How to calculate starting current in a motor

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  • JesseDC
    New Member
    • Mar 2022
    • 2

    #1

    How to calculate starting current in a motor

    Hey all
    How do you calculate the starting current for iec motors, I can't find a clear answer, but nema motor manufacturers have clear guidelines for their motors so there must be some sort of method
  • GCE
    Platinum Member

    • Jun 2017
    • 1473

    #2
    Manufactures specifications/charts will be the only way.

    Each make/size has a different ratio and the reactance will need to be calculated for which you will also require the specifications

    What size motor are you talking about ?

    General start up is 6 times full load as a thumb suck

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    • AndyD
      Diamond Member

      • Jan 2010
      • 4946

      #3
      We always measure the inrush / start current using a suitable clamp meter, unfortunately a cheap clamp meter can't give an accurate measurement of this. The inrush current can vary greatly with voltage so a small motor may have an inrush current of 4 times the run current at 230v but when the same motor starts up after loadshedding when the voltage is 205v it might have a start current 20% higher.

      Manufacturers may publish data on start current ranges but these figures will often be with the motor at full load or zero load. In the real world the actual load will be somewhere in between so I always recommend taking a live measurement.
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