Mitsubishi Electronics FR-A500 ユーザーズマニュアル
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2
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).
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)
* 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.
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.
Motor Capacity
(kW)
Rated Motor
Current (A)
Leakage Current (mA)
Motor: SF-JR 4P
Carrier frequency: 14.5kHz
Cable used: 2mm
2
4-core cab
tyre cable
Wiring length 50m
Wiring length 100m
0.4
1.8
310
500
0.75
3.2
340
530
1.5
5.8
370
560
2.2
8.1
400
590
3.7
12.8
440
630
5.5
19.4
490
680
7.5
25.6
535
725
NFB
Line-to-Line Leakage Current Path
Power
supply
supply
Thermal relay
Line static capacitances
Motor
Inverter
IM