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Bearing currents in an inverter-fed machine arise from

  • A.common mode voltage driving current through the bearing
  • B.the mechanical unbalance of the rotor at high speed
  • C.the reduced cooling of the machine at low speed
  • D.the harmonic torque produced by the inverter

Answer: A

Switching produces a common mode voltage that appears across the thin lubricant film and discharges through it, pitting the races. Insulated bearings, shaft grounding brushes and common mode chokes are the standard countermeasures.

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