Starting current of an induction motor

currentinduction motormotorpower

How to calculate the Starting current of an induction motor if I have the power in kilo watts and the voltage? I have looked up formulas , but I got confused.

I calculated the full load current by dividing the power by the voltage (p=VI). I'm not sure about the full load current calculation as well.

Also I know that the starting motor current is higher than the full load current, is that correct?

Best Answer

There is no way to calculate the Starting Current or Locked Rotor Current (LRA) without more information!

Single-phase or three-phase? NEMA Motor Design B, C or D?

What does academic education's sake mean? A voltage of 15V with a power of 132kW is meaningless for an induction motor. You just can't make up numbers. You are also using \$P = V\ I\$, which is DC power.

You'd be better off looking up a motor nameplate and going from there.

Take a 150hp, 1789rpm, 460V, Design B, Code G, 3-phase induction motor. So rated current is 163A, with a power factor of 0.897 lagging and an efficiency of 96.2%. enter image description here Code G gives you locked rotor kVA on a per hp basis. Locked rotor kVA will allow you to calculate LRA. Code G = 5.6 up to but not including 6.3. Worst case = 6.3. enter image description here $$150hp \times 6.3 = 945 kVA$$

$$ S = \sqrt {3}\ V_{Line}\ I_{Line} $$ $$ I_{Line} = \frac {S} {\sqrt {3}\ V_{Line}} = \frac {945 kVA} {\sqrt {3} \times 460V} = 1,186A $$

LRA will be between 1,102A and < 1,186A vs 163A or 676% to 728% of full-load current.