Protecting A Contactor Controlled Motor - ABB REM 610 Technical Reference Manual

REM 610
5.2.3.
126
Motor Protection Relay
Technical Reference Manual

Protecting a contactor 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
Two cold starts allowed
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:
2000A 5A
--------------- - × ------ - 1.212 1.21
=
1650A 5A
For a direct-on-line started motor, p = 50%.
At an ambient temperature of 50°C the internal FLC is 0.9 x FLC of the motor. Thus,
the start-up current of the motor is 6.0/0.9 = 6.67 x the internal FLC.
The safe stall time setting, t
load 1 x FLC. A safe stall time setting of 25 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 20 seconds, which will allow two cold starts.
As the operate time when no prior load is shorter than the safe stall time of 21
seconds, no start-up supervision will be required to protect the motor against single
start ups. Still, start-up supervision is recommended in order to shorten the operate
time in case of a locked rotor condition.
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. Thus t
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 9 s/20 s
restart inhibit level, θ
>, should be set to below 55 per cent, e.g. to 50 per cent.
i
The prior alarm level, θ
a
The time constant multiplier, K
900 kW
380 V
1650 A
6.0 x I
nm
9 s
21 s
50°C
2000/5 A (relay input = 5 A)
≈
, is calculated or selected from the trip curves at prior
6x
≈
45% of the thermal capacity of the motor, the
>, is set to 80...90 per cent of the trip level.
, is set to 4...6.
c
1MRS 752263-MUM
≈
> is set to 9 s x 1.1 10 s.
s