Different Types of Bearing Currents – The Fundamentals (plus)

1. Different Types of Bearing Currents – The Fundamentals (plus) Mike Melfi

2. Different Types of Bearing Currents – The Fundamentals • Background • Bearing Damage / Failure Mechanisms • Causes • Line-Fed • Inverter-Fed • Solutions • Line-Fed • Inverter-Fed • Summary & Conclusions

3. Background Q: Since bearing currents in rotating machinery have been documented for at least 75 years, why is this a contemporary issue?

4. Background Q: Since bearing currents in rotating machinery have been documented for at least 75 years, why is this a contemporary issue? A: Modern PWM inverters create both common mode voltages (CMV) and common mode currents (CMC) which provide new opportunities for current to flow through rotating bearings (along with couplings, gears, etc)

5. Background While the 75 year old sources of bearing currents are well understood and solutions exist, it is important to keep them in mind to avoid resurrecting them in trying to solve the challenges brought on by common mode voltages and currents.

6. Bearing Damage / Failure Mechanisms Individual arc damage spots Fluting in outer race, from prolonged operation after damage from current flow

7. Bearing Damage / Failure Mechanisms Fluting on inner race, from prolonged operation after damage from current flow Fluting in outer race

8. Bearing Damage / Failure Mechanisms • Interrupted current causes melting and “re-hardening” of the race material, creating untempered martensite, which is brittle and prone to fatigue

9. Bearing Damage / Failure Mechanisms Quenched and tempered ball bearing inner ring White Layer, untempered martensite

10. Bearing Damage / Failure Mechanisms • Interrupted current causes melting and “re-hardening” of the race material, creating untempered martensite, which is brittle and prone to fatigue • The normal bearing loads are then capable of breaking off small pieces of this brittle material • Subsequent running on this brittle surface and in the presence of the damage “trash” material creates the “fluting”

11. Bearing Damage / Failure Mechanisms • If the damaged material does not progress to a fluted pattern from subsequent running, two other patterns may be seen • A “frosted” surface may appear, or • A number of “pits” may be visible under high magnification • The verification of current flow as the root cause requires more than visual inspection

12. Sine Wave Bearing Currents • “If it were possible to design a perfectly balanced and symmetrical machine, both theory and practice indicate that no bearing current could exist” - C. T. Pearce, Bearing Currents - Their Origin and Prevention, The Electric Journal, August 1927.

13. Sine Wave Bearing Currents • Alternating flux “linking” the shaft … • Net flux encircling the shaftis typicallydue to asymmetric magnetic properties of stator or rotor core • Bearing current created by transformer action in “single turn” secondary (shaft, bearings, frame)

14. Flux path Shaft Boyd and Kaufman, 1959 Sine Wave Bearing Currents

15. Sine Wave Bearing Currents • Currents flow thru shaft, bearings, endshields, and frame • Axial voltage on shaft can be measured if a bearing is insulated (IEEE Std 112 - 1996) • Small shaft voltage (500 mV) can lead to bearing currents above 20 amps • Bearing damage is more likely to occur in larger machines

16. Common Mode Voltage / Current • Modern PWM drives create switching patterns where instantaneous average voltage to ground is not zero. • Voltage has a rapid change of magnitude with respect to time (dV/dt) • High dV/dt results in capacitively coupled currents from motor windings to ground through several paths I = C x dV/dt

17. Common Mode Voltage / Current PHASE VOLTS CMV

18. Common Mode Voltage / Current PHASE VOLTS CMV

19. Common Mode Voltage / Current PHASE VOLTS CMV

20. Common Mode Voltage / CurrentHigh Frequency Current Paths I = C x dV/dt

21. Common Mode Voltage / CurrentHigh Frequency Current Paths

22. Common Mode Voltage / CurrentHigh Frequency End-End Circulation

23. Common Mode Voltage / CurrentRotor Discharge Current

24. Stator Winding Rotor + + CRF CSF Cb VCM - - Frame Bearing Csr VCM Bearing Voltage : Vb = Csr + Cb + Crf Common Mode Voltage / CurrentHigh Frequency Current Paths – Capacitive Charging of Rotor / Bearing CSR

25. Common Mode Voltage / CurrentRotor Discharge Current

26. Common Mode Voltage / CurrentTransient Frame Voltage Discharge

27. Common Mode Voltage / CurrentBearing Current Relative Magnitude

28. Bearing Current Solutions • Eliminate or reduce common mode voltage / current (Drive design issue) • Create best high frequency ground paths between drive,motor, and load • Electrostatic shielded induction motor • Insulated bearings • Shaft grounding brush

29. Bearing Current SolutionsInsulated Opposite Drive-End Bearing for Circulating Type Currents

30. Bearing Current Solutions - PrecautionsShaft Brush Without Opposite End Insulated Bearing (Larger Motor)(One Bearing at Increased Risk)

31. Bearing Current Solutions - PrecautionsInsulated Opposite Drive-End Bearing and Drive-End Shaft Brush(Bearings in Coupled Equipment Still at Peril)

32. Bearing Current SolutionsInsulated Opposite Drive-End Bearing, Drive-End Shaft Brush, and Coupled Equipment Bond Strap

33. Bearing Current SolutionsTwo Insulated Bearings, Drive-End Shaft Brush, and Coupled Equipment Bond Strap

34. Bearing Current Solutions – Faraday (Electrostatic) Shield • Add grounded conductive layer between stator and rotor • Eliminates stator to rotor coupling • Will not eliminate stator winding to frame coupling • Still need good high frequency ground current path from motor to drive ground

35. Bearing Current SolutionsFaraday Shield to Prevent Rotor Charging / Discharging (Bearings Still at Peril from Transient Frame Voltage Discharge when Shaft is Conductively Coupled to Grounded Equipment)

36. Bearing Current SolutionsFaraday Shield and Coupled Equipment Bond Strap

37. Bearing Current Solutions • Internal, end-end from magnetic asymmetry • Insulate opposite drive-end bearing • Insulate both bearings

38. Bearing Current Solutions • Shaft Extension Current (stray ground current) • Insulate coupling • Insulate bearings • Bond strap from motor to load • Better low impedance ground in cable from inverter to motor

39. Bearing Current Solutions • Discharge of voltage on rotor • Faraday (electrostatic) shield • Shaft brush

40. Measurements (Voltage) Common Mode Voltage Shaft Voltage 250 V/div 12.5 V/div

41. Measurements (Current) Internal End-End Circulation 2 A/div

42. Measurements (Current) Shaft Extension Current(30 Amp Pulse)

43. Measurements • Other than internally-sourced circulating currents, all data is at high frequency • Data tends to be non-repetitive • Oscilloscope triggering technique strongly influences perceived results

44. Conclusions • Current flow in rotating bearings is not new • Common mode voltages and currents from modern inverters can cause current flow through bearings (plus couplings, gears, etc)

45. Conclusions • Corrective actions are dependent upon the particular type of current flow • Transient (high frequency) nature of the voltages and currents imposes different requirements than traditional 60 Hz waveforms

46. Conclusions • Since the sources of the currents as well as the paths are typically outside the machine whose bearings are taking the hit, a thorough understanding of the system is key • Grounding is important, but more in the sense of point to point (low impedance) “bonding” rather than “earthing”

47. Plus ... • Bonus material