Mitsubishi Electric FR-A700 Series, Technical Manual

Page: 494 / 550

528
SELECTION
3.6.1 Moulded case circuit breaker
(1) Protective coordination (Breaking
capacity)
The MCCB is used to protect the wiring from damage
caused by overload or short-circuit currents. Install
the MCCB to shut off any accidental current passing
through the inverter input circuit, such as overload or
short-circuit thereby minimizing the influence of the
accident.
Select the MCCB of which breaking capacity is
appropriate for the estimated to the overall
impedance of the power supply. (For full information,
refer to the Mitsubishi no-fuse breaker technical
information)
The master MCCB and inverter MCCB must be fully
coordinated for protection. Should a low-impedance
short circuit occur, for example, if the transistors in
the inverter circuit of the inverter are damaged or the
diodes in the converter circuit are broken, the master
MCCB may be tripped. Hence, it is necessary to
make a pre-check using the operational
characteristic curve.
When the overall impedance of the power supply line
is small, the peak value of the inverter input power
supply increases. Therefore, the current peak value
must be reduced by the current limiting action of the
power-factor improving reactor.
The power-factor improving reactor must be inserted
in the input or DC circuit when the power transformer
is large.
(2) Setting the rated current of inverter
primary MCCB
The MCCB in the inverter primary circuit is used to
protect the inverter primary wiring from overload and
short circuit.
Since the inverter has a converter circuit and a large-
capacity smoothing electrolytic capacitor, it serves as
a capacitor-input rectifier with respect to the power
supply and a pulse-shaped current flows in the
inverter input circuit to charge the capacitor.
The effective value of the inverter input current varies
according to its form factor which is under the
influence of the power supply impedance. Hence, the
input current is not determined only by the load
capacity of the motor and changes under the
influence of the reactance of the power supply and
wiring. The rated currents of the MCCB listed on the
previous page have been selected in relation to the
effective value of the inverter input power supply
found by using the power supply impedance value on
the assumption that the power supply capacity is
about 200kVA to 500kVA, with the influence of the
temperature rise and other factors such as harmonic
components taken into consideration. Select a larger
current rating version when an electro-magnetic type
MCCB is used because its operational characteristic
may change due to harmonic current. By inserting
the power-factor currents is improved and the current
effective value is reduced.