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FLORIDA

Loop Current Balanceand

Setting Electrical NeutralDC Machines – AC Method

Rick Scherer, PETechnical Training Advisor

June 11, 2015

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Loop Current Balance

•Why is it important?

Mechanical Reliability

Mechanical Torque Balance

Electrical Dynamic / TransientResponse

•What is an acceptable tolerance?

OEM’s Recommend <10%

•How do we correct loop imbalance?

Let’s Explore……

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Electrical-Mechanical Relationships

• Speed of DC Motor is proportional to:

Armature Voltage (Va)(CEMF)

• Speed of DC Motor is “inversely” to:

Shunt Field Current (1/If)

• Torque of DC Motor is proportional to:

Armature Current (Ia)

• Torque of DC Motor is proportional to:

Shunt Field Current (If)

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PrimeMover

Resistance = .01W

Ward Leonard LoopFundamental Construction

ACSynchronous

Motor

CEMF

EMF = K x if

Speed = K xCEMF

i f

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Typical 2 Loop Configuration

Loop 1

ArmatureCurrent

Flow

Loop 2

ArmatureCurrent

Flow

Each Motor and each Generatorrepresents a variable DC VoltageSource

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Equivalent Circuit

8 Variables

G1 G2

M1

M2

G3 G4

M3

M4

Loop 1 Loop 2

Balance motor shunt field flux (i.e. field amps)

How might we reduce the variables?

Electrical neutral influences motor CEMF performance

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Equivalent Circuit

G1 G2

M1

M2

G3 G4

M3

M4

Loop 1 Loop 2

How might we furtherreduce the variables?

4 Variables

Electrical neutral influencesgenerator EMF performance

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DrawingCompliments ofDCS

Let’s Explore aMethod to Set

Electrical Neutral

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What is Electrical Neutral ?

Of the following definitions, which BESTdescribes Electrical Neutral?

1. Point of Optimum Commutation / Film

2. Point of Zero Induced Voltage in Armature Coil

3. Point of Optimum Performance

Answer:

Point of Zero Induced Voltage in ArmatureCoil

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DC Motors:

–Typically DC Motors are set to PreciseElectrical Neutral

• This provided for balanced reversing performance

DC Generators:

–Typically, Intentionally set ¼ - ¾ CommutatorBar (Segment) Against Rotation

• DC Generators Operate one Direction of RotationOnly

• Boost commutating field strength resulting inimproved commutation

DC Motors – vs – DC Generators

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•PVN (Pencil Volt Neutral)

•DC “Kick” or “Flash” Method

•Bi-Directional Speed Matching

•Generators, Voltage Drop at Stall

•AC Null Method

•Brush Potential Method

•Tram Markings / Mechanical Method

•Cover the Spark Method ☺

•AC Curve Method

Various Methods of SettingElectrical Neutral

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The Question?

Considering all factors, whichmethod of setting the electricalneutral on a DC Machine is best foryour application?

Let us explore the AC Plot method,theory and safety……then youdecide!

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AC Plotting Method

•Safe – electrical exposure is 120 VAC viaGFCI

•Safe – no flash hazard while testing

•No moving parts

•Very accurate

•Repeatable / Dependable

•Easy to learn, easy to implement

•> 15 Minutes for experienced techniciansfollowing test set up

•Let’s explore this method……

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DC Machine Simplified Drawing

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Single Ø Transformer ModelDC Machine Equivalent Circuit

Transformer Primary:

Typical 6 pole DC Machine: 200 +/- turns / pole , approximately 1200 total turns

Transformer Secondary:

Each commutator bar represents a tap of a single turn secondary winding

Iron core

Secondary voltage: Vs = Vp * Ns/Np Vs = 120V * 1/1200 ~ 100mV

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ALL:

Electrical, Mechanical, Hydraulic and Stored

Energy sources must be properly disabled,

locked, blocked and tagged, per your company

policy…..

Lock / Tag / Try

• Let’s explore the testing procedure….

SAFETY - First and Always

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Step # 1

VERIFY and CORRECTas necessary to obtain

EqualizedCircumferentialSpacing (Span)

TOLERANCE:

1/32”

(0.8 mm)

Equal Circumferential Spacing

Symmetry / Symmetry / Symmetry

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Measuring CircumferentialSpacing “Span”

Tolerance 1/32” MaximumDifference between all Arms

Maintenance PracticesSetting Span in the Field

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Step # 2

VERIFY and CORRECTas necessary to obtain

proper

Brush Box Elevation

TOLERANCE:

.070” to .080”

(1.8 mm to 2.0mm)

Brush Holder Elevation

Symmetry / Symmetry / Symmetry

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Test Procedure Outline:

1. Determine the total number of commutator bars

2. Identify the total number of poles

3. Calculate the Span and Throw (see following example)

4. Number the commutator bars (see following illustrations)

5. Lift all the brushes to isolate the armature from thearmature circuit components

6. Apply a GFCI protected, 120 VAC - 1Ø (F1 to F4)

7. Measure and record at the commutator, theinduced AC voltage into the Armature

8. Plot the results and adjust brush yoke accordingly

Proceed Only after verifying Span and Brush Box Elevation, Subsequent Adjustments

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Convenient Location for which to apply

GFCI Protected 120 Vac

Photo:

Complements ofDCS

Don Pattersonoffers the Ghost atno additionalcharge!

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Span Calculation

1. Throw = Commutator Bars ÷ Number of Poles

2. Span = Throw + 1

Total number of commutator bars: 288

Total number of main field coils:6

Throw: (288 ÷ 6) 48

Span: 48 + 11 and 49

Practical Example:

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Numbering the Commutator Bars

21 43 5

12:00 Position Brush

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“As Found” Neutral Setting

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“As Found” Position

12:00 Position Brush

543

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Data Acquisition Procedure

Test Results:

1-49 = .094V or 94mV2-50 = .050V or 50mV3-51 = .006V or 6mV4-52 = .038V or 38mV5-53 = .081V or 81mV

Commutator Bars = 288Main Shunt Field Poles = 6Throw = 48Span = 1 and 49

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Document:

A. Equipment I.D.

B. Machine I.D.

C. Date

D. Technician

Plot Step 1

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Plot Step 2

Document:

Actual measuredinducedvoltage

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Plot Step 3

Scaling:

Determine verticalscaling basedupon highestinduced voltage

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Plot Step 4

Data:

Place the first of fivepoints on theproperly scaledgraph

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Plot Step 5

Data:

Place the second offive points on theproperly scaledgraph

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Plot Step 6

Data:

Place the fourth andfifth points on theproperly scaledgraph

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Plot Step 7

Data:

Using a straightedge, draw a linethrough the fourdata points

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Plot Step 8

Quality Control:

The fifth data pointwill now fall directlyon the line to serveas quality assurancethe recorded data isvalid

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Plot Step 9

ElectricalNeutral:

Precise ElectricalNeutral is now definedas the point at whichthe drawn lineintersects the zeroinduced voltage axisNow it is necessary totransfer this graphicalrepresentation ontothe actual commutatorsurface

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Graphically

“As Found”

VS.

“Precise”

Or

“Hard”

Neutral

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“As Found” Neutral Setting

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“As Found” Position

12:00 Position Brush

543

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“Desired” Neutral Setting

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12:00 Position Brush

543

“Desired” Position

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“Final” Electrical Neutral

51 2 3 4

“Final” and “Precise”Electrical Neutral Setting

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What if….Throw is not a whole number ?

• A few DC Machines have armaturewindings that result in a throw that is not awhole number, i.e. not evenly divisible

• In this situation it is necessary to obtain twosets of data and subsequently plot two lines(see following Data Sheet)

– 1 plus throw less ½

– 1 plus throw plus ½

• Electrical neutral will be ½ the distancebetween the two parallel plotted lines

• Q/A, if plotted correctly, the two lines will beperfectly parallel to each other

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Data Sheet for

Multiple Data

Two Sets of Dataare Necessary:

The data sheet allowsfor 2 sets of data

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Conclusion…

• Extremely accurate, combined with Q/C

• Safe…eliminates PVN and the inherent danger ofworking on generator while rotating

• Serves both Generators and Motors

• Safe…energized with GFCI protected 120 Vac

• Eliminates brush seating as potential error

• Fast…performed with minimal down time

• Does not require un-coupling the motor

• Very effective procedure in correcting loop currentimbalances

The AC Plotting Method for Setting Electrical Neutral is:

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Questions?

Thank You

God Blessand