Electric motor - Locked Rotor Current
Electric motor - Locked Rotor Current
(OP)
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!
Is it the same as the start current?
The data sheet for a 400V / 15kW / 33A electric motor specifies:
120V D 34.9A
Thanks!






RE: Electric motor - Locked Rotor Current
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
RE: Electric motor - Locked Rotor Current
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!
RE: Electric motor - Locked Rotor Current
RE: Electric motor - Locked Rotor Current
RE: Electric motor - Locked Rotor Current
"Venditori de oleum-vipera non vigere excordis populi"
RE: Electric motor - Locked Rotor Current
RE: Electric motor - Locked Rotor Current
Best regards
RE: Electric motor - Locked Rotor Current
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.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Electric motor - Locked Rotor Current
RE: Electric motor - Locked Rotor Current
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