Electromagnetic Compatibility (Emc); Leakage Currents And Countermeasures - Mitsubishi Electric FR-F740 EC Instruction Manual

Electromagnetic compatibility (EMC)

3.8 Electromagnetic compatibility (EMC)
3.8.1

Leakage currents and countermeasures

Mains filters, shielded motor cables, the motor, and the inverter itself cause stationary and var-
iable leakage currents to PE. Since its value depends on the 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 measures. Select the earth leakage breaker
according to its rated sensitivity current, independent 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 earth (ground) cable, etc. These leakage currents may operate earth (ground) leak-
age 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 selec-
tion" 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 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.
– Shielded motor cables significantly increase the leakage current to PE (approx. double
the value generated with unshielded motor cables of the same length).
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. When the wiring length is long (50m or
more) for the 400V class small-capacity model (FR-F700-00170 or less), the external thermal
relay is likely to operate unnecessarily because the ratio of the leakage current to the rated motor
current increases.
Example Line-to-line leakage current data example
Dedicated motor: SF-JR 4P
Carrier frequency: 14.5kHz
Used wire: 2.0mm², 4 cores, cab tyre cable
Motor Capacity [kW]
Tab. 3-12: Line-to-line leakage current data example
3 - 44
Rated Motor Current [A]
0.4
0.75
1.5
2.2
3.7
5.5
7.5
Wiring length 50m
1.1
1.9
3.5
4.1
6.4
9.7
12.8
Leakage Currents [mA]
Wiring length 100m
620
680
740
800
880
980
1070
Wiring
1000
1060
1120
1180
1260
1360
1450