Engineering innovation through Advanced Materials and Technology. A wholly owned subsidiary of The Morgan Crucible Company PLC

Transcription

1 Engineering innovation through Advanced Materials and Technology A wholly owned subsidiary of The Morgan Crucible Company PLC

2 Unraveling the Commutation Mystery

3 DC Machine Basics

4 Machine Adjustments

5 Carbon Brushes

6 Brush Holder Types

7 N g N = Number of Turns g = Air Gap

8 N e i φ Flux φ α N x i

9 SATURATION CURVE Flux in φ Air Gap Air Air Gap Gap Region Region Iron Saturation Region N x I (Ampere F Turns)

10 When Flux or Current Changes N e I F φ e = - N dφ d t e = - L d I F d t

11 Generated Volts Volts = E. M. F. = B x l xv where B = Flux Density ( φ / area) l = Length of the conductor V = Velocity of the conducto N V l I F e

12 Force on Conductor F = B x I A x l where B = Flux Density ( (φ / area) I A = Current l = Length of the conductor V N I F l F R e I A

13 Generator V α B x l x V V α B α I Field

14 Motor Speed V = B x l x V l V α B x RPM RPM α Volts α Volts B I Field Torque F α B x I A x l Torque = Force x Radius Torque α B x I A r B ( Flux Density )

15 Commutation Frame N S Armature Current 0 Armature Rotation North Pole + I A Δ t Δ I A - I A South Pole V = - L d I A d t = - L Δ I A Δ t This is called Reactance Voltage Time

16 Frame N S Armature Rotation ( V ) V = B x l x V V α B x l x RPM V = - L Δ I A Δ t B x l x V = - L Δ I A Δ t B α I A

17 Frame N C φ S Flux due to main field

18 N Frame C φ S φ C Flux due to main field Flux due to armature conductor current

19 N Frame C φ S φ C Flux due to main field Flux due to armature conductor current C φ Net flux due to main field and armature current

20 Pole Face or Compensating Winding N Frame S φ - Cancels Effects of Armature Reaction - Reduces Bar - to - Bar Voltage - Improves Some Output Characteristics

21 Main Pole or Field Pole and Windings Commutator Pole or Interpole and Windings Frame N S Pole Face or Compensating Windings Armature Windings

22 Commutating Pole or Interpole Backgap g o Frame Shims Flux Armature g Frontgap φ Flux α N x I A

23 Brush Neutral Position N Frame S Brush Commutation Zone

24 Adjustments 1. Brush Position 2. Commutating Field Strength

25 Factory Method Field Method

26 Excavator MG Set

27 DC Drops.. Easy to Measure

28 AC Drops.. More Sensitive To Shorted Turns

29 Main Field AC Drop Test Volts Volts Volts Voltages Should Agree Within 15%

30 Typical Data AC Drops - Main Field Pole 12:00 2:00 4:00 6:00 8:00 10:00 Good Coil Volts

31 Typical Data AC Drops - Main Field Pole Good Coil Volts Shorted Coils Volts 12: Bad Coil 2:00 4:00 6: : :

32 Commutating Pole AC Drop Test

33 Typical Data AC Drops - Commutating Field Pole 1:00 3:00 5:00 7:00 9:00 11:00 Good Coil Volts

34 AC Voltage Drops If Voltage Drops Vary More Than 15% Between Coils, Replace Coil With Low Voltage Drop

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

36 Brush Arm

37 Brush Spacing

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

39 Uneven and Tapered Airgaps Pole Frame Armature Pole Frame

40 Airgap Taper Gauge

41 Airgap Measurement

42 MMS 6000

43 Other Things To Check Electrical Connections Vibration

44 Methods to Set Electrical Neutral DC Kick AC Null Reversability (Speed & Voltage) Black Band Brush Potential

45 For additional information on tuning up DC machines see the WMEA web site in Papers, NECP Tuning Up DC Motors and Generators Jun 07

46 Another static method for setting neutral can be found on the WMEA web site in Papers, Setting Neutral via the AC curve method on DC machines Flanders Nov 05

47 Adjustable.015 Commutating Pole Shim Fixed.125 Commutating Pole Shim

48 Adjusting interpole strength in the field is difficult at best and has safety concerns. It is recommended that on machine disassembly, the shims be taped together and the side facing the pole and frame and pole location be marked and the shims be replaced exactly as they came out.

49 Commutating Pole Shim Order Frame Commutating Pole Thin Steel Thin Aluminum 1/8 Inch Aluminum 1/8 Inch Steel

50 Theory of Brush Operation

51

52 Carbon Brushes Transfer Electrical Current from Stationary Parts To Rotating Parts Aids In The Commutation Process

53 Base Carbon

54 Electrographitic Family Commutating Grade Strength Resistivity Ability Life A Increasing Increasing Increasing Increasing B C D E

55 Treatment

56 Treatment Improves Brush Life Improves Filming Provides Low Humidity Protection Allow High Temperature Operation Reduces Copper Drag Contaminated Atmospheres Minimizes Commutator Wear Reduces Friction

57 Electrographitics Wear Rate Untreated Treated Brush Temperature Humidity (Decreasing) (Increasing)

58 Electrographitics Treatment Lubricates Treatment Increases Mechanical Strength Some Treatments Improve Commutation which Lowers Temperatures and Increases Life

59 Friction Brush Temperature 175C Coefficient of Friction

60 Friction Coefficient of Friction Friction is highest at low brush temperatures (light loads) Brush Temperature 175C

61 Friction Other Factors affect friction - Chemical contamination Coefficient of Friction - Low humidity - Ring material (copper, brass, steel) -Treatments can increase or decrease friction Brush Temperature 175C

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

63 Lubrication All surfaces require lubrication to prevent excessive wear. This is very true for carbon, which is abrasive without water or some other ingredient to lubricate the brush/commutator interface. This is normally referred to as the commutator film which is composed of: Copper Oxide 75% Graphite % Contaminants 5% Water High brush temperature and low humidity reduce the amount of water in the film, which then reduces lubrication between the brush and commutator film. Seizure between the brush and commutator film can result, which eventually leads to a raw commutator surface.

64 Humidity Explanation

65 Humidity Relative Humidity- The amount of water vapor in the air as a percentage of what the air could hold at that temperature. This is what the weather man reports

66 Humidity Absolute Humidity- The mass of water vapor per mass of dry air. Absolute humidity is independent of temperature (until you fall below the dew point). Absolute humidity is what brushes care about.

67 Humidity Units Relative Humidity - % Absolute Humidity grains / lb 1 grain = lbs dry air

68 Humidity Dew Point - Temperature at which relative humidity is 100% (saturated). If temperature decreases further moisture will condense out of the air. (dew)

69 % Relative Humidity Absolute Humidity Temperature (Degrees F) Grain / lb Dry Air (20 F to 105 F - 18 C to +41

70 % Relative Humidity Psychometric Chart - 40 F to + 40 F - 40 C to + 4 C Grain / lb Dry Air Temperature (Degrees F) 30

71

72 Absolute Humidity 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 A Wear vs Brush Pressure Wear Rate D B C Brush Pressure Intensity

74 Wear Rate A Wear vs Brush Pressure AB- Spring Pressure too light. Sparking, electrical erosion and high temperature accelerate brush wear. D B C Brush Pressure Intensity

75 Wear vs Brush Pressure A CD Spring pressure too high Wear Rate High abrasive frictional forces accelerate brush wear. D B C Brush Pressure Intensity

76 Wear vs Brush Pressure A Wear Rate BC Balance to achieve minimal wear D B C Brush Pressure Intensity

77 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 Application Range 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

78 Do not sandblast springs or brush holders! Solvent clean

79 Single vs Multiple Wafer F F Rotating Surface Rotating Surface - Limited brush contact points on irregular surfaces - Increased cross resistance - Better brush face contact for better commutation

80 Steel Clip or Rubber Hardtop F F Rotating Surface Rotating Surface - Distributes force evenly if spring is off center - Allows movement between wafers - Absorbs shock and vibration

81 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 R A V R i c i c i c = V R - 2 V CD R A + R B Becomes less as R B increases Use high resistivity grade brush for difficult to commutate machines.

82 I A Load Amps R B V CD R A V R V CD i c R W V R = i c x R A i c + V CD + i c x R B i + c x R W + V CD V R - 2 V CD R A + R B + R W i c = Becomes less with R w adder Use 2 or 3 wafer brush construction for difficult to commutate machines.

83 Radial Holder Rotation

84 Radial Holder Rotation

85 Modified Reaction Holder Rotation

86 Modified Reaction Holder Friction Chatter Rotation

87 Reaction Holder Trailing Rotation

88 Reaction Holder Stubbing or Leading Rotation

89 Reaction Holder Stubbing or Leading Friction Chatter Rotation

90 Reaction Holder Stubbing or Leading Rotation

91 Questions