1.6 Precautions for Using Inverters
1-27
Chapter 1 BEFORE USE
(5) Molded case circuit breaker (MCCB)
Install a recommended molded case circuit breaker (MCCB) or an earth leakage circuit breaker
(ELCB) (with overcurrent protection function) in the primary circuit of the inverter to protect wiring.
Since using an MCCB or ELCB with a lager capacity than recommended ones breaks the protective
coordination of the power supply system, be sure to select recommended ones. Also select ones
with short-circuit breaking capacity suitable for the power source impedance.
If no zero-phase current (earth leakage current) detective device such as a ground-fault relay is
installed in the upstream power supply line in order to avoid the entire power supply system's shutdown
undesirable to factory operation, install an earth leakage circuit breaker (ELCB) individually to inverters
to break the individual inverter power supply lines only.
Otherwise, a fire could occur.
(6) Magnetic contactor (MC) in the inverter input (primary) circuit
Avoid frequent ON/OFF operation of the magnetic contactor (MC) in the input circuit; otherwise, the
inverter failure may result. If frequent start/stop of the motor is required, use FWD/REV terminal
signals or the
/ keys on the inverter's keypad.
The frequency of the MC's ON/OFF should not be more than once per 30 minutes. To assure
10-year or longer service life of the inverter, it should not be more than once per hour.
• From the system's safety point of view, it is recommended to employ such a sequence that
shuts down the magnetic contactor (MC) in the inverter input circuit with an alarm output
signal ALM issued on inverter's programmable output terminals. The sequence minimizes
the secondary damage even if the inverter breaks. When the sequence is employed,
connecting the MC's primary power line to the inverter's auxiliary control power input makes
it possible to monitor the inverter's alarm status on the keypad.
• The breakdown of a braking unit or misconnection of an external braking resistor may
trigger that of the inverter's internal parts (e.g., charging resistor). The breakdown of a
braking unit or misconnection of an external braking resistor is possible if a DC link voltage
establishment signal is not issued within three seconds after the MC is switched on. Even in
such cases, a sequence that shuts down the magnetic contactor (MC) in the inverter input
circuit minimizes the impact of failure. For the braking transistor built-in type of inverters,
assign a transistor error output signal DBAL on inverter's programmable output terminals to
switch off the MC in the input circuit.
(7) Magnetic contactor (MC) in the inverter output (secondary) circuit
If a magnetic contactor (MC) is inserted in the inverter's output (secondary) circuit for switching the
motor to a commercial power or for any other purposes, it should be switched on and off when both
the inverter and motor are completely stopped. This prevents the contact point from getting rough
due to a switching arc of the MC. The MC should not be equipped with any main circuit surge killer
(Fuji SZ-ZM, etc.).
Applying a commercial power to the inverter's output circuit breaks the inverter. To avoid it, interlock
the MC on the motor's commercial power line with the one in the inverter output circuit so that they
are not switched ON at the same time.
(8) Surge absorber/surge killer
Do not install any surge absorber or surge killer in the inverter's output (secondary) lines.
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