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Locked Rotor Protection
Functional Description
The Locked-rotor protection function is a integral part of the thermal model and is used to protect the motor in the event
that the motor fails to start or accelerate after being energized. The heating in the motor during this period of time can
be significantly higher than the heating at rated current, ranging from 10 to 50 times the normal rated heating. The time
that a motor can remain at a standstill after being energized varies with the applied voltage and has an I
When determining the heat in the motor during this period of time, both the negative and positive sequence currents are
used in the equation that approximates the heat generated in a locked rotor condition. The heat can be approximated
by the equation:
I
2
= I
2
+ K I
2
H
1
2
where :
=
the per unit stator positive sequence current;
I
1
K
=
weighting factor for the value of I
sequence current component due to skin effect in the rotor bar; and
=
per unit stator negative sequence current.
I
2
Settings for the LRC (Locked Rotor Current) can be found under the System Parameters. The LRC value is a
multiplier of the Full Load amps (FLA) setting and ranges from 300 to 1200 % of FLA.
The value of K = 6.01 should be used to mimic the thermal model of Eaton's MP3000 and MP4000 motor relays.
EMR-4000
resulting from the disproportionate heating caused by the negative
2
www.eaton.com
IM02602009E
2
T limit.
453
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