Toshiba S2E21 User Manual
Page 62
T O S H I B A
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
of the motor.
In providing protection, the
simulates temperature rises of the
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
and for higher load
currents, the rotor produces most of the heat,
For this reason. the
performs the temperature rise simulation for the
rotor as follows:
(1)
When the load current is less than 2.5 times the rated current
I 2.5 x :
The temperature rise R of the rotor is made to converge into the
temperature rise S of the
by the heating time constant (TR)
(i) When
S:
R(t) =
S (1
. . . . . . . . . . . . . . . . . . . . . . . . . . ..*.......
e s
(ii) When
. . . ..I.................. (4)