Generally, the prior alarm level is set to 80…90 per cent of the trip level.
6.1.2.7. Time constant dial, K
c
The time constant dial, K
c
, is the ratio of the cooling time (at motor standstill) and
the heating time constant:
K
cooling
heating
c
=
τ
τ
(10)
Generally, the time constant dial of the motor is set to 4...6. However, when
protecting non-rotating objects, e.g. feeder cables or transformers, for instance, the
time constant dial is generally set to one.
6.1.3. Start-up supervision
Start-up supervision is based on thermal stress calculation by default but can also be
set to be based on definite-time overcurrent protection, especially in other than
motor applications.
6.1.3.1. Start-up supervision based on thermal stress calculation
The start-up current, 48/14 PICKUP, is set to equal the start-up current of the motor
and the start-up time, 48/14 TDLY, to approximately ten per cent above the start-up
time of the motor in order to leave a safety margin for operation.
For instance, if the start-up current of the motor is 6.2 x FLA and the start-up time
11 seconds, 48/14 PICKUP = 6.2 and 48/14 TDLY = 11 s x 1.1 = 12 s.
6.1.3.2. Checking the need for speed switch
When protecting motors of ExE-type, for instance, the safe stall time may be shorter
than the start-up time of the motor, which is why a speed switch on the motor shaft
is required to give information on whether the motor is accelerating during motor
start up. In this case, the start-up time is set to slightly below the safe stall time.
The speed switch should be open at standstill and closed during acceleration. When
the input is activated, element (48/14 PICKUP)
2
x (48/14 TDLY)/(48/14) will be
blocked. If the motor does not start to accelerate, element (48/14 PICKUP)
2
x (48/
14 TDLY) will trip when the reference value, (48/14 PICKUP)
2
x (48/14 TDLY), is
exceeded. If start-up supervision is based on overcurrent protection, element 48/14
will trip on expiration of the set operate time
However, if the safe stall time is longer than the start-up time of the motor when no
prior load, a speed switch will not be required.
134
REM 610REM 610
Motor Protection Relay
Technical Reference Manual - ANSI Version
1MRS755537
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