Loop Current Balance - Mining Electrical FLORIDA Loop Current Balance and Setting Electrical Neutral...
Transcript of Loop Current Balance - Mining Electrical FLORIDA Loop Current Balance and Setting Electrical Neutral...
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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
21
“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
21
“As Found” Position
12:00 Position Brush
543
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“Desired” Neutral Setting
21
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