Checking The Set Safe Stall Time For A Single Start; Restart Inhibit Level, Θ; Prior Alarm Level, Θ; Time Constant Multiplier, K - ABB REM 610 Technical Reference Manual

1MRS 752263-MUM
5.1.2.4.
5.1.2.5.
5.1.2.6.
5.1.2.7.
5.1.3.
5.1.3.1.
Motor protection Relay
Technical Reference Manual

Checking the set safe stall time for a single start

If the safe stall time of the motor is shorter than the operate time when no prior load,
a single motor start up should instead be protected by the start-up supervision.
Restart inhibit level, θ
The restart inhibit level can be calculated as follows:
start up time of the motor

θ ----------------------------------------------------------------------------------------------- -
= 100% –

i
operate time when no prior load
For instance, if the start-up time of the motor is 11 seconds and the calculated
operate time of the thermal protection stage when no prior load 25 seconds, one
motor start up will use 11 s/25 s = 45% of the thermal capacity of the motor.
Therefore, the restart inhibit level must be set to below 100% - 45% = 55%, e.g. to
50%.
Prior alarm level, θ
Tripping due to a beginning overload can be avoided by reducing the load of the
motor at a prior alarm.
The prior alarm level can be set to a level, which will allow the use of the motor's
full thermal capacity without causing a trip due to long-time overload.
Generally, the prior alarm level is set to 80...90 per cent of the trip level.

Time constant multiplier, K

The time constant multiplier, K
and the heating time constant:
τ
cooling
-----------------
K =
c
τ
heating
Generally, the time constant multiplier 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 multiplier is generally set to one.

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.

Start-up supervision based on thermal stress calculation

The start-up current, I >, is set to equal the start-up current of the motor and the
s
start-up time, t >, to approximately ten per cent above the start-up time of the motor
s
in order to leave a safety margin for operation.
i
–
a
c
, is the ratio of the cooling time (at motor standstill)
c
REM 610

×
100% + margin

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