How to Calculate the Starting Current of an Induction Motor?

Induction motor starting on full voltage (known as direct on-line starting) has the undesirable effect of drawing five to ten times or more of their full load current. Usually this starting current persists till the motor reaches close to its synchronous speed (rated speed). Induction motors under starting conditions have extremely low power factor of around 10-30%. The combination of large starting current and poor power factor will cause large voltage drop across the system impedance.

The starting current, (many times the nominal one) is variable depending on the motor design.

Starting-current (also called inrush) is the current drawn by a motor during its runup, or acceleration, to normal operating speed. Its value is in Amperes. Typically it can be from 4 to 8 times the rated current. Ignoring the transients of the first 2 or 3 cycles, then starting-current lasts from the time the motor is first energized (breakaway) until the motor nears full speed, often defined as starting-time.

The generally accepted quick field calculation is 225% of the FLA (Full Load Amps) or if not available RLA (Running Load Amps) for a period of 0.5 to 2 seconds duration.