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Thread: Motor Starting Current

  1. #1
    Join Date
    Dec 2006
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    Motor Starting Current

    Is there is quick simple way to calculate the starting current of an electric motor?
    Bob Kraemer
    RK Electric/Ohio Licensed Contractor

    "Some people are like Slinkies. They have no practical use whatsoever, but they still bring a smile to your face when you push them down a flight of stairs."

  2. #2
    Join Date
    Feb 2003
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    Typical starting currents range from 5 to 7 times the FLC or rated current of the motor. Locked-Rotor current can be calculated per Section 430.7(B) of the NEC.
    Bryan P. Holland, MCP

  3. #3
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    Thanks Bryan
    Bob Kraemer
    RK Electric/Ohio Licensed Contractor

    "Some people are like Slinkies. They have no practical use whatsoever, but they still bring a smile to your face when you push them down a flight of stairs."

  4. #4
    Join Date
    Jan 2010
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    Here is an example from a book.

    Motor starting current/code letters:

    Motor branch circuit short-circuit and ground fault protection is based upon the type of motor and the Code letter found on the motor's nameplate.
    The higher the code letter, the higher the starting current.

    NEC Table 430.7(B) lists Code letters for electric motors. Because this table is based upon kVA per horsepower, it is easy to calculate the range of the motor's starting current in ampere.


    Ex: Calculate the starting current for a 240 volt, three phase. Code letter J. 7.5 hp motor.

    1- The minimum starting current would be:

    I = (kVA x 1000) / ( E x 1.732) = ( 7.1 x 7.5 x 1000 )/(240 x 1.732) = 128 ampere


    2- The maximum starting current would be

    I = (kVA x 1000)/(E x 1.732) = (7.99 x 7.5 x 1000) / (240 x 1.732) = 144 ampere.


    Here is another example which has been calculated by someone in other forum. I am not sure regarding this. It is as below



    I have a 90 kW 380V 50 Hz induction motor directly coupled with a refrigeration compressor. Starter is star/delta and full load current is 160A. I found the following data on the motor data sheet.



    I need others comment regarding this calculation.

    IA/IN = 7.5
    MA/MN = 2.4
    MK/MN = 3
    Mass moment of inertia = 0.52 kgm2
    "Imagination is more important than knowledge."

  5. #5
    Join Date
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    Quote Originally Posted by Hameedulla-Ekhlas View Post
    Here is an example from a book.




    Here is another example which has been calculated by someone in other forum. I am not sure regarding this. It is as below



    I have a 90 kW 380V 50 Hz induction motor directly coupled with a refrigeration compressor. Starter is star/delta and full load current is 160A. I found the following data on the motor data sheet.



    I need others comment regarding this calculation.

    IA/IN = 7.5
    MA/MN = 2.4
    MK/MN = 3
    Mass moment of inertia = 0.52 kgm2
    here his calculation as above data given.
    Your starting current is:

    160*7.5/1.73=693.6 A peak
    "Imagination is more important than knowledge."

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