Leakage Currents And Countermeasures - Mitsubishi Electric FR-A520-0.4K Instruction Manual

2.3.4

Leakage currents and countermeasures

Leakage currents flow through static capacitances existing in the inverter I/O wiring and motor. Since their
values depend on the static capacitances, carrier frequency, etc., take the following measures.
(1) To-earth (ground) leakage currents
Leakage currents may flow not only into the inverter's own line but also into the other line through the earth
(ground) cable, etc. These leakage currents may operate earth (ground) leakage circuit breakers and earth
(ground) leakage relays unnecessarily.
•
Countermeasures
⋅ Decrease the carrier frequency (Pr. 72) of the inverter.
Note that motor noise increases. Selection of Soft-PWM (Pr. 240) will make it unoffending.
⋅ By using earth(ground) 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 current
⋅ Note that a long wiring length will increase leakage currents. Decrease the carrier frequency of the
inverter to reduce leakage currents.
⋅ Higher motor capacity leads to larger leakage currents. Larger leakage currents occur in 400V class than
in 200V class.
(2) 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 (7.5kW or less), 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 Data Example (200V class)
Motor Capacity Rated Motor
(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
* Leakage current of the 400V class is about twice larger.
•
Countermeasures
⋅ Use the electronic overcurrent protection (Pr. 9) of the inverter.
⋅ Decrease the carrier frequency. Note that motor noise increases. Selection of Soft-PWM (Pr. 240) will
make it unoffending.
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.
Leakage Current (mA)
Wiring length 50m
310
340
370
400
440
490
535
NFB
Power
Inverter
supply
Line-to-Line Leakage Current Path
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INSTALLATION AND WIRING
Wiring length 100m
500
Motor: SF-JR 4P
530
Carrier frequency: 14.5kHz
560
Cable used: 2mm
590
tyre cable
630
680
725
Thermal relay
Motor
IM
Line static capacitances
41
2
2
4-core cab