Electro-magnetic interference (EMI) and leakage currents
60
PRECAUTIONS FOR USE OF THE CONVERTER UNIT
3.1
Electro-magnetic interference (EMI) and
leakage currents
3.1.1
Leakage currents and countermeasures
Capacitances exist between the I/O cables or other cables of the inverter or the converter unit and earth, and in the motor,
through which a leakage current flows. The amount of current leakage depends on the size of the capacitance. Take the
following precautions to prevent current leakage. Earth leakage circuit breakers should be selected based on their rated
current sensitivity.
To-earth (ground) leakage currents
Leakage currents may flow not only into the power system of the inverter and converter unit but also into the other power
systems through the earthing (grounding) cable, etc. These leakage currents may operate earth leakage circuit breakers and
earth leakage relays unnecessarily.
Precautions
• Use an earth leakage circuit breaker with a weak sensitivity in a high frequency range.
The output current of the inverter or the converter unit contains a high-frequency leakage current component, which gives
relatively low impacts to human bodies. The detention level for this high-frequency leakage current component can be set
weaker to prevent unnecessary operations.
• Minimize the stray capacitance to the earth.
Use the cables insulated with low dielectric constant material, and perform wiring to make the wiring length between the
inverter and the motor to be as short as possible.
To-earth (ground) leakage currents
• Take caution as long wiring will increase the leakage current.
• Increasing the motor capacity increases the leakage current.
Line-to-line leakage currents
Harmonics of leakage currents flowing in static capacitances between the output cables of the inverter or the converter unit
may operate the external thermal relay unnecessarily.
Precautions
• Use
Pr.9 Electronic thermal O/L relay
.
• 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.
• Increase the external thermal overload relay setting by the amount of the leakage current.
• Minimize the stray capacitance between the lines.
Use the cables insulated with low dielectric constant material, and perform wiring to make the wiring length between the
inverter and the motor to be as short as possible.
Installation and selection of the molded case circuit breaker
Install a molded case circuit breaker (MCCB) on the power receiving side to protect the wiring at the input side of the
inverter or the converter unit. Select an MCCB according to the inverter input side power factor, which depends on the
power supply voltage, output frequency and load. Especially for a completely electromagnetic MCCB, a slightly large
capacity must be selected since its operation characteristic varies with harmonic currents. (Check it in the data of the
corresponding breaker.)
As an earth leakage current breaker, use the Mitsubishi earth leakage current breaker designed for harmonics and surge
suppression.
Power
supply
Thermal relay
Line-to-line static
capacitances
Line-to-line leakage currents path
Motor
Inverter/
converter
M
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