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3
Selection and application
3-5 Motor circuit applications
3-5 Motor circuit applications
3-5-1 Description
Individual or tandem overcurrent protective devices are
installed in motor circuits to provide the motor with overload
and locked rotor protection and to provide the wiring with
overcurrent protection. These protective devices must operate
at or below current-time characteristics for the motor windings
to reach the allowable temperature. Any of the combinations
in Fig. 3-16 would provide adequate protection for actual
motor circuits depending on the motor capacity, operating
characteristics, frequency of operation, switching durability and
short-circuit capacity.
(a) Motor protection MCCB only
(b) Motor protection MCCB plus magnetic contactor
(c) Magnetic motor starter plus line protection MCCB
(d) Magnetic motor starter plus instantaneous trip type MCCB
The MCCBs in (a) and (b) provide both overcurrent and short-
circuit protection. With configurations (c) and (d), the motor
starter provides overload protection while a line protection or
instantaneous trip type MCCB provides short-circuit protection.
Combination (d) acts as a single overcurrent circuit breaker for
one panel.
(1) Motor starting current
Unlike the situation with loads like lamps, starting current and
inrush current exceeding the full load current flow when motor
circuits start up. Therefore, motor circuits need overcurrent
protection devices that will not be tripped by these starting
currents.
(a) Direct-on-line starters (Full voltage starting)
These are some of the problems to be solved when starting a
squirrel-cage induction motor directly-on-line.
1) An asymmetrical current flows at the time the motor starts
due to the symmetrical AC component and DC components.
This causes the instantaneous trip mechanism to operate.
2) The inverse time-delay trip mechanism will operate due to
the longer starting time.
The magnitude of the starting currents (symmetrical AC
component) varies according to the type of motor, outputs,
and the number of poles. However, overcurrents generally
equal to 500% to 800% of the full load current will flow. For
FUJI standard motors, approximately a 600% overload can be
expected.
A few cycles immediately after starting the DC component will
overlap.
The magnitude of the asymmetrical current can be obtained
from the relations given in Fig. 3-14 and 3-15.
These two diagrams are used as follows. For instance, for a
55kW induction motor, the starting power factor cos
M
will be
0.22. The effective value of
D
, including the DC component, is
1.23. Therefore, the asymmetrical currents can be expressed
as follows.
Symmetrical starting current
u
1.23 (effective value). In this
example, assuming that the starting current’s multiplication
factor is 600%, the asymmetrical currents are approx. 750%. If
the factor is 800% the latter is approx. 1000%.
The MCCB’s instantaneous trip value will have to exceed this
value. The starting period of a motor depends on the GD
2
of
the load. Strictly speaking, this must be calculated for each
motor. However, the starting period is generally less than 10
secs. Pump motors require a shorter starting time, while fans
and blowers require a longer time to reach operating speed.
Fig. 3-14 Starting power factor example of induction motors
0.5 1 2 3 4 5 10 20 30 40 50 100 200 300 500
80
70
60
50
40
30
20
10
Motor output (kW)
Star
ting po
w
er f
actor (%)
Fig. 3-15 DC component overlap ratio
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
3.0
2.0
1.0
2.0
1.5
1.0
: Max. asymmetrical instantaneous coefficient
: Max. asymmetrical effective value coefficient
Circuit power factor (cos )
(b) Star-delta starters
Although it takes little time to changeover from star to delta
connection, the inrush current at this time is significant. This
inrush current occurs when voltage higher than the power
supply voltage (1 +
3
1
|
1.58 times in the worst case
scenario) is applied to the motor because of residual voltage
generated in the motor stator winding and by the phase
differential with the power supply voltage when a delta
connection is performed.
The amount of inrush current in the worst case scenario is 1.1
to 1.3 times the starting current
u
1.58, which is direct-on-line
starting. If the starting current momentarily reaches 800% of
the full load current, then the inrush current in the worst case
scenario is 800% of the full load current
u
1.3
u
1.58
|
1700%.
The instantaneous trip device in the MCCB may trip if its
setting is exceeded for even a 1/2 cycle, so an MCCB must be
selected where the instantaneous trip current is higher than
the inrush current described above.