Toshiba S2E21 - Page 62

TOSHIBA
6F9EOlll
7.2 Locked Rotor Unit
The Locked Rotor Unit (L.R) has a main function of protecting the rotor of
a motor as opposed to the Overload (OL) which protects the stator of the motor.
In providing protection, the S2E21 simulates temperature rises of the stator
and rotor in a motor independently because each has different thermal
characteristics.
For the load currents that are below about 2.5 times the rated current, the
heat of the motor is produced mainly by the stator, and for higher load
currents, the rotor produces most of the heat,
For this reason. the S2E21 performs the temperature rise simulation for the
rotor as follows:
(1) When the load current is less than 2.5 times the rated current
(0 < I (0 < 2.5 x IN) :
The temperature rise 0 R of the rotor is made to converge into the
temperature rise 8 S of the stator by the heating time constant (TR)
(i) When OR(t) < 0 S:
e R(t) = e S (1 - e-9
. . . . . . . . . . . . . . . . . . . . . . . . . . ..*....... (3)
e
1‘
es
(ii) When OR(t) > 0s:
-_
8 R(t)<TR(O) - es) e-“TR + es . . . ..I.................. (4)
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