Understanding Shaft Current Problems in AC Motors
 

Understanding Shaft Current Problems in AC Motors

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Understanding Shaft Current Problems in AC Motors Understanding Shaft Current Problems in AC Motors Presentation Transcript

  • L&S Electric, Inc. Understanding Shaft Current Problems in AC Motors
  • Shaft Current History
    • Shaft currents are not a new problem.
    • Technical papers written on the subject date back to the 1930s
    • Shaft currents come from multiple sources
      • Electromagnetic Asymmetries
      • Variable Frequency Drives (PWM VFD)
      • Capacitive coupling
  • Shaft Current Damage
    • When shaft currents flow the result is damage to the bearing races and rolling element.
    • The damage can be minimal (frosting) or more severe creating a pattern of fluting on the inner and outer races.
    • Either way, the bearing life is shortened.
  • Severity of Bearing Current Damage
    • The photograph below shows signs of the pitting that can occur.
  • Severity of Bearing Current Damage
    • Severe cases of shaft bearing currents produce a fluting pattern on the inner and outer races.
  • Severity of Bearing Current Damage
    • Fluting on inner race caused by shaft bearing currents.
  • Shaft Currents
    • We refer to these currents as shaft currents as it is the current that causes the damage
    • There is no practical method to measure these currents, so we measure the magnitude of the voltage instead.
    • How much voltage is acceptable?
  • Acceptable Shaft Voltages
    • Most manufacturers use a rule of thumb of 100 mV for ball bearing applications and 200mV for sleeve bearing applications.
    • NEMA MG-1 part 31.4.4.3 suggests a limit of 300 mV measured end to end on the shaft.
  • Measuring Shaft Voltages
    • Test Equipment:
    • Oscilloscope: Fluke Scope meter 199C
    • 600 MHz 2.5GS/s
    • Probe: Fluke VP 210
    • 200 MHz 10 to 1
    Sources : Electro-Static Technology , L&S Electric
  • Electromagnetic Asymmetries
    • Circular residual fluxes resulting from electromagnetic asymmetries in the construction of AC motors can generate voltages end to end in the shaft during operation.
    • 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.
    • This condition primarily exists in larger frame sizes (above 500 frames).
  • Current Path
    • 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.
    Source: : Electro-Static Technology
  • VFD Produced Bearing Currents
    • Variable frequency drives (PWM VFD).
    • VFD rectifies incoming AC voltage.
    • Resulting DC voltage is applied to the DC Bus.
    • The DC is “chopped” into positive and negative pulses to simulate an AC sine wave.
    • The DC pulse width is varied simulating a variable AC sine wave which changes in frequency.
    • The change in frequency allows us to change the motor speed.
  • VFD Produced Bearing Currents
    • Common Mode Voltages
      • A three phase motor operating on true sine wave power is balanced and has a common mode voltage that is always zero.
      • With VFD produced voltages the balance no longer exists.
      • DC is either positive or negative so at any point in time the three phases are ++- or +--
  • Common Mode Voltages
    • Unbalanced line voltage
    • Voltages should sum to zero but don’t if phases are unbalanced.
    • Neutral might reach 20–30 volts with severe unbalance
    Sources : Electro-Static Technology , Marathon Electric
  • Common Mode Voltages
    • VFD voltage is inherently unbalanced.
    • Problem is worse on 460 volt VFD power.
    • Motor neutral reaches 375 volts.
    Sources : Electro-Static Technology , Marathon Electric
  • Capacitive Coupling
    • VFDs are generators of high frequency common mode voltages.
    • Common mode voltages are coupled to the rotor through parasitic capacitances between the stator winding and the rotor.
    • The currents generate circular time varying magnetic fluxes which induce shaft end to end voltages.
  • Capacitive Coupling
    • A static charge builds up over the entire rotor surface.
    • The rotor charge discharges to ground through the bearings and returns to the VFD.
    Sources : Electro-Static Technology , Marathon Electric
  • What effect does this have on the bearings?
    • Rotor charge builds up until it exceeds the insulation level of the bearing’s oil film
    • The stored energy produces an arc as the rotor voltage collapses
    • Current flow is concentrated in a pinpoint sized area where the ball meets the races
    • Such high energy concentration melts the bearing metal creating microscopic pits in the surface of the races
    Sources : Electro-Static Technology , Marathon Electric
  • Factors Affecting Shaft Currents
    • Long cable runs from drive to motor.
    • Poor grounding connections.
    • Higher switching frequencies on the PWM drive.
    • Later designs of VFDs have higher switching frequencies.
  • Solutions for Eliminating Shaft Currents
    • Insulating of bearing housings.
    • Installation of grounding brushes.
    • Use of insulated bearings.
  • Insulating of bearing Housings
    • Both housings need to be insulated to eliminate the effects of capacitive coupling.
    • Housings wear with usage and must be replaced.
    • Cost to insulate housing is high.
    • Insulation can be compromised by contamination, etc.
  • Grounding Brushes
    • Is an effective method to mitigate all types of currents.
    • Some brush designs are good for the life of the motor.
    • Two brushes may be necessary on larger frame motors.
    • Two brushes required to eliminate effects of capacitive coupling.
  • Insulated Bearing Designs
    • Ceramic rolling elements.
    • Bearing life is as good as standard bearings or better.
    • SKF Insocoat bearing.
    • Races are coated with an insulating material.
    • Rolling element same as standard bearing.
    • Cost for these types of bearings can be as much as double that of standard bearings.
  • Comparison of Solutions
    • Comparison of Solutions
    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
  • L&S Electric: we continually work hard to keep you running!