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.5.4 Leakage current and installation of earth leakage circuit breaker
Due to static capacitances existing in the inverter I/O wiring and motor, leakage
currents flow through them. Since their values depend on the static capacitances,
carrier frequency, etc., take the following counter measures.
(1) To-ground leakage currents
Leakage currents may flow not only into the inverter's own line but also into the
other line through the ground cable, etc.
These leakage currents may operate earth leakage circuit breakers and earth
leakage relays unnecessarily.
Counter measures
If the carrier frequency setting is high, decrease the carrier frequency (Pr. 72) of
the inverter.
Note that motor noise increases. Selection of Soft-PWM control (Pr. 70) will make
it unoffending. (Factory setting)
By using earth leakage circuit breakers designed for harmonic and surge
suppression (e.g. Mitsubishi's Progressive Super Series) in the inverter's own line
and other line, operation can be performed with the carrier frequency kept high
(with low noise).
(2) Line-to-line leakage currents
Harmonics of leakage
currents flowing in static
capacities between the
inverter output cables
may operate the external
thermal relay
unnecessarily.
Line-to-Line Leakage Current Path
Inverter
Power
supply
IM
Thermal relay
Line static
capacitances
NFB
Motor
Counter measures
Use the electronic overcurrent protection of the inverter.
Decrease the carrier frequency. Note that motor noise increases. Selection of
Soft-PWM (Pr. 70) makes it unoffending.
To ensure that the motor is protected against line-to-line leakage currents, it is
recommended to use a temperature sensor to directly detect motor temperature.
Installation and selection of no-fuse breaker
On the power receiving side, install a no-fuse breaker (NFB) to protect the primary
wiring of the inverter. Which NFB to choose depends on the power supply side
power factor (which changes with the power supply voltage, output frequency and
load) of the inverter. Especially as the completely electromagnetic type NFB
changes in operational characteristic with harmonic currents, you need to choose
the one of a little larger capacity. (Check the data of the corresponding breaker.)
For the earth leakage circuit breaker, use our product designed for harmonic and
surge suppression (Progressive Super Series). (Refer to page 13 for the
recommended models.)
CAUTION
Choose the NFB type according to the power supply capacity.