General Catalogue 2007 - Ibemo Kazakhstan Ltd

Fig 14
Time in Seconds
10000
6000
4000
3600
2000
1000
600
400
200
100
60
40
20
10
6
4
2
1
0,6
0,4
0,2
0,1
0,04
0,02
0,01
0,004
1 1,5 2 3 4 6 8 10 15 20 30 40
1,13
1,45
Multiples of In
C
S
M
Feeder
circuit
breaker
Remote
starter
Motor
Direct start
Full load Full load Starting Starting
speed current current torque
kW hp rev/min A x FLC x FLC
2800 3.2 6.75 3
1400 3.5 5.5 2.5
1.5 2 900 3.8 4.5 2.2
700 4.3 4.0 2.0
Table 27
Fig 15
Hager Catalogue 2007 • Technical
MCCB Motor Power Circuit Protection
Motor Power Circuit Protection
The selection of the circuit protective device for motor power supply
circuits depends in the first instance on the relative physical position
of the various circuit elements. The feeder circuit breaker in the
switchboard, panelboard or distribution board, the starter with its
contactor and thermal overload relay, with perhaps its own isolator or
short-circuit protective device (SCPD) and of course the motor.
The feeder circuit breaker, which can be a perfectly standard thermal
magnetic breaker, must protect the cable feeding the starter so the
normal selection criteria apply. In addition, however, it must be able
to withstand the inrush and starting currents of the motor without
nuisance tripping. The inrush current, which should not be confused
with the starting current, appears at the instant of switch on and
could be as great as 10 times the full load current (FLC) of the motor,
but with a relatively short pulse time of 20 to 30 milliseconds.
The starting current of a direct on line (DOL) start squirrel cage
motor does vary with the designed speed of the motor - the higher
the speed the higher the starting torque and the starting current as
a ratio of the FLC. However the FLC is inversely proportional to the
design speed of the motor. Table 27 shows typical performance data
for average 1.5kw/2hp three phase squirrel cage motors.
The run-up time can vary between one and fifteen seconds
depending on the surge of the motor and the type of load the motor
is driving.
Clearly then, to accurately select the correct circuit breaker for a
motor power supply circuit it is essential to know the correct FLC, the
starting current and the run-up time. This information is then plotted
against the time/current characteristic curve of the type of circuit
breaker (or fuse) selected.
Example
Select an appropriate feeder circuit breaker to supply a 1.5kw 3
phase motor DOL start. FLC 3.5A, starting current 5.5 x FLC, run-up
time 6 secs. The circuit breaker must be suitable for fitting into a 3
phase MCB Distribution Board.
Starting current: 3.5 x 5.5 = 19.25A for 6 secs
Inrush current : 3.5 x 10 = 35A
Comparing the data against the time/current characteristics of a type
C MCB, Fig 15, we see that at 6 secs the breaker will carry 2 x In
without tripping. Therefore a 10A MCB would carry 20A for 6 secs.
The minimum instantaneous trip for this type C MCB would be 50A.
Therefore the closest protection for this motor feeder circuit would
be a 3 Pole 10A type C MCB. A 10A type D could be used providing
the 100A maximum instantaneous trip was not a problem. The inrush
current would preclude the use of a 10A type B because the
minimum instantaneous trip is only 30A. In this case use the next
size up, i.e. 16A.
ibemo Kazakhstan - 090301 Republic of Kazakhstan, West Kazakhstan Oblast, Aksai, Pramzone, BKKS office complex
Phone: +7 71133 93077 ; Fax: +7 71133 93074 ; E-Mail: info@ibemo-kz.com
3.43
Protection
Devices