unraveling the commutation mystery.pdf

7/24/2019 Unraveling the Commutation Mystery.pdf

1/91


2/91

UnravelingUnravelingthethe

Commutation MysteryCommutation Mystery


3/91

DCDC

MachineMachine

BasicsBasics


4/91

MachineMachineAdjustmentsAdjustments


5/91

CarbonCarbonBrushesBrushes


6/91

BrushBrushHolderHolder

TypesTypes


7/91

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


8/91


9/91


10/91


11/91


12/91

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


13/91

GeneratorGenerator

VV B xB x ll xx V

VV BB II FieldField


14/91

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 )


15/91

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


16/91

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


17/91

FrameFrame

NN SS

Flux due to main fieldFlux due to main field

C


18/91


19/91


20/91


21/91


22/91

Commutating Pole or InterpoleCommutating Pole or InterpoleFrameFrame

BackgapBackgap ggoo

ShimsShims

ArmatureArmature

FluxFlux

gg FrontgapFrontgap

FluxFlux N x IN x IAA


23/91


24/91


25/91

Factory MethodFactory Method

Field MethodField Method


26/91


27/91


28/91


29/91

Main Field AC Drop TestMain Field AC Drop Test

VoltsVoltsVoltsVoltsVoltsVolts

Voltages Should Agree Within 15%Voltages Should Agree Within 15%


30/91


31/91


32/91


33/91


34/91


35/91


36/91


37/91


38/91


39/91


40/91


41/91


42/91


43/91


44/91

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


45/91

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


46/91


47/91


48/91


49/91

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


50/91

Theory ofTheory of

Brush OperationBrush Operation


51/91


52/91


53/91


54/91


55/91

TreatmentTreatment

T t t


56/91

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


57/91

ElectrographiticsElectrographitics

W

earRate

W

earRate

(Increasing)(Increasing)(Decreasing)(Decreasing)HumidityHumidity

Brush TemperatureBrush Temperature

UntreatedUntreated

TreatedTreated


58/91

Friction


59/91

Friction

Brush Temperature 175C

Coeffic

ientofFriction

Friction


60/91

Friction

Brush Temperature 175C

CoefficientofFriction

Friction is highest at low brush

temperatures (light loads)

FrictionFriction


61/91

FrictionFriction

Brush Temperature175C

CoefficientofFriction

Other Factors affect friction

- Chemical contamination

- Low humidity

- Ring material (copper, brass, steel)

-Treatments can increase or decrease friction


62/91


63/91


64/91

HumidityHumidity


65/91

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


66/91

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


67/91

HumidityHumidity

UnitsUnits

Relative HumidityRelative Humidity -- %%

Absolute HumidityAbsolute Humidity grains / lbgrains / lbdry airdry air

1 grain = 0.000143 lbs1 grain = 0.000143 lbs


68/91

180

170

% Relative Humidity


69/91

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


70/91

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


71/91

Absolute HumidityAbsolute Humidity


72/91

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


73/91


74/91

Wear vs Brush PressureWear vs Brush Pressure


75/91

W

earRate

Brush Pressure Intensity

A

B C

D

CD Spring pressure too high

High abrasive frictional forcesaccelerate brush wear.


76/91

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


77/91

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


78/91

Single vs Multiple Wafer


79/91

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


80/91

F F

Rotating

Surface

Rotating

Surface

- Distributes force evenly

if spring is off center

-Allows movement betweenwafers

-Absorbs shock and vibration


81/91

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


82/91

RadialRadial


83/91

HolderHolder

RotationRotation

RadialRadial


84/91

HolderHolder

RotationRotation


85/91

ModifiedModifiedReactionReaction

HolderHolder

RotationRotation


86/91

ReactionReaction

H ldH ld


87/91

HolderHolder

TrailingTrailing

RotationRotation

ReactionReaction

H ldH ld


88/91

HolderHolder

StubbingStubbing

or Leadingor Leading

RotationRotation

ReactionReaction

H ldH ld


89/91

HolderHolder

StubbingStubbing


RotationRotation

Friction

Chatter

ReactionReaction

H ldH ld


90/91

HolderHolder

StubbingStubbing


RotationRotation


91/91

Questions

unraveling the commutation mystery.pdf

Documents