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Electro-magnetic interference (EMI) and leakage currents
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PRECAUTIONS FOR USE OF THE INVERTER
3.1 Electro-magnetic interference (EMI) and
leakage currents
3.1.1 Leakage currents and countermeasures
Capacitances exist between the inverter I/O cables, other cables and earth and in the motor, through which a leakage current
flows. Since its value depends on the static capacitances, carrier frequency, etc., low acoustic noise operation at the
increased carrier frequency of the inverter will increase the leakage current. Therefore, take the following countermeasures.
Select the earth leakage current breaker according to its rated sensitivity current, independently of the carrier frequency
setting.
To-earth (ground) leakage currents
Leakage currents may flow not only into the inverter's own line but also into the other lines through the earthing (grounding)
cable, etc. These leakage currents may operate earth leakage circuit breakers and earth leakage relays unnecessarily.
Countermeasures
• If the carrier frequency setting is high, decrease the Pr.72 PWM frequency selection setting.
Note that motor noise increases. Selecting Pr.240 Soft-PWM operation selection makes the sound inoffensive.
• By using earth leakage circuit breakers designed for harmonic and surge suppression in the inverter's own line and other
line, operation can be performed with the carrier frequency kept high (with low noise).
To-earth (ground) leakage currents
• Take caution as long wiring will increase the leakage current. Decreasing the carrier frequency of the inverter reduces 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 inverter output cables may operate the external
thermal relay unnecessarily.
Countermeasures
•Use Pr.9 Electronic thermal O/L relay.
• If the carrier frequency setting is high, decrease the Pr.72 PWM frequency selection setting.
Note that motor noise increases. Selecting Pr.240 Soft-PWM operation selection makes the sound inoffensive.
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 the molded case circuit breaker
Install a molded case circuit breaker (MCCB) on the power receiving side to protect the wiring at the inverter input side.
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
MCCB MC
Line-to-line leakage currents path
Motor
Inverter
M