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How to estimate the Startup torque of a pump motor

  1. Jul 13, 2010 #1
    Hi Guys,

    I have a problem about the starting current of a pump's electrical motor.

    I have the working condition of the pump (torque, power, speed, P and Q)

    But how could I calculate the start up current of an IEC standard motor?
    Seems I has to gather a Torque-speed curve from the supplier, right?

    Also, in case it is not provided, how can I estimate the starting current and how to protect my motor to burn (since the starting current is 3 times more!? than the operation current )

    p.s. since it's a a 10" pump, starting with inverter would not an economic choice. Direct on line would be better, I think.

    Thanks in advance.

  2. jcsd
  3. Jul 13, 2010 #2


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    The manufacturer should be able to provide this information to you. However, you can also find it by testing the motor with a locked rotor. You can then find the starting current by multiplying that value by the ratio of the line (rated) voltage vs. the line (locked-rotor test) voltage.

  4. Jul 13, 2010 #3
    Thanks alot. I wonder which manufacturer, the pump or motor.

    Is the equation is "start up current" = "normal current" * [Normal V / Start up V]?

    Also, I wonder is there any method to know how to calculate the current theoritically without perform a test?
    p.s. is that include as a specification of a motor?

    Thanks in advence.
  5. Jul 13, 2010 #4


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    The motor for sure. If it were a NEMA motor the name plate would have a Code Letter designation that could be used to determine the starting current.

    The equation I gave you would be if you tested the motor and had some empirical data. In that case it would be:

    [tex]I_{starting} = I_{lr} \cdot \frac{V_{line,rated}}{V_{line,lr}} [/tex]

    If you don't test it yourself or have the data on the name plate (I'm not sure what IEC motor nameplate have on them off hand) then you will need to know the impedance. You can calculate the starting current using the impedance. Just note that their are two components to the current, the transient and the steady-state. The total can be found (again assuming you know the impedance) with this approximation:

    [tex]i_{lr} = \sqrt{2} \left[ \frac{V_{phase}}{Z_{in}} \right]_{s=1.0} sin(2\pi ft - \theta_z) + A\epsilon^{-(\frac{R}{L})t}[/tex]

    This is obviously in the time domain so the current will be dependent on where the waveform is when the motor starts (i.e. the peak current occurs at a zero voltage crossing).

    Also note that the exponential term on the far right is the transient component.

    This calc is a lot easier if you have the right name plate data...so buy a NEMA motor if you have the choice (unless IEC nameplates have the same type data)!

    Hope this helps.

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