Understanding Shaft Current Problems in AC Motors


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Understanding Shaft Current Problems in AC Motors

  1. 1. L&S Electric, Inc. Understanding Shaft Current Problems in AC Motors
  2. 2. Shaft Current History <ul><li>Shaft currents are not a new problem. </li></ul><ul><li>Technical papers written on the subject date back to the 1930s </li></ul><ul><li>Shaft currents come from multiple sources </li></ul><ul><ul><li>Electromagnetic Asymmetries </li></ul></ul><ul><ul><li>Variable Frequency Drives (PWM VFD) </li></ul></ul><ul><ul><li>Capacitive coupling </li></ul></ul>
  3. 3. Shaft Current Damage <ul><li>When shaft currents flow the result is damage to the bearing races and rolling element. </li></ul><ul><li>The damage can be minimal (frosting) or more severe creating a pattern of fluting on the inner and outer races. </li></ul><ul><li>Either way, the bearing life is shortened. </li></ul>
  4. 4. Severity of Bearing Current Damage <ul><li>The photograph below shows signs of the pitting that can occur. </li></ul>
  5. 5. Severity of Bearing Current Damage <ul><li>Severe cases of shaft bearing currents produce a fluting pattern on the inner and outer races. </li></ul>
  6. 6. Severity of Bearing Current Damage <ul><li>Fluting on inner race caused by shaft bearing currents. </li></ul>
  7. 7. Shaft Currents <ul><li>We refer to these currents as shaft currents as it is the current that causes the damage </li></ul><ul><li>There is no practical method to measure these currents, so we measure the magnitude of the voltage instead. </li></ul><ul><li>How much voltage is acceptable? </li></ul>
  8. 8. Acceptable Shaft Voltages <ul><li>Most manufacturers use a rule of thumb of 100 mV for ball bearing applications and 200mV for sleeve bearing applications. </li></ul><ul><li>NEMA MG-1 part suggests a limit of 300 mV measured end to end on the shaft. </li></ul>
  9. 9. Measuring Shaft Voltages <ul><li>Test Equipment: </li></ul><ul><li>Oscilloscope: Fluke Scope meter 199C </li></ul><ul><li>600 MHz 2.5GS/s </li></ul><ul><li>Probe: Fluke VP 210 </li></ul><ul><li>200 MHz 10 to 1 </li></ul>Sources : Electro-Static Technology , L&S Electric
  10. 10. Electromagnetic Asymmetries <ul><li>Circular residual fluxes resulting from electromagnetic asymmetries in the construction of AC motors can generate voltages end to end in the shaft during operation. </li></ul><ul><li>Examples of electromagnetic asymmetries are gaps in the iron (segmented laminations), uneven air gap, and circulating currents in the parallel circuits of a three phase winding. </li></ul><ul><li>This condition primarily exists in larger frame sizes (above 500 frames). </li></ul>
  11. 11. Current Path <ul><li>The current path is from the motor frame through a bearing to the motor shaft, down the shaft, and through the other bearing back to the motor frame. </li></ul>Source: : Electro-Static Technology
  12. 12. VFD Produced Bearing Currents <ul><li>Variable frequency drives (PWM VFD). </li></ul><ul><li>VFD rectifies incoming AC voltage. </li></ul><ul><li>Resulting DC voltage is applied to the DC Bus. </li></ul><ul><li>The DC is “chopped” into positive and negative pulses to simulate an AC sine wave. </li></ul><ul><li>The DC pulse width is varied simulating a variable AC sine wave which changes in frequency. </li></ul><ul><li>The change in frequency allows us to change the motor speed. </li></ul>
  13. 13. VFD Produced Bearing Currents <ul><li>Common Mode Voltages </li></ul><ul><ul><li>A three phase motor operating on true sine wave power is balanced and has a common mode voltage that is always zero. </li></ul></ul><ul><ul><li>With VFD produced voltages the balance no longer exists. </li></ul></ul><ul><ul><li>DC is either positive or negative so at any point in time the three phases are ++- or +-- </li></ul></ul>
  14. 14. Common Mode Voltages <ul><li>Unbalanced line voltage </li></ul><ul><li>Voltages should sum to zero but don’t if phases are unbalanced. </li></ul><ul><li>Neutral might reach 20–30 volts with severe unbalance </li></ul>Sources : Electro-Static Technology , Marathon Electric
  15. 15. Common Mode Voltages <ul><li>VFD voltage is inherently unbalanced. </li></ul><ul><li>Problem is worse on 460 volt VFD power. </li></ul><ul><li>Motor neutral reaches 375 volts. </li></ul>Sources : Electro-Static Technology , Marathon Electric
  16. 16. Capacitive Coupling <ul><li>VFDs are generators of high frequency common mode voltages. </li></ul><ul><li>Common mode voltages are coupled to the rotor through parasitic capacitances between the stator winding and the rotor. </li></ul><ul><li>The currents generate circular time varying magnetic fluxes which induce shaft end to end voltages. </li></ul>
  17. 17. Capacitive Coupling <ul><li>A static charge builds up over the entire rotor surface. </li></ul><ul><li>The rotor charge discharges to ground through the bearings and returns to the VFD. </li></ul>Sources : Electro-Static Technology , Marathon Electric
  18. 18. What effect does this have on the bearings? <ul><li>Rotor charge builds up until it exceeds the insulation level of the bearing’s oil film </li></ul><ul><li>The stored energy produces an arc as the rotor voltage collapses </li></ul><ul><li>Current flow is concentrated in a pinpoint sized area where the ball meets the races </li></ul><ul><li>Such high energy concentration melts the bearing metal creating microscopic pits in the surface of the races </li></ul>Sources : Electro-Static Technology , Marathon Electric
  19. 19. Factors Affecting Shaft Currents <ul><li>Long cable runs from drive to motor. </li></ul><ul><li>Poor grounding connections. </li></ul><ul><li>Higher switching frequencies on the PWM drive. </li></ul><ul><li>Later designs of VFDs have higher switching frequencies. </li></ul>
  20. 20. Solutions for Eliminating Shaft Currents <ul><li>Insulating of bearing housings. </li></ul><ul><li>Installation of grounding brushes. </li></ul><ul><li>Use of insulated bearings. </li></ul>
  21. 21. Insulating of bearing Housings <ul><li>Both housings need to be insulated to eliminate the effects of capacitive coupling. </li></ul><ul><li>Housings wear with usage and must be replaced. </li></ul><ul><li>Cost to insulate housing is high. </li></ul><ul><li>Insulation can be compromised by contamination, etc. </li></ul>
  22. 22. Grounding Brushes <ul><li>Is an effective method to mitigate all types of currents. </li></ul><ul><li>Some brush designs are good for the life of the motor. </li></ul><ul><li>Two brushes may be necessary on larger frame motors. </li></ul><ul><li>Two brushes required to eliminate effects of capacitive coupling. </li></ul>
  23. 23. Insulated Bearing Designs <ul><li>Ceramic rolling elements. </li></ul><ul><li>Bearing life is as good as standard bearings or better. </li></ul><ul><li>SKF Insocoat bearing. </li></ul><ul><li>Races are coated with an insulating material. </li></ul><ul><li>Rolling element same as standard bearing. </li></ul><ul><li>Cost for these types of bearings can be as much as double that of standard bearings. </li></ul>
  24. 24. Comparison of Solutions <ul><li>Comparison of Solutions </li></ul>Sources : Electro-Static Technology , Marathon Electric No No No No No Yes Maintenance Free Operation No No No No No Yes Effective at any RPM No No No No No Yes Low Lifetime Cost - High Return on Investment No No No No No Yes Contamination Proof No No No No No Yes Easy to install No No No No No Yes Long-term Effectiveness No No No No No Yes Protects Motor and Attached Equipment Conductive Grease Carbon Black Brush Copper or Bronze Metal Brush Ceramic/ Hybrid Bearing Insulating Sleeve AEGIS SGR TM
  25. 25. L&S Electric: we continually work hard to keep you running!