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Machine Riddle No.33 – Shaft voltage of motor

  • Creator
  • #674
    Sinnadurai Sripadmanaban

      Significance of shaft voltage/current in induction motors …
      How does a voltage or current gets induced in a motor shaft,what are the causes and disadvantages,how to measure the values?

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    • #2385

        Bearing damage resulting from Electrical Discharge Machining (EDM) isn


          Voith , the famous manufacturer of geared and hydraulic couplings , suggests the shaft voltage must be less than 300 mV and NEMA advises less than 1 Volt . These figuers seem somewhat conservative . Would you please explain the reason ?


            It seems the permitted amount of shaft voltage is related to some design criteria which shall be recommended by manufacturers. Normally, voltage levels below 0.5 V will not cause harmful currents. When expected or measured shaft voltage is above 0.5 V, provisions are made to open or break the circuit to prevent current flow. The normal method is to insulate the motor bearing at the end opposite to the coupling end where the motor is coupled to the driven equipment. And, of course, other connections to the motor shaft at the end opposite the drive coupling must also be insulated to maintain the open circuit. Special provisions are necessary in special cases to maintain an open circuit, such as insulated drive couplings in large tandem motor arrangements, or when driving in auxiliary oil pump or other device on the outboard end of the motor.

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