Mitsubishi Electric 800 Series Instruction Manual page 79

Checkpoint
When using the electronic bypass
operation, electrical and
mechanical interlocks are
provided between the electronic
bypass contactors MC1 and MC2.
A countermeasure is provided for
power restoration after a power
failure.
A magnetic contactor (MC) is
installed on the inverter's input
side.
The magnetic contactor on the
inverter's output side is properly
handled.
When using a PM motor, a low-
voltage manual contactor is
installed on the inverter's output
side.
An EMI countermeasure is
provided for the frequency setting
signals.
A countermeasure is provided for
an overload operation.
The specifications and rating
match the system requirements.
78
3. Precautions for Use of the Inverter
3.6 Checklist before starting operation
When using a switching circuit as shown below, chattering due to misconfigured
sequence or arc generated at switching may allow undesirable current to flow in
and damage the inverter. Miswiring may also damage the inverter. (Note that a
PM motor cannot be driven by the commercial power supply.)
Power
R/L1
supply
S/L2
T/L3
Inverter
If switching to the commercial power supply operation while a failure such as an
output short circuit has occurred between the magnetic contactor MC2 and the
motor, the damage may further spread. If a failure has occurred between the
MC2 and the motor, a protection circuit such as using the OH signal input must
be provided.
If the machine must not be restarted when power is restored after a power
failure, provide an MC on the inverter's input side and also make up a sequence
which will not switch ON the start signal. If the start signal (start switch) remains
ON after a power failure, the inverter will automatically restart as soon as the
power is restored.
On the inverter's input side, connect an MC for the following purposes:
• To disconnect the inverter from the power supply at activation of a protective
function or at malfunctioning of the driving system (emergency stop, etc.).
• To prevent any accident due to an automatic restart at power restoration after
an inverter stop made by a power failure.
• To separate the inverter from the power supply to ensure safe maintenance
and inspection work.
To use an MC to perform an emergency stop during operation, select the MC
conforming to JEM 1038-AC-3 rated current for the inverter rated input current.
Switch the magnetic contactor between the inverter and motor only when both
the inverter and motor are at a stop.
A PM motor is a synchronous motor with high-performance magnets embedded
inside. High-voltage is generated at the motor terminals while the motor is
running even after the inverter power is turned OFF. In an application, such as
fan and blower, where the motor is driven by the load, a low-voltage manual
contactor must be connected on the inverter's output side, and wiring and
inspection must be performed while the contactor is open. Otherwise you may
get an electric shock.
If electromagnetic noise generated from the inverter causes the frequency
setting signal to fluctuate and the motor rotation speed to be unstable when
changing the motor speed with analog signals, the following countermeasures
are effective:
• Do not run the signal cables and power cables (inverter I/O cables) in parallel
with each other and do not bundle them.
• Run the signal cables as far away as possible from the power cables (inverter
I/O cables).
• Use shielded cables.
• Install a data line filter to signal cable (example: ZCAT3035-1330 by TDK).
When performing frequent starts/stops by the inverter, rise/fall in the
temperature of the transistor element of the inverter will repeat due to a repeated
flow of large current, shortening the life from thermal fatigue. Since thermal
fatigue is related to the amount of current, the life can be increased by reducing
current at locked condition, starting current, etc. Reducing current may extend
the service life but may also cause torque shortage, which leads to a start failure.
Adding a margin to the current can eliminate such a condition. For an induction
motor, use an inverter of a higher capacity (up to two ranks). For a PM motor,
use an inverter and PM motor of higher capacities.
Make sure that the specifications and rating match the system requirements.
Countermeasure
MC1
Interlock
U
V
MC2
W
Undesirable current
Refer to
page
IM
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Check by
user