ABB RELION REX640 Technical Manual page 1004

Section 4
Protection functions
998
The activation of the BLOCK input signal deactivates the MOT_START output.
Thermal stress calculator
Because of the high current surges during the start-up 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 start-
up causes a rapid heating of the rotor.
This module calculates the thermal stress developed in the motor during start-up. The
heat developed during the starting can be calculated with the equation.
t
2
W R i t dt
( )
=
∫
s s
0
GUID-002232B6-FE55-4FFD-BD7D-3FCD17D3C388 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 protection relays 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.
Stall protection
This module is activated only when the selected Operation mode setting value is "IIt
& stall" or "IIt & stall, CB".
The start-up current is specific to each motor and depends on the start-up method used,
such as direct online, autotransformer and rotor resistance insertion. The start-up time
is dependent on the load connected to the motor.
1MRS759142 C
(Equation 237)
REX640
Technical Manual