Toshiba GR-200 Series Instruction Manual page 352

Locked rotor protection
2.28.6
The locked rotor protection is a function for protecting the rotor of the motor as opposed to the
thermal overload protection function that protects the stator of the motor. The locked rotor
protection simulates the temperature rise of the rotor, because the stator and the rotor have
different thermal characteristics.
Element of locked rotor relay (Mot-LKRT)
Thermal state determination
(i)
The stator produces the most of the heat when the motor current is below about 2.5 times the
motor rated current. If the motor current is higher than that, the rotor produces the most of
the heat. Because of this, the locked rotor protection simulates the motor temperature as
follows.
Case where the motor current is less than 2.5 times the motor rated current
The thermal state of the rotor (θr) is used to converge into the thermal state of the stator
(θs) by the heating time constant (τ
motor rated current, the thermal state of the rotor is calculated in Equation (2.28-8):
where
θr thermal state of rotor (percentage), showing allowable thermal capacity
θs thermal state of stator (percentage), showing allowable thermal capacity

thermal heating time constant of the stator, defined by setting [T-Mot-THM]
s
When the motor is being operated, the thermal state of the rotor exponentially rises to a
certain temperature that is determined by the current magnitude. When the motor has
stopped, the rotor temperature decreases to the ambient temperature. Therefore, the
thermal state is calculated in Equation (2.28-9):
where
 thermal heating time constant at the stator, defined by setting [T-Mot-THM-rad]
s2
Case where the motor current is larger than 2.5 times the motor rated current
If the rotor is being locked, rising temperature on the rotor is simulated with heating
). When the motor current is less than 2.5 times the
s
−��
⁄
��
θr = θs (1 − �� )
��
−��
⁄
θr = θs (1 − �� �� )
��2
- 333 -
6F2T0200 (0.15)
(0 < I(t) < 2.5×[Mot-Ir])
(2.28-8)
(2.28-9)
(I(t) ≥ 2.5×[Mot-Ir])
GRE200 (1,2)