Instruction manual

INSTALLATION AND WIRING
30
2.3.2 Leakage currents and countermeasures
Due to the static capacitance existing in the inverter I/O wiring and motor, leakage currents flow through
them. Since their values depend on the static capacitance, carrier frequency, etc., take the following
measures.
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1
)
To-ground leakage currents
Leakage currents may flow not only into the inverter's own line but also into the other line through the ground
cable, etc. These leakage currents may operate earth leakage circuit breakers and earth 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 leakage circuit breakers designed for harmonics and surges (e.g. Mitsubishi's Progressive
Super Series) in the inverter's own line and other line, operation can be performed with low noise (with
the carrier frequency kept high)
•
••
• To-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 575V class than
in 200V and 400V class.
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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 (164.04 feet) or more) for the
small-capacity model (7.5kW (10HP) 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
Leakage Current (mA)
Motor Capacity
(kW (HP))
Rated Motor
Current (A)
Wiring length 50m
(164.04 feet)
Wiring length 100m
(328.08 feet)
0.4 (1/2) 1.8 930 1500
0.75 (1) 3.2 1020 1590
1.5 (2) 5.8 1110 1680
2.2 (3) 8.1 1200 1770
3.7 (5) 12.8 1320 1890
5.5 (7.5) 19.4 1470 2040
7.5 (10) 25.6 1605 2175
Motor: Mitsubishi Electric
SF-JR 4P
Carrier frequency: 14.5Hz
Cable used: 2mm
2
4-core
cable
IM
NFB
Line-to-Line Leakage Current Path
Power
supply
Inverter
Thermal relay
Line static capacitances
Motor
•
••
• 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.
2