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

R(t) =

S (1

. . . . . . . . . . . . . . . . . . . . . . . . . . ..*.......

e s

(ii) When

. . . ..I.................. (4)