236
1MRS751802-MUM
Protection Relay
Technical Reference Manual, Standard Configurations
REX 521
• For other basic setting parameters, the factory default values can be used as a
starting point. The Gen&Trafo
τ setting applies only if the Device type setting is
set to be a generator or transformer.
11.7.3.2. Start-up supervision (stall protection)
The thermal stress during any single start-up condition is monitored by the start-up
supervision (Is2t n<), which monitors the thermal stress equivalent product I
2
xt.
The Start current setting is set directly to motor starting current value, whereas the
Start time setting is set about 10% above the normal start-up time in order to give a
safety margin for the operation.
For example, if the motor starting current is 6.2 x FLC and the starting time is 11
seconds, the Start current setting = 6.2 and Start time setting =11 s x 1.1
≈ 12
seconds.
Check-up of the need of speed switch
In case the start-up time of the motor is smaller than the maximum stall time for cold
condition, no speed switch information is needed for protection.
In many applications, for example in ExE-type drives, the motor is not allowed to
be in a stalled condition as long as its own start-up time, that is, the maximum stall
time is shorter than the start-up time. To find out whether the motor is speeding up
or not, a speed switch on the motor shaft is used. Refer to Fig. 11.7.1.-2 for an
example of speed switch connection to the inputs of REX 521.
The speed switch information is used to control the stall protection (the operation
mode of Is2t n< is set to “I
2
t & stall”). In case the motor starts accelerating, the speed
switch will inhibit the stall protection trip and leave the protection to the thermal
unit. If the motor does not speed up, tripping will be carried out after the Stall time.
11.7.3.3. Cumulative start-up counter
A motor manufacturer may give a statement of how many times a motor may be
restarted within a certain time interval. The totalling check of start-up time acts as a
backup to the thermal protection by ensuring that restarts cannot be made too
frequently, in other words that the manufacturer recommendations are not exceeded.
The start-up time counting possibility is integrated in the function of Is2t n< so that
no extra timers are needed. To use the start-up time counter, you have to determine
two settings.
Firstly, you have to define the restart inhibit level (Time limit setting) in start
seconds. Secondly, the module has to be told how rapidly the accumulated amount
of start seconds should be counted down.
The equation for restart inhibit level is:
where
n
=required number of starts
t
=starting time of the motor (in seconds)
margin
=safety margin, about 10...20%
()
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