Toshiba GR-200 Series Instruction Manual page 347

Thermal overload protection
2.28.4
The temperature at the motor stator will rise according to the I
overload protection provides a good protection against stator damage caused by sustained
overloading.
Element of thermal overload relay (Mot-THM)
Thermal state determination
(i)
Mot-THM simulates the change of thermal state using an algorithm; Mot-THM can detect an
overload condition. Thermal state of the stator (θs) is shown in Equation below:
where
θs stator thermal state (percentage), showing allowable thermal capacity
I motor current
I allowed overload current at the stator, defined by the setting [Mot-THM].
AOL
 thermal time constant at the stator, defined by the setting [T-Mot-THM].
s
Thermal state 0% is the cold state, whereas 100% reflects the thermal limit—which is the point
at which no further temperature rise can be tolerated safely and the system should be
disconnected. The thermal limit for any given system is fixed by the I
The Mot-THM relay will issue the operating signal when the value of θs is at 100%.
The period for operating to occur depends not only on the degree of overload, but also on the
degree of load current as far as reaching the overload state. The period of the operating, that
is, depends on whether the overload is applied from 'cold' or 'hot'. Mot-THM has thresholds for
alarm and trip; hence, the two thresholds exist independently.
Operating time
(ii)
The operating time is determined with two equations. Equation (2.28-5) is used to define the
cold state. Equation (2.28-6) is used to define the hot state:
2
��
−��
⁄
��
θs = (1 − �� ) × 100⁡(%)
��
2
��
������
2
��
t = �� ∙ Ln [
��
2
2
�� − ��
������
2
�� − ��
t = �� ∙ Ln [
��
2
2
�� − ��
������
- 328 -
t function. The thermal
2
AOL
]
2
��
]
6F2T0200 (0.15)
(2.28-4)
(setting [Mot-THM]).
(2.28-5)
(2.28-6)
GRE200 (1,2)