Commutation Assembly and Adjustment Details Make the Difference Gary Lozowski Morgan Advanced Materials June 13, MEMSA Technical Symposium

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1 Commutation Assembly and Adjustment Details Make the Difference Gary Lozowski Morgan Advanced Materials June 13, MEMSA Technical Symposium

2 COMMUTATION Commutation is the Reversal of Armature Current in the Armature Coils The following slides and comments will assume that the electrical design of the motor or generator is sound and a proven design.

3 COMMUTATION Commutation is the Reversal of Armature Current in the Armature Coils Good Commutation or SPARKLESS operation occurs when the parts are machined, assembled and adjusted correctly Sparking at the brushes occurs when parts are machined, assembled or adjusted incorrectly. Rough comms., weak springs, sandblasted BH or excessive vibration can also cause sparking

4 Main Purpose of a Brush To conduct electrical current from it, to a rotating Commutator or Slip Ring In the case of a DC Motor with a Commutator, it needs to conduct current to each and every commutator segment It needs to have intimate contact with the comm. segments to conduct current without arcing ( Arcing or sparking is current going through air )

5 Good Commutation is a team effort of the brush, commutator, brush holder and springs. It takes proper mechanical adjustment of these parts. Uniform air gap settings of the poles is also very important. Electrical settings like neutral and interpole strength also need to be correct for good operation of DC Motors & Generators.

6 Commutation / Brush Demands Since The Beginning (e.g KW) Rotational Speed 1200 RPM - 20 Rev per Second!

7 Commutation / Brush Demands Since The Beginning (e.g KW) Bars Pass each brush at a rate of 3000 commutator bars per second! And the brushes need to contact every one of them to prevent arcing

8 Commutation / Brush Demands Since The Beginning (e.g KW) Sliding Speed 70 Miles per Hour 6,123 ft / minute Equivalent to 14,000 RPM of a typical car engine piston

9 Commutation / Brush Demands Since The Beginning (e.g KW) Revolutions per day - 1,728,000 ( 630,720,000 / year )

10 Commutation / Brush Demands Since The Beginning (e.g KW) Distance a brush slides in 6 months- 305,000 miles ( only 274,000 miles at 90% running time )

11 Commutation / Brush Demands Since The Beginning (e.g KW) Commutation Cycles (current reversals) 120 per second (6 per revolution)

12 Commutation / Brush Demands Since The Beginning (e.g KW) Current Switched Per Commutation Cycle 733 Amperes at Rated Current 1466 Amperes at Stall Current

13 Commutation / Brush Demands Since The Beginning (e.g KW) Time to Switch the Current seconds 0.84 milli seconds

14 Most Draglines in The USA Have Been Doing This For More Than 25 Years No Small Accomplishment!!

15 What Has Been Done Since The Machines Were New? Larger Buckets Added - Higher RMS Loads - More Heat - Switching Higher Currents Greater % of the Time

16 What Has Been Done Since The Machines Were New? Static Motor Field Exciters Higher Rope Speeds Increased RMS Current Faster Average Motor Speeds

17 What Has Been Done Since The Machines Were New? New Controls - Faster Response - Higher Rate Of Current Change (di / dt)

18 What Has Been Done Since The Machines Were New? - Many Draglines Have Greatly Increased Productivity Since New - Often With Less Maintenance People - And More Pressure for Less Down Time for Maintenance

19 A1 A2 F1 F2 S S N NZ Approx 30% CZ NZ CZ = Commutating Zone N - N NZ = Neutral Zone N S S

20 A1 A2 F1 F2 S S N - N N N S S

21 Straightened-out Connection View Frame Stator Armature Coils S N S N Main Pole Main Pole Main Pole S Commutator Segments A1 Brush Rotation Brush A2

22 Straightened-out Connection View Frame Stator Armature Coils S N S N Main Pole Main Pole Main Pole S Commutator Segments Brush Rotation Brush A2 A1

23 Linear Commutation of a Coil That is Correctly Compensated Graph of Current A I Time - 200A t Armature Coils 200A 200A 200A 200A 200A 200A 200A 200A Rotation Commutator Segments Brush 400A V = - K d I d t This is the Voltage generated as one armature coil is commutated.

24 Machine Adjustments 1. Brush Position ( neutral ) 2. Commutating Field Strength

25 Factory Method vs Field Method

26 Excavator MG Set

27 Methods to Set Electrical Neutral DC Kick AC Null Reversability (Speed & Voltage) Black Band Brush Potential Pencil Volt Neutral AC Curve Method

28 Black Band Set Up B.B. Gen Comm Field Field ARM Load

29 Buck Amps Boost Amps Buck Boost Curve Load Amps (%)

30 Buck Boost Curve Buck Amps Boost Amps 0 X X Band Center on Boost Side (Weak) x No sparking in black area, sparking outside black area x x Band Center x Corrective Action Remove nonmagnetic shims, add magnetic shims x x Load Amps (%)

31 Buck Boost Curve Buck Amps Boost Amps 0 X X x Corrective Action Remove magnetic shims, add non magnetic shims x 150 Band Center x Band Center on Buck Side (Strong) Sparking with no buck or boost x x Load Amps (%) x

32 Buck Amps Boost Amps Buck Boost Curve Near perfect black band centered at all loads X x x x X x x x Load Amps (%)

33 Adjustable.015 Commutating Pole Shim Fixed.125 Thick Commutating Pole Shim GE uses non- Magnetic comm Pole bolts

34 Commutating Pole & Shims Magnet Frame Commutating Pole Thin Steel Thin Aluminum 1/8 Inch Aluminum 1/8 Inch Steel Order of shims is very important!

35 Brush Spacing F A B A = B = C = D = E = F = Max. Spacing Diff. = E D C Target is.030 On Westinghouse Equipment Max. Spacing diff =.050 on GE equip.

36 Brush Arm Spacing Example A F B A = 8.90 B = 8.96 C = 9.02 D = 8.94 E = 9.01 F = 8.85 E D This will take some work to get all 6 measurements within.050 C Max. Spacing Diff. =.170 Target is.050 On GE

37 Brush Arm

38 Brush Spacing Clearance hole Commutator

39 Brush Holder Height

41 BRUSH Brush Holder Spacing BRUSH BRUSH HOLDER This gap between brush holder and commutator is usually between.070 and.093 for most industrial motors. Only on rather small or very large motors is the recommended gap beyond this range. This gap is not always adjustable on newer motor designs. Readjusting this gap should only be necessary after turning or stoning a substantial amount off the commutator or slip ring diameter. Slip ring motors typically have a.125 maximum gap. Too large of a brush holder setting is unstable and can promote a friction chatter condition.

42 Brush Holder Alignment Brush Holder alignment OK, exactly perpendicular to the slip ring surface or commutator surface Commutator or slip ring Rotation

43 Brush Holder alignment, not the best, but 1 or 2 degrees trailing is OK Commutator or slip ring Rotation

44 Brush Holder alignment needs adjustment. This is called leading or stubbing and can cause fast wear or friction chatter. Just 1 or 2 degrees makes a big difference. Commutator or slip ring Rotation

45 Pole Tip Spacing Frame A B Difference Between A and B is 1/8 Maximum

46 Uneven And Tapered Airgaps Pole Frame Armature Airgap Pole Frame Airgaps should be equal within.015

47 Airgap Taper Gauge

48 Airgap Measurement ( Equal within.015 )

49 All This Mechanical & Electrical Symmetry Was Important When The Machines Were New With The Upgrades and Productivity Improvements, it is Even MORE SO!!

50 Circulating currents within single wafer Brush I A Load Amps V R = i c x R A + V CD + i c x R B + V CD R B V CD V CD i c = V R - 2 V CD R A + R B R A V R i c i c Becomes less as R B increases Use high resistivity grade brush for difficult to commutate machines.

51 Reducing circulating currents with more wafers I A Load Amps V R = i c x R A + V CD + i c x R B +V CD + i c x R w R B V CD V CD R A V R i c R W i c = V R - 2 V CD R A + R B + R W i c Becomes less as R B increases Use a 2 or 3 wafer brush construction for difficult to commutate machines.

52 Commutator Repair Indicators Situation Runout ( TIR ) Bar to Bar Variance Undercut Depth NEW Less than.0015 Less than or more IN SERVICE Less than.003 Less than or more NEEDS REPAIR More than.003 More than or less

53 Radial Scale ( +/- 2.0 Mils) 2.0 Profiler MAS - Commutator - Radial Plot Brush Path Brushes Commutator Recording Plot Rec# Recording start (Actual) Size TIR Max BTB Mean BTB Std dev Suspect Hi Mica Remark Brush Path Mar 2003 ( 8:48:43 am) (209) 209 Bars 1.18 mil : : These brushes have an easy job of conducting current to each Commutator segment with 1.18 mils TIR.

54 Radial Scale ( +/- 2.0 Mils) Profiler MAS - Commutator - Radial Plot Track Recording Plot Rec# Recording start (Actual) Size TIR Max BTB Mean BTB Std dev Suspect Hi Mica Remark Track Mar 2002 ( 4:06:18 pm) (211) 211 Bars 3.39 mil : : These brushes will have a harder job trying to contact each Commutator segment with 3.39 mils TIR &.44 Max. BTB

55 Radial Scale ( +/- 2.0 Mils) Profiler MAS - Commutator - Radial Plot Brush Path Recording Plot Rec# Recording start (Actual) Size TIR Max BTB Mean BTB Std dev Suspect Hi Mica Remark Brush Path Jun 2003 ( 7:47:42 am) (213) 213 Bars 3.94 mil : : These brushes have an impossible task of trying to contact each commutator segment with 3.94 mils TIR & 2.32 MBTB

56 Radial Scale ( +/- 2.0 Mils) Profiler MAS - Commutator - Radial Plot 1 * 200 * 20 Brush Path * * * * 140 * * Recording Plot Rec# Recording start (Actual) Size TIR Max BTB Mean BTB Std dev Suspect Hi Mica Remark Brush Path Jun 2003 ( 7:47:42 am) (213) 213 Bars 3.94 mil : : You can hear brush clatter and see sparking when comm. TIR is excessive. Poor brush life is also realized.

58 Advanced Pitch Bar Burning

59 Off Center Commutator Re-surfacing

60 These brushes are from a motor with a very rough comm. Brush wear is different since TIR of each brush path is different.

61 Commutator Undercutting

62 How Much Brush Pressure? The proper amount of brush pressure against the commutator or slip ring depends on the application and/or the brush grade. To calculate the brush pressure; you need to know or measure the spring force and the brush thickness and width. Brush T x W = the cross sectional area. Brushes that contact the commutator at an angle do have more contact area that the product of their T x W, but it is usually not a significant difference so the easier to calculate cross sectional area is used. Recommended Brush PSI Range Application 3.5 to 6 General Industrial Motors 3 to 8 Fractional HP Motors 5 to 10 Traction Motors 3 to 4 Slip Rings Low Speed, Graphite Grade 2 to 2.75 Slip Rings High Speed, Graphite Grade 3 to 5 Metal Graphite Grades

63 Wear Rate Total Brush Wear Optimum Range Brush Pressure ( PSI )

64 Do not sandblast springs or brush holders! Solvent clean

65 Do Not mix brush grades or brush springs

67 Airborne Contaminants 5% Carbon Graphite from the brush 15-20% Carbon Brush Grains of Moisture Copper Oxide 75% Copper Commutator Commutator Film Makeup

68 Electrographitic Brush Materials and treatments Treatment can lubricate ( reduce friction ) Treatment increases Mechanical Strength Some Treatments Improve Commutation which Lowers Temperature And Increases Life Treatment help brushes survive harsh environments

69 Contaminants Gear Lubricants Vapors or mist can contribute to copper drag, insulation degradation and brush wear

70 Blower Inlet Grease splash drawn into Motor and on commutator

71 Copper Drag

72 Thank you for your attention Questions?

Carbon Brush Performance on Slip Rings

Carbon Brush Performance on Slip Rings Richard D. Hall Morgan AM&T Roland P. Roberge Morgan AM&T Gary Lozowski Morgan AM&T Mining Electrical Maintenance and Safety Association September 9, 2010 Clearwater,