ABB SPAM 150 C, Техническая информация

Страница: 9 / 24

9
Example 2
Determining
suitable setting
values for a
standard,
contactor
controlled direct
started motor
Data on the motor to be protected:
Type = squirrel cage motor with direct start,
totally enclosed with fan cooling
Rated power P
nm
900 kW
Rated voltage U
nm
380 V
Rated current I
nm
1650 A
Motor directly started, i.e. p = 50%
Starting current I
s
6.0 x I
nm
Starting time ts 9 s
Two starts from cold required
Max. stall time from cold condi.
21 s
Ambient temperature 20
°
C
C.T. current ratio 2000/1A
Network solidly earthed
Earth-fault sensitivity required
20% of I
nm
Calculation of settings:
Due to the ambient temperature <40
°
C, the full
load current (FLC) is increased by 5%:
Thus FLC = 1 .05 x 1650 A = 1733 A
The relay setting I
θ
is then:
I
θ
/ I
n
= = 0.87
The motor is direclty started, i.e. p = 50%.
The setting t
6x
is selected from the time/current
characteristic corresponding to a hot motor con-
dition. First the ratio between the starting cur-
rent and the full load current is calculated :
I
s
/I
θ
= ( 6.0 x 1650 A)/(0.87 x 2000 A) = 5.7
The starting current 6.0 x 1650 x 1/2000
≈
5.0 is
used together with the starting time 9 s + 10 …15%
safety margin = 10 s to set up the startup supervi-
sion, based on I
s
2
x t
s
.
As the start-up time is smaller than the maxi-
mum motor safe stall time of 21 seconds, no
speed switch inhibited stall protection is needed.
A setting t
6x
= 15 s is selected from the hot
curve (see Fig. 5), permitting a starting time
which is a bit longer than the given motor start-
ing time. The corresponding time read out from
the cold curve gives a trip time of about 16
seconds.
The prior alarm setting can be selected e.g.
Θ
a
= 85 ...90 % for a reasonably early alarm.
As the start-up consumes 9s/15s
≈
60% of the
full thermal capacity, the restart inhibit level
Θ
i
must be set at 40% or lower.
The cooling factor k
c
is set at 4...6 beacuse the
motor is a normal, totally enclosed unit, cooled
by a fan on the the rotor shaft.
As the drive is contactor-controlled and the con-
tactor cannot break a high current fault, the
high-set overcurrent unit must be set out of op-
eration.
The earth-fault setting is calculated as:
I
0
/I
n
= 0.2 x 1650 A / 2000 A = 16%.
1.05 x 1650 A x 1 A
2000 A x 1 A
50
10
20
5
1
2
3
4
30
40
1 2 3 4 5 10 6 8 I/I
θ
1.05
t 6x
[s]
120
60
30
20
15
10
5
2.5
Fig. 5. How to find the t
6x
setting from the hot
thermal curve
The trip time for an earth-fault is set at 50 ms
as the network is solidly earthed.
Because the drive is contactor controlled, the
earth-fault unit must be blocked at high cur-
rents. Whenever the phase currents exceed e.g.
6 times the setting I
θ
, the earth-fault unit is
blocked in order not to operate the contactor
at too high current levels. During the high cur-
rent conditions the protection is based on the
backup fuses. The limit for the blocking cur-
rent is set by means of the switches SGF/3 and
SGF/4.
The thermal unit with a set value of p = 50% is
normally used to describe the thermal capacity
of a directly started motor.
The feeding network is solidly earthed and thus
the operation of the earth-fault protection is se-
lected to be tripping.