Electro-Magnetic Interference (Emi) And Leakage Currents - Mitsubishi FR-A846-00023 Instruction Manual

Electro-magnetic interference (EMI) and leakage currents

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.
Suppression technique
• 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. When the wiring length is long (50 m or more) for small-capacity models (FR-A846-00250(7.5K)
or lower), the external thermal relay is likely to operate unnecessarily because the ratio of the leakage current to the rated
motor current increases.
Line-to-line leakage current example
Motor Rated motor
capacity (kW) current (A)
0.4 1.8
0.75 3.2
1.5 5.8
2.2 8.1
3.7 12.8
5.5 19.4
7.5 25.6
Power
supply
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.
70
PRECAUTIONS FOR USE OF THE INVERTER
Leakage current (mA)
Wiring length 50 m Wiring length 100 m
620
680
740
800
880
980
1070
MCCB MC
Inverter
Line-to-line leakage currents path
• Motor: SF-JR 4P
• Carrier frequency: 14.5 kHz
• Cable: 2 mm
1000
• Cabtyre cable
1060
1120
1180
1260
1360
1450
Thermal relay
Motor
Line-to-line static
capacitances
2
, 4 cores
M