IMM 8001/8002 – Digital Motor Protection Relay
ICE / CEE
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IMU307-1 GB Ind. B
79
5.13. UNDER VOLTAGE PROTECTIONS [27] [27ST]
Function
Voltage drop-off can be detrimental to the motor since they are generally accompanied by an increase in
absorbed currents. Though the motors are protected against overloads by the thermal unit, possible
tripping of the machine is interesting in two cases :
During start periods, where these conditions may result either in the notable extension of the starting
time, or sometimes in motor ‘chattering’.
During operation, where these conditions require power load-shedding of some machines before
restarting to avoid a network collapse, or high inertia machine stop before subsequent complete
restart.
Therefore, the IMM 8000 relays are fitted with a under voltage unit during the starting period [27ST],
and a under voltage unit [27] during motor normal operation.
Main characteristics
Operating levels Ust< , U<
20 – 120 % Un in steps of 1 %Un
Time-delays
t(Ust<),t(U<)
0.1-100 seconds in steps of 0.1 second
Note : The Ust < unit is active during the start period only (time equal to 2tst> or td). The U<
unit is inhibited during this period.
The U< unit is active, motor operating, and after the start period. The Ust< unit is then
inhibited.
