ABB RED615 Technical Manual page 531

1YHT530004D05 D
615 series
Technical Manual
the MOT_START output. In this mode of operation, the value of the setting is
in the range of around 100 milliseconds.
• In the "IIt, CB" or "IIt & stall, CB" modes, the purpose of this setting is to
check for the life of the protection scheme after the CB_CLOSED input has
been activated. Based on the values of the phase currents, the completion of
the startup period cannot be judged. So in this mode of operation, the value of
the time delay setting can even be as high as within the range of seconds, for
example around 30 seconds.
The activation of the BLOCK input signal deactivates the MOT_START output.
Thermal stress calculator
Because of the high current surges during the startup period, a thermal stress is
imposed on the rotor. With less air circulation in the ventilation of the rotor before
it reaches its full speed, the situation becomes even worse. Consequently, a long
startup causes a rapid heating of the rotor.
This module calculates the thermal stress developed in the motor during startup.
The heat developed during the starting can be calculated with the formula.
t
2
( )
W = R i ∫ t dt
s s
0
GUID-7A92209F-7451-415B-8C3D-276A6ED4A44B V1 EN
R combined rotor and stator resistance
s
i starting current of the motor
s
t starting time of the motor
This equation is normally represented as the integral of I²t. It is a commonly used
method in protective IEDs to protect the motor from thermal stress during starting.
The advantage of this method over the traditional definite time overcurrent
protection is that when the motor is started with a reduced voltage as in the star-
delta starting method, the starting current is lower. This allows more starting time
for the motor since the module is monitoring the integral of I²t.
The module calculates the accumulated heat continuously and compares it to the
limiting value obtained from the product of the square of the values of the Motor
start-up A and Motor start-up time settings. When the calculated value of the
thermal stress exceeds this limit, the OPR_IIT output is activated.
The module also measures the time START_TIME required by the motor to attain
the rated speed and the relative thermal stress IIT_RL. The values are available in
the monitored data view.
The activation of the BLOCK input signal resets the thermal stress calculator and
deactivates the OPR_IIT output.
Section 4
Protection functions
(Equation 72)
525