Electric motor - Locked Rotor Current

[screendyke]

Please explain what is meaned by the "Locked Rotor Current" for a electric motor.

Is it the same as the start current?

The data sheet for a 400V / 15kW / 33A electric motor specifies:

120V D 34.9A

Thanks!

 

[mta2003]

Hi!

The locked rotor current is usually the starting current and is 6 to 7 times higher than the normal rated current.
The formula for calculating LRA is:

LRA = kva per hp X hp X 1000 X 1/k X 1 / line volts

value of "k" is 1 for 1-phase, 2 for 2-phase and
1.732 for 3-phase.
The LRAs are usually standardized and is given by a letter on the name plate.
God bless.
Tahir

 

[screendyke]

Hi! Thanks for your reply!

However what I still don't understand is that on the test report of the motor is stated:

Locked rotor:
120V 34,4A

While normal operation is:
400V 33A

Does it mean that the voltage is lower during locked rotor status and that therefore the (starting) current is lower??

Thanks in advance!

 

[DanDel]

mta2003, unless you're using fault current kVA in your equation, I beieve you will simply be calculating FLA by a round-about method.

 

[edison123]

120 V, 34.4 A indicates that with the rotor locked againstg rotation, a stator voltage of 120 V will result in a stator current of 34.4 A (i.e. near about rated stator load current).

 

[jraef]

Seems like an odd bit of nameplate info. Of what possible use is that information? Why would ayone want to know what the LRA is when a lower voltage is applied? I would want to know the LRA at rated voltage!

"Venditori de oleum-vipera non vigere excordis populi"

 

[buzzp]

New to me too, maybe they are thinking the supply will brown out under a locked rotor condition, I don't know.

 

[timohearn]

I believe your are reviewing the results of a locked rotor test. This is a common test where the manufacurer prevents the rotor from turning (locked rotor) and increases the terminal voltage until the line current is approximately full load current. This test is described in IEEE Standard 112. The results can be used to verify the starting current for the motor at full voltage. For more information I would recommend contacting the original equipment manufacturer.

Best regards

 

[electricpete]

I think it is safe to assume that locked rotor current will vary almost linearly with voltage up to operating voltage.

You can expect your starting current (disregarding the exponentially decaying dc component) will initially be at 400/120 * 34.4 if motor terminal voltage was 400v during start.

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[edison123]

LRC will be linear at 400 V in the ratio 34.4*400/120, as pete says.

 

[mta2003]

Hi DanDel!

Please note that the 'kva per hp'given in the equation is the locked rotor kva, as defined in NEC. The letter / alphabet given on a motor can be used to determine the kva per phase. e.g. Letter 'A' means 0-3.14 kva/hp.
Hope it will clarify.
God bless.

mta2003