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UnravelingUnravelingthethe
Commutation MysteryCommutation Mystery
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DCDC
MachineMachine
BasicsBasics
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MachineMachineAdjustmentsAdjustments
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CarbonCarbonBrushesBrushes
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BrushBrushHolderHolder
TypesTypes
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N = Number of TurnsN = Number of Turns
g = Air Gapg = Air Gap
NN
gg
N = Number of TurnsN = Number of Turns
g = Air Gapg = Air Gap
NN
gg
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NN
ee
Force on ConductorForce on Conductor
ll
FF
F = B x IF = B x IAA xx llwherewhere
B = Flux Density (B = Flux Density ( / area)/ area)
IIAA = Current= Current
ll = Length of the conductor= Length of the conductor
V
RIIAA
IIFF
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GeneratorGenerator
VV B xB x ll xx V
VV BB II FieldField
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MotorMotorVV B xB x ll xx V
VV
B x RPMB x RPM
RPMRPM
VoltsVolts
VoltsVolts
BB IIFieldField
SpeedSpeed
TorqueTorque
FF
B x IB x IAA xx ll
Torque = Force x RadiusTorque = Force x Radius
TorqueTorque
B x IB x IAA
BB
ll
rr
( Flux Density )( Flux Density )
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CommutationCommutationFrameFrame
NN SS
Armature RotationArmature Rotation
tt
00
Armatur
eCurrent
ArmatureCurrent
North PoleNorth Pole
+ I+ IAA
-- IIAA
South PoleSouth Pole
IIAA
This is called Reactance VoltageThis is called Reactance Voltage
V =V = -- LL dd IIAAd td t == -- LL
IIAA tt
TimeTime
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VV B xB x ll xx V
VV B xB x ll x RPMx RPM
B xB x ll xx ==
BB IIAA
V =V = -- LL IIAA
tt-- LL IIAA
tt
FrameFrame
NN SS
Armature Rotation (Armature Rotation (V
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FrameFrame
NN SS
Flux due to main fieldFlux due to main field
C
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Commutating Pole or InterpoleCommutating Pole or InterpoleFrameFrame
BackgapBackgap ggoo
ShimsShims
ArmatureArmature
FluxFlux
gg FrontgapFrontgap
FluxFlux N x IN x IAA
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Factory MethodFactory Method
Field MethodField Method
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Main Field AC Drop TestMain Field AC Drop Test
VoltsVoltsVoltsVoltsVoltsVolts
Voltages Should Agree Within 15%Voltages Should Agree Within 15%
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Methods to Set Electrical NeutralMethods to Set Electrical Neutral
DC KickDC Kick
AC NullAC Null
ReversabilityReversability (Speed & Voltage)(Speed & Voltage)
Black BandBlack Band
Brush PotentialBrush Potential
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For additional information ontuning up DC machines see
the WMEA web site
http://wmea.net
in Papers , NECP Tuning
Up DC Motors and Generators
Jun 07
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Commutating Pole Shim OrderCommutating Pole Shim Order
CommutatingCommutating
PolePole
Thin SteelThin Steel
Thin AluminumThin Aluminum
1/8 Inch Aluminum1/8 Inch Aluminum
1/8 Inch Steel1/8 Inch Steel
FrameFrame
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Theory ofTheory of
Brush OperationBrush Operation
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TreatmentTreatment
T t t
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TreatmentTreatment
Improves Brush LifeImproves Brush Life
Improves FilmingImproves Filming
Provides Low Humidity ProtectionProvides Low Humidity Protection
Allow High Temperature OperationAllow High Temperature Operation
Reduces Copper DragReduces Copper Drag Contaminated AtmospheresContaminated Atmospheres
Minimizes Commutator WearMinimizes Commutator Wear
Reduces FrictionReduces Friction
El t hiti
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ElectrographiticsElectrographitics
W
earRate
W
earRate
(Increasing)(Increasing)(Decreasing)(Decreasing)HumidityHumidity
Brush TemperatureBrush Temperature
UntreatedUntreated
TreatedTreated
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Friction
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Friction
Brush Temperature 175C
Coeffic
ientofFriction
Friction
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Friction
Brush Temperature 175C
CoefficientofFriction
Friction is highest at low brush
temperatures (light loads)
FrictionFriction
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FrictionFriction
Brush Temperature175C
CoefficientofFriction
Other Factors affect friction
- Chemical contamination
- Low humidity
- Ring material (copper, brass, steel)
-Treatments can increase or decrease friction
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HumidityHumidity
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HumidityHumidity
Relative HumidityRelative Humidity -- The amount ofThe amount of
water vapor in the air as awater vapor in the air as apercentage of what the airpercentage of what the air couldcould
hold at that temperature.hold at that temperature.
This is what the weather manThis is what the weather man
reportsreports
HumidityHumidity
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Absolute HumidityAbsolute Humidity-- The mass ofThe mass of
water vapor per mass of dry air.water vapor per mass of dry air.Absolute humidity isAbsolute humidity is
independent of temperatureindependent of temperature(until you fall below the dew(until you fall below the dew
point). Absolute humidity ispoint). Absolute humidity is
what brushes care about.what brushes care about.
HumidityHumidity
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HumidityHumidity
UnitsUnits
Relative HumidityRelative Humidity -- %%
Absolute HumidityAbsolute Humidity grains / lbgrains / lbdry airdry air
1 grain = 0.000143 lbs1 grain = 0.000143 lbs
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180
170
% Relative Humidity
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Temperature (Degrees F)Temperature (Degrees F)20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 Grain / lb Dry Ai r
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
100 8060
4020 10
(20 F to 105 F
- 18 C to +41
A
bsoluteH
umidity
A
bsoluteH
umidity
% Relative Humidity Psychometric Chart
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Temperature (Degrees F)
-40 -30 -20 -10 0 10 20 30
40
Grain / lb Dry Air
24
22
20
18
16
14
12
10
8
6
4
2
0
100
50
10
- 40 F to + 40 F- 40 C to + 4 C
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Absolute HumidityAbsolute Humidity
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yy
Where When Temp. R.H. Abs. H
LA Summer 90 F (32C) 90% 178 gr/lb
AZ Summer 100F (38C) 10% 28 gr/lb
TX Winter 32F (0C) 100% 24 gr/lb
WY Winter - 20F (- 26C) 100% 1.8 gr/lb
AB Winter - 40F (- 40C) 100% 0.5 gr/lb
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Wear vs Brush PressureWear vs Brush Pressure
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W
earRate
Brush Pressure Intensity
A
B C
D
CD Spring pressure too high
High abrasive frictional forcesaccelerate brush wear.
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How Much Brush PressureHow Much Brush Pressure ?
The proper amount of brush pressure against the commutator orThe proper amount of brush pressure against the commutator or
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The proper amount of brush pressure against the commutator orThe proper amount of brush pressure against the commutator or
slip ring depends on the application and/or the brush grade.slip ring depends on the application and/or the brush grade.
To calculate the brush pressure; you need to know or measureTo calculate the brush pressure; you need to know or measure
the spring force anthe spring force andd the brush thickness and width. Brush T xthe brush thickness and width. Brush T x
W = the cross sectional area. Brushes that contact theW = the cross sectional area. Brushes that contact the
commutator at an angle do have more contact area that thecommutator at an angle do have more contact area that the
product of their T x W, but it is usually not a significantproduct of their T x W, but it is usually not a significant
difference so the easier to calculate cross sectional area is usdifference so the easier to calculate cross sectional area is used.ed.
Recommended
Brush PSIRange
Application
3.5 to 6 General Industrial Motors3 to 8 Fractional HP Motors
5 to 10 Traction Motors
3 to 4 Slip Rings Low Speed, Graphite Grade2 to 2.75 Slip Rings High Speed, Graphite Grade
3 to 5 Metal Graphite Grades
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Single vs Multiple Wafer
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Rotating
Surface
Rotating
Surface
F F
- Limited brush
contact points on
irregular surfaces- Increased cross resistance- Better brush face contact
for better commutation
Steel Clip or Rubber Hardtop
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F F
Rotating
Surface
Rotating
Surface
- Distributes force evenly
if spring is off center
-Allows movement betweenwafers
-Absorbs shock and vibration
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VVRR
RRBB
VVCDCD VVCDCD
RRAA
VVRR == iicc x Rx RAA + V+ VCDCD ++ iicc x Rx RBB + V+ VCDCD
iicc
VVRR -- 2 V2 VCDCD
RRAA + R+ RBBiicc ==
iicc Becomes less as RBecomes less as RBB increasesincreases
Use high resistivity grade brushUse high resistivity grade brush
for difficult to commutate machines.for difficult to commutate machines.
II AA Load AmpsLoad Amps
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RadialRadial
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HolderHolder
RotationRotation
RadialRadial
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HolderHolder
RotationRotation
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ModifiedModifiedReactionReaction
HolderHolder
RotationRotation
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ReactionReaction
H ldH ld
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HolderHolder
TrailingTrailing
RotationRotation
ReactionReaction
H ldH ld
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HolderHolder
StubbingStubbing
or Leadingor Leading
RotationRotation
ReactionReaction
H ldH ld
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HolderHolder
StubbingStubbing
or Leadingor Leading
RotationRotation
Friction
Chatter
ReactionReaction
H ldH ld
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HolderHolder
StubbingStubbing
or Leadingor Leading
RotationRotation
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Questions