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for a project i must calculate the exact energy that used for start of motor until the motor return to normal work in other words how much energy use for normal work of this motor equals the starting energy ? the motor is 3 phase 20 Ampere 9KW will be different when motor is under load ?

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    \$\begingroup\$ You have not provided enough information to calculate much of anything. To calculate what you want, you need the motor's performance curves including speed vs. torque, current and input power. You also need detailed information about the load including speed vs torque and moment of inertia reflected to the input shaft. \$\endgroup\$
    – user80875
    Oct 31, 2020 at 16:49
  • \$\begingroup\$ Starting power * start time. What's the starting current (stall current) of your motor? (Hint : not 20A!) and how long does it take to spin up? Actual figure will be less than the product of these numbers : you might make a linear speed (constant acceleration) approximation and guess at half the product. \$\endgroup\$ Oct 31, 2020 at 16:53
  • \$\begingroup\$ You could probably estimate it from the kinetic energy of the attached load mass at full speed, then apply a factor for motor efficiency in the start-up phase. \$\endgroup\$ Oct 31, 2020 at 17:00
  • \$\begingroup\$ @CharlesCowie unfortunately i have no complete details but overestimately could you tell me will 10 second of normal work equals the starting energy ? \$\endgroup\$
    – hossein
    Oct 31, 2020 at 17:03
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    \$\begingroup\$ @hossein: can you estimate how long the motor takes to start? As a guess, I would say that the motor uses twice the rated power during starting.That would mean one second of start time is like 2 seconds of normal work energy assuming the motor uses the rated power for normal work. \$\endgroup\$
    – user80875
    Oct 31, 2020 at 17:31

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Look on the motor's nameplate.

There should be a figure called Locked Rotor Amperage (LRA).

If you're looking for a way to derive this figure based on other data about the motor, don't bother. Motors differ, which is why LRA has to be put on the nameplate.

Needless to say, if LRA cannot be computed from other data, neither can total energy needed for a machine to start up and reach speed. However you can derive the rotational mass and friction drag of the machine, and compute the energy needed to get it to target speed.

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  • \$\begingroup\$ thanks for your useful information , but for now i have no connect to motor's nameplate , then for a guess could i say that one second of motor under load equals three second or 4 second of normal work ? \$\endgroup\$
    – hossein
    Oct 31, 2020 at 17:47

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