R
s
combined rotor and stator resistance
i
s
starting current of the motor
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.
Based on the motor characteristics supplied by the manufacturer, this module is
required if the stalling time is shorter than or too close to the starting time. In such
cases, a speed switch must be used to indicate whether a motor is accelerating
during start-up or not.
At motor standstill, the STALL_IND input is active. It indicates that the rotor is not
rotating. When the motor is started, at certain revolution the deactivation of the
STALL_IND by the speed switch indicates that the rotor is rotating. If the input is
not deactivated within
Lock rotor time
, the OPR_STALL output is activated.
The module calculates the duration of the motor in stalling condition, the STALL_RL
output indicating the percent ratio of the start situation and the set value of
Lock
rotor time
. The value is available in the Monitored data view.
The activation of the BLOCK input signal resets the operation time and deactivates
the OPR_STALL output.
Cumulative start-up protection
This module protects the motor from an excessive number of start-ups.
Whenever the motor is started, the latest value of START_TIME is added to the
existing value of T_ST_CNT and the updated cumulative start-up time is available
at T_ST_CNT. If the value of T_ST_CNT is greater than the value of
Cumulative time
Lim
, the LOCK_START output is activated and lockout condition for the restart of
motor is enabled during the time the output is active. The LOCK_START output
remains high until the T_ST_CNT value reduces to a value less than the value of
Protection functions
1MRS757644 H
762 620 series
Technical Manual
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