Application Examples; Protecting A Circuit-Breaker Controlled Motor; Prior Alarm Level, Θ A - ABB REM 610 Technical Reference Manual

REM 610
5.2.
5.2.1.
124
Motor Protection Relay
Technical Reference Manual

Application examples

Protecting a circuit-breaker controlled motor

Data of the squirrel cage motor stated by the manufacturer:
Rated power, P
nm
Rated voltage, U
nm
Rated current, I
nm
Start-up current of the motor
Start-up time of the motor
Safe stall time
Ambient temperature
CT current ratio
Setting calculations
The protected unit scaling factor is calculated as follows:
1000A 5A
--------------- - ------ -
× 1.075 1.08
=
930A 5A
For a direct-on-line started motor, p = 50%.
At an ambient temperature of 40°C the internal FLC equals the FLC of the motor.
Thus the start-up current of the motor is 6.2 x the internal FLC.
The safe stall time setting, t
load 1 x FLC. A safe stall time setting of 30 seconds is selected, permitting a
start-up time slightly longer than the one stated by the motor manufacturer.
By selecting the correct trip curve from the trip curves at no prior load according to
the previously selected or calculated safe stall time setting, the total start-up time of
the motor can be read from the curve. In this case, the thermal protection stage will
trip in approximately 28 seconds, which will allow two cold starts.
However, as the operate time when no prior load is longer than the safe stall time of
nineteen seconds, single motor start ups should instead be protected by the start-up
supervision. The start-up current, I
and the start-up time, t >, to approximately ten per cent above the start-up time of
s
the motor in order to leave a safety margin for operation. Thus t
≈
12 s.
As the safe stall time is longer than the start-up time of the motor, no speed switch
will be required.
As one motor start up uses 11 s/28 s
restart inhibit level, θ
>, should be set to below 61 per cent, e.g. to 55 per cent.
i
The prior alarm level, θ
a
The time constant multiplier, K
4500 kW
3300 V
930 A
6.2 x FLC
11 s
19 s
40°C
1000/5 A (relay input = 5 A)
≈
, is calculated or selected from the trip curves at prior
6x
>, is set to equal the start-up current of the motor
s
≈
39% of the thermal capacity of the motor, the
>, is set to 80...90 per cent of the trip level.
, is set to 4.
c
1MRS 752263-MUM
> is set to 1 s x 1.1
s