4.10.3
Functionality
The motor start-up supervision function STTPMSU is designed for protection
against excessive starting time and locked rotor conditions of the motor during
starting. For a good and reliable operation of the motor, the thermal stress during
the motor starting is maintained within the allowed limits.
The starting of the motor is supervised by monitoring the TRMS magnitude of all
the phase currents or by monitoring the status of the circuit breaker connected to
the motor.
During the start-up period of the motor, STTPMSU calculates the integral of the I²t
value. If the calculated value exceeds the set value, the operate signal is activated.
STTPMSU has the provision to check the locked rotor condition of the motor using
the speed switch, which means checking if the rotor is able to rotate or not. This
feature operates after a predefined operating time.
STTPMSU also protects the motor from an excessive number of start-ups. Upon
exceeding the specified number of start-ups within certain duration, STTPMSU
blocks further starts. The restart of the motor is also inhibited after each start
and continues to be inhibited for a set duration. When the lock of start of motor is
enabled, STTPMSU gives the time remaining until the restart of the motor.
STTPMSU contains a blocking functionality. It is possible to block function outputs,
timer or the function itself, if desired.
4.10.4
Operation principle
The function can be enabled and disabled with the
Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of STTPMSU can be described with a module diagram. All the
modules in the diagram are explained in the next sections.
Protection functions
1MRS757644 H
758
620 series
Technical Manual
