# Starting current of an induction motor

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?

• It's useful to know the winding resistances here... – Brian Drummond Aug 6 '15 at 10:23
• @BrianDrummond Ok, so the starting current equals to voltage divided by the resistance of the windings ? – user3052793 Aug 6 '15 at 10:31
• To a first approximation, yes. That's why the lights go dim for a moment! At least that allows you to place an upper bound on starting current. (Other factors : Inductance will reduce starting current somewhat. And you have to consider each winding separately, and remember the phase of the current in each winding is different, so to add them you need a vector sum) – Brian Drummond Aug 6 '15 at 10:35
• I see @BrianDrummond. So if inductance is a given parameter how would I take it into consideration in the formula? – user3052793 Aug 6 '15 at 10:39
• In short : measure it with the rotor locked, and consider it part of the (complex) winding impedance. Or if it's less than 10% or 30% of the resistive component at your power frequency, ignore it. – Brian Drummond Aug 6 '15 at 10:44

## 4 Answers

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%. 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. $$150hp \times 6.3 = 945 kVA$$

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

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

You need to check the spec sheet for the motor. The number you are looking for is called Locked Rotor Amps (LRA).

This number is usually also on the spec plate attached to the motor, along with the voltage, frequency and running current.

For What It's Worth, the LRA current often is NOT directly related to the running current. Two different motors with similar specs can have radically different starting current.

• I don't actually have a data sheet for the motor. It's just like a technical question that I came across. I don't know what is the right formula to calculate the starting motor current. @Dwayne Reid – user3052793 Aug 6 '15 at 4:15

The start current depends on the motor start type.

• DOL direct on line from 5 to 9 times motor current.
• S D star delta 4 times
• soft starter from 2 to 4
• VSD variable speed driver from 0 to I
• Hello Ahmad! Please try to clarify a bit what you mean in the answer, it's pretty hermetic – clabacchio Nov 22 '17 at 10:36

If you don't have the specifications for the actual motor, the best way to estimate the starting and full load current is to look up published specifications for a motor that is similar. The starting current is usually much higher than the full load current, up to 600% and sometimes higher.

• I actually have some numbers for the power and voltage. 15 volts and 132 KW and the current I calculated is 8800 Amps. But I don't know how to calculate the starting current. Or if I need more information about the motor in order to calculate it. @Charles Cowie – user3052793 Aug 6 '15 at 4:21
• @user3052793 those numbers sound fishy. Not totally implausible but quite unlikely. 9 kA of alternating current will require a tube of copper about 100 mm diameter, 10 mm thick, possibly smaller if water cooled. Is this the size of the cables? Is it not perhaps 15 kV at 9 A? – tomnexus Aug 6 '15 at 4:52
• @tomnexus Yes these numbers are just for just acadamic education's sake. But what I want to find is a formula that for the starting current given two parameters; the power and the voltage. – user3052793 Aug 6 '15 at 5:24
• Read above... "the LRA current often is NOT directly related to the running current. Two different motors with similar specs can have radically different starting current." The complete formula would need consider X and R of stator and rotor, coupling through the air gap, and iron saturation. – tomnexus Aug 6 '15 at 10:12
• For installing wiring for motors in the USA and other places that follow USA standards, there are tables listing full-load and maximum locked-rotor current. A 132 kW motor is between 150 and 200 horsepower. The lowest available voltage is 200 volts, 3-phase. For 150 & 200 Hp, FLA is 415 & 552 A and max LRA is 3335 & 4307 A. There may be similar tables for other standards. Based only on power and voltage, you can only survey what is available on the market. – Charles Cowie Aug 6 '15 at 16:45