ABB REM 610, Technical Reference Manual

Page: 141 / 158

141
6.4. Protecting a contactor controlled motor
Data of the squirrel cage motor stated by the manufacturer:
Rated power, P
nm
900 kW
Rated voltage, U
nm
380 V
Rated current, I
nm
1650 A
Start-up current of the motor 6.0 x I
nm
Two cold starts allowed
Start-up time of the motor 9 s
Safe stall time 21 s
Ambient temperature 50°C
CT current ratio 2000/5 A (relay input = 5 A)
Setting calculations
The protected unit scaling factor is calculated as follows:
2000
1650
5
5
1 212 1 21
A
A
A
A
× = ≈
. .
(16)
For a direct-on-line started motor, p = 50%.
At an ambient temperature of 50°C the internal FLA is 0.9 x FLA of the motor.
Thus, the start-up current of the motor is 6.0/0.9 = 6.67 x the internal FLA.
The safe stall time setting, t
6x
, is calculated or selected from the trip curves at prior
load 1 x FLA. 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 element
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, 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. Thus 48/14 TDLY
is set to 9 s x 1.1
≈
10 s.
As the safe stall time is longer than the start-up time of the motor, no speed switch
will be required.
Motor Protection Relay
Technical Reference Manual - ANSI Version
REM 610 REM 610
1MRS755537