring pressure

e) Brush support

f) Disposition of the brushes

2. ELETRICAL FACTORS

a) Contact drops

b) Commutation

c) Distribution of current in the brushes

d) Current density

e) Resistively

3. PHYSICAL AND CHEMICAL FACTORS

a) Humidity

b) Corrosive gases or vapours

c) Oils and hydrocarbons

d) Dust

APPROXIMATE NORMAL VALUES

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APPLICATIONS OF CARBON BRUSHES


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1. STATIONARY COMMUTATOR MACHINES

A. Direct currents

a) Old acines without interpoles ----- electrographite grades

b) Low voltage machines ------- graphite and electrographite grades

c) Industrial machines ------ electrographite grades

B. Alternating current

a) Single phase repulsion motors ------- eletrographite grades

b) 3 phase motors ------ eletrographite and natural graphite

2. TRACTION COMMUTATOR MACHINES

A. Direct current

a) Light traction – eletrographite grades

b) Heavy traction – metal-impregnated and electrographite

c) Diesel electric traction – metal – impregnated and electrographite

d) Battery electric vehicle motors – graphite and electrographite grades

B. Rectified current

a) Heavy traction and modern motors – metal – impregnated graphite

C. Alternating current – electrographite and metal impregnated graphite.

3. SLIPRING MACHINES

A. DC AND AC – metal graphite grades.

BEDDING OR SEARING OF CARBON BRUSHES

It is necessary to match the curvature of the brush face to that of the commutator in

order to establish maximum contact area for collection of current, stable positioning

of the brush on the commutator and to ensure even wear of the brushes. This

seating of carbon brushes can be accomplished in two ways :–

1. SAND PAPER METHOD –


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a) Cut a 25-30 cm long strip of coarse sand paper so that its approximates the

width of the brushes.

b) With the abrasive side towards the brush, the strip is slipped between the

collector and the brush and is pulled back and forth untill the brush's

commutator end closely conforms to the collector.

c) Now a strip of fine sand paper is inserted under the brush in the same may. If

the machine is unidirectional, the sand paper is drawn only in the direction of

rotation, lifting the brush to return the sand paper strip of another cutting

stroke.

b) Additional passes are made with the sand paper untill the brush collector

contours match.

e) After bedding, holders and brushes are thoroughly cleaned, brushing or

wiping and compressed air jet are useful methods. There are three

precautions to be observed –

a) It is to be ensured that only SAND PAPER AND NOT EMERY PAPER IS

USED as electrically conductive emery particles can lodged between the

commutator bars and cause short circuiting.

c) The black of the sand paper is kept firmly against the commutator as one

goes. Carelessness here spells rounded brush edges and decreased brush

area.

2. SEATING WITH SEATING STONE –

Another method for a complete set of new brushes is to use a suitable grade of

brush bedding stone also known as a brush seater.


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A brush bedding stone is a loosely bonded artificial pumice which readily releases its

abrasive particles when rubbed on the commutator. Bedding of the brushes is partly

accomplished by the slight roughening effect of the stone on the commutator surface

but chiefly by the passage underneath the brushes of the fine abrasive particles

which are detached from the bedding stone.

Before the stone is used, the commutator should be rubbed with a clean rag

moistened with petrol. The merest trace of oil or grease is sufficient to clog the

bedding stone and render it ineffective. Avoid commercially available grease

solvents which could damage the bond of micanite inter segmental insulation. If not

dealing wiht a newly surfaced commutator, it is as well to remove the old skin at this

stage, using a strip of worn abrasive cloth on the rotating commutator.

The pressure on the brushes should be adjusted to the normal and machine runs up

to speed. It is advisable to raise the speed of a large machine slowly and if there is

any chattering likely to break the brush contact edges, to transverse the commutator

immediately with the bedding stone. The bedding stone should be applied closely

behind the one line of brushes and should be moved from side to side across the

commutator, using sufficient pressure to give a steady flow of particles but not a

cloud.

The brushes should be inspected after about 20 sec. and the process repeated untill

al are perfectly bedded.

Those brushes immediatly in front of the stone naturally bed quicker than the others.

On completion, the machine should be shut down and all the stone and carbon dust


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blown out with compressed air and brushes and brush holder boxes thoroughly

cleaned.

Bedding by this method has a number of advantages. The full set of brushes is

bedded at operational speed with each brush occupying its normal running position

in its holder. The bedding is therefore to correct radius and over the entire arc of

contact. Small machines with small end inaccessible commutators can usually dealt

with by bedding stone.

FAULTS AND TROUBLE SHOOTINGS

BRUSH SPEAKING:

Sparking is always the result of iterrupted contact. This can be an interruption of the

load current and also sparking as a result of transverse currents at high potentials.

The following are the cause of faults relating with sparking & remedies :

01. Incorrect setting of brush gear: Finding the neutral zone by indective method

and adjust brush position.

02. Incorrect setting of interpole : Suitable adjustment of the machine by the

maker (alteration of the air gap).

03. Unsymmetrical positioning of the fixing bolts : Lay paper strips of the length of

the commuator and marked with the exact position, around the commutator.

Set the brushes to the marks.

04. Commutator not round : skim off the commutator, remove the ridge at the

edges of the segments.

05. Commutator segments too loose : Tighten up the fixing bolts and skim off the

commutator.

06. Mica insulation projecting : Undercut the insulation and slightly chamfer the

edges of the grooves.

07. Brush pressure too light : Adjust according to the tables.

08. Brushes have too much play in the brush box : fit new brush holder.


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09. Distance between the lower edge of the brush box and the commutor surface

too great : Set the distance to 2 mm.

10. Thick insulating film on the surface of the commutator, due to atmospheric

effects : Clean the commutator. Use recommended brush grades.

11. Faulty soldered connections : Re-solder leads.

12. Defective winding : Repair or nenew the winding.

13. Armature out of balance, faulty bearing : Remove the faults.

B. FORMATIN OF GROOVES

The fact that the occurance of individual grooves and flats on the commutator

surface and slpring is due to many factors, some of which act directly -

a) Chemically active gases,

b) Atmospheric humidity,

c) Oil vapour and

d) Dust particles.

Further causes for groove formation are :

a) The material of the commutator or slipring.

b) The brush material and its specific current loading.

c) The bearings of the machines.

C. MARKING AND BURNT SPOTS ON SLIRRINGS AND COMMUTATOR.

Marking or brunt spots occur if there is an interruption of contact or a loose contact

between brush and rotating slipring or commutator. The anodic vaporisation in the

momentary sparking gives rise to spot marks on the commutator or slipring.

The following are the possible cause of troubles with remedies :

01. Partial jamming of brushes in their holder (partial overloading of some

brushes) : Remove the dust that has collected between brush and brush box.

Check the dimensional tolerance of the brush box.


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02. Bad soldering at the commutator or at the equalising connections.

03. Unsuitable brush material (not sufficient hard contact-making particles in the

running surface of the brushes) : Fit the recommended brush grade. In case

of difficult commutation, use twin carbons.

04. Element formation (iron/carbon) with DC-loaded steel rings when at rest :

raise the brushes when at rest.

D. UNEQUAL BRUSH WEAR

Unequal brush wear can be due to both mechanical and electrical factors.

01. Defective brush fittings.

02. Brush pressure unequal or too low.

03. Commutator or slipping not round.

04. Armature out of balance.

05. Not using a uniform brush grade.

E. BRUSH CHATTERING AND BREAKING UP OF CARNON BRUSH

The visible and audible chattering or brushes, due to rough running depends

primarily on the brush quality and the peripheral speed of the commutator.

01. Dust in the air : Throttling of the air supply, taking into account the permissible

heating of the machine.

02. Unsuitable brush material.

03. Underloading of brushes (greater friction) : Use more suitable brush types.

04. Commutator not round.

05. Commutator segments loose.

06. Brushes have too much play in their boxes : Renew the brush holders.

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38649431 Carbon Brush