How to Calculate Locked Rotor Current

I doubt there is a calculation for this, as it depends on the specific motor manufacture. However, the nameplate usually contains a Code Letter that designates a (range of) multiplier that tells how many times larger the LRA (locked rotor amps) is compared to the FLA (full load amps).

The locked rotor current is the same as the starting current. The electric motor cannot draw more current than the starting current or lock rotor current. This is the absolute maximum.

Rule of thumb for starting current or lock rotor current 6 X full load current.

Regards

Using a standard formula I=E/R and the resistance through the windings will get you in the same zip code but will not be accurate as even a locked rotor will have some inductive Reactance. I personally haven't tested this formula, but it should get you close.

http://www.reliance.com/mtr/flaclcmn.htm

Locked Rotor Current (I_L) From Nameplate Data

Three Phase: I L =	577 x HP x KVA/HP E		See: KVA/HP Chart
Single Phase: I L =	1000 x HP x KVA/HP E

EXAMPLE:

Motor nameplate indicates 10 HP, 3 Phase, 460 Volts, Code F. I L = 577 x 10 x (5.6 or 6.29) 460 I L = 70.25 or 78.9 Amperes (possible range

It is normally given in the motor data sheet.

locked rotor current is calculated during design.It depends upon so many factors like rotor bar configuration,Inductance of windings on stator ,Flux density and so on.It is approximately 6 times the Full load current of the motor on DOL starting.The satrting current values are subject to tolerance of 20%.You can peruse any catalogue of motor manufcaturer and see the starting current given in performance chart .

can anyone tell me how to calculate locked rotor current? is there a formula?

eg. i need to find the locked rotor current of a 3phase 45kW 415volt motor.

Hi,

There is no known calculation for lock rotor current. We can, however measure it. One previous CR4 member said it is the maximum starting amps,but I need to explain further.

The lock rotor amps is the current draw of the motor when the motor is switched on and before the rotor starts to turn. This is the stator current but it is called lock rotor current for 2 reasons.

(1) The measured current is at a time (0 sec) ie when the rotor is at standstill,thus the rotor is considered "LOCKED" and thus the term LOCK ROTOR AMPS.

(2) You can mechanically strap down the rotor (LOCK ROTOR),press the start button and measure the stator current.

This lock rotor amps is the highest current draw of the motor because when the rotor is locked impedance is the lowest.

Item 2 above You can use a clip on to measure the Lock rotor amps.

Another method for measuring lock rotor amps is by using a storage oscilloscope and a passive resistor.Let me know if you are interested in this method.

Regards

Let me know the method of measurement.

thankyou all for your help, problem solved.....

Rule of thumb.

Locked rotor is usually 6 times the RLA (run load amperage).

Hi Benocrash.

It is not always correct, the locked motor draws 6 times more current than it's nominal current. This is only an estimation.

Because of the nature of my business, I achieve locked shaft motor tests on the motors, taken for maintenance.

Checks during maintenance is also covering the tests in the matter of to inspect if any section of the squirrel cage is damaged.

How we do is very similar to the short Circuit tests for the transformers.

We have a 1000 VA Variac transformer, of we can adjust the voltage in 0 to 50 volts to achieve the tests.

We apply a certain AC voltage (42 Volts AC in general) across one phase coil of the motor. And a clamp meter on the 42 Volts line. (We adjust the voltage to obtain an easy readible current value.

Because the rotor is not turning, it is considered as a transformer, of which the secondary winding is shorted.

We check the locked motor current by using the clamp meter on each phase winding, to check if all three phase windings are at the same impedance. If yes, there is not any shortage loop inside any winding and the windings are O.K

If you multiply this with the Nominal Voltage/test Voltage ratio, then you find the real locked rotor current rating for the nominal working voltage. ( This is almost the same with the transformer SC calculations)

This test could be only done with low voltage applications without resulting a damage on the motor, as described above.

During the tests, we turn the motor shaft slowly by hand to see if the squirrel cage is homogenous or some interruptions to indicate the aluminium cage have broken somewhere inside the rotor lamination.

Hope above way of calculation is useful for you.

Kindest Regards.

Just clarify also, what is the standard range of motors that used DOL starting and for wye delta starting? And what is the correct reference or formula when calculating the overcurrent protection device, overload device and motor circuit conductor and feeder cable? Please assist, thanks.

<...what is the correct reference or formula when calculating the overcurrent protection device, overload device and motor circuit conductor and feeder cable...>

In this country, the protocol is called British Standard 7671.

just attacth the clamp meter to your wire which is connected to the motors mains now when you starts the motors it shows the starting current its so simple method dont worry about formulass

Good idea

Direct measuring is the best way to go directly to the target.

It depends how rapidly the clamp meter is varying.

Applicable for the motors driving the slow accelerating loads.

Measure the terminal resistance in ohms, and divide it into the applied voltage.