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I am looking for the formula, not a rule of thumb. Is there a way to calculate the no-load current?

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    \$\begingroup\$ It depends on the frequency, eddy current losses, leakage inductance, no-load mechanical power due to friction and windage, any saturation artifacts and stator inductance etc.. Have you studied the equivalent circuit? \$\endgroup\$ – Andy aka Sep 10 '19 at 10:49
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    \$\begingroup\$ If only everything in the world could be described with a formula. Understanding how something works is crucial in being able to determine when you can use a formula and when you can't. \$\endgroup\$ – Bimpelrekkie Sep 10 '19 at 11:18
  • \$\begingroup\$ @Andy aka. It even depends on speed. Heavily. \$\endgroup\$ – Janka Sep 10 '19 at 11:38
  • \$\begingroup\$ @Janka the OP asked for no-load current and no-load speed can be reasonably estimated. \$\endgroup\$ – Andy aka Sep 10 '19 at 11:50
  • \$\begingroup\$ At steady state. \$\endgroup\$ – Janka Sep 10 '19 at 12:04
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As both Andy aka and Bimpelrekkie have commented, there is no useful formula because there are far too many variables both known and unknown. It depends on the design and type (not all motors are induction) of the motor and how well it has been maintained. By far the easiest way of finding out is to get yourself a clamp on ammeter (readily available from multiple sources) and actually measure it on the particular motor in which you are interested.

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  • \$\begingroup\$ I accept that as an answer. Back on my head I had feeling that the best way is to measure it. I \$\endgroup\$ – Midas Sep 10 '19 at 12:07

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