Leakage Currents And Countermeasures - Mitsubishi Electric FR-A700 Technical Manual

3.5.4

Leakage currents and countermeasures

Due to capacitances existing in the inverter I/O lines and
ground, leakage currents flow through them, in addition to
the motor current.
These leakage currents are determined by the magnitudes
of switching frequency (fc) (carrier frequency) and line-to-
line and to-ground capacitances:
(1) Influence of leakage currents
1) An earth leakage circuit breaker is actuated by to-
ground leakage currents
Compared to a case where a motor is driven by a
commercial power
produced by inverter operation includes more
high-frequency
leakage current in this high frequency band are
higher than the operating current of the earth
leakage circuit breaker, actuating the earth
leakage circuit breaker.
Power
2) External thermal relay is tripped by line-to-line
leakage currents
If the wiring distance on the inverter output side is
long, line-to-line leakage currents
the effective value of the current flowing in the
thermal relay, operating the thermal relay.
Power
(2) Leakage current data
To-ground leakage current example
(Total current measured at 100kHz or less
using FFT.)
Carrier Leakage Current
Frequency (mA)
2kHz
14.5kHz
(running frequency: 60Hz, wiring length:
20m)
(Motor capacity: 3.7kW 4-pole)
* Leakage currents in commercial power
supply operation are approximately 1mA.
supply, leakage current
components and to-ground
NV1
Earth leakage
circuit breaker
NV 2
Earth leakage
circuit breaker
To-ground leakage current paths
may increase
A)
MCCB
FR-BIF
B
Motor
Capacity
90
(kW)
210
0.4
0.75
1.5
2.2
(Carrier 14.5kHz)
(Wire used: 3.5
1) When the carrier frequency increases, the
leakage current of the inverter increases.
2) If the wiring length is large, the line-to-line and to-
ground capacitances increase, increasing the
leakage current.
Therefore, independent of the manufacture and inverter
type, a low acoustic noise inverter using high-carrier
frequency PWM control tends to increase leakage current.
(a) The earth leakage circuit breaker (NV1) is
(b) The earth leakage circuit breaker NV2 or NV1
Inverter
B A
E
A smaller-capacity model whose rated current is
less than several amperes is more liable to be
affected by leakage currents.
A leakage current
potential at the terminal of the radio noise filter
(FR-BIF) but it is not a fault.
Inverter
Thermal relay
Line-to-line leakage current example
Total current measured at 100kHz or less using FFT.
200V Class
Leakage current value
Rated
(A)
current
Wiring
(A)
length 50m length 100m
1.8 0.31
3.2 0.34
5.8 0.37
8.2 0.40
4-core type cable)
524
actuated when leakage current flow through
to-ground capacitances C in paths A) and B)
indicated by dotted lines and exceed the
setting of the earth leakage circuit breaker in
the same line.
in the other line is actuated when leakage
current flows in paths ,
C)
by dotted line.
Motor
C
C
Motor
C
D
may produce a several volts
B)
A
Line-to-line
capacitance
400V Class
Leakage current value
Rated
current
Wiring Wiring
(A)
length 54m
0.50 1.1 0.62
0.53 1.9 0.67
0.56 3.5 0.74
0.59 4.1 0.80
SELECTION
etc. indicated
D), E),
Motor
(A)
Wiring
length 100m
1.00
1.05
1.12
1.18