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EMC and leakage currents
3.1 EMC and leakage currents
3.1.1 Leakage currents and countermeasures
Capacitances exist between the drive unit I/O cables, other cables and earth and in the motor, through which a leakage
current flows. Therefore, take the following measures. Select the earth leakage current breaker according to its rated
sensitivity current, independently of the carrier frequency.
(1) To-earth (ground) leakage currents
Leakage currents may flow not only into the drive unit's own line but also into the other lines through the earthing cable, etc.
These leakage currents may operate earth (ground) leakage circuit breakers and earth leakage relays unnecessarily.
Suppression technique
Use an earth leakage circuit breaker with a weak sensitivity in the high frequency range.
The output current of the drive 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 to-earth stray capacitance.
Use the cables insulated with low dielectric constant material, and perform wiring to make the wiring length between the
drive unit and the motor to be as short as possible.
(2) Line-to-line leakage currents
Harmonics of leakage currents flowing in static capacitances between the drive unit output cables may operate the external
thermal relay unnecessarily.
Suppression technique
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
drive unit and the motor to be as short as possible.
Installation and selection of moulded case circuit breaker
Install a moulded case circuit breaker (MCCB) on the power receiving side to protect the wiring of the drive unit input side.
Select the MCCB according to the drive unit input side power factor (which depends on the power supply voltage, output
frequency and load). Especially for a completely electromagnetic MCCB, one of 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.
Line-to-line leakage currents path
Power
supply
Thermal relay
Line-to-line static
capacitances
MCCB MC
Motor
Drive
unit
M