35
EMC and leakage currents
3
3 PRECAUTIONS FOR USE OF THE INVERTER
3.1 EMC and leakage currents
3.1.1
Leakage currents and countermeasures
Capacitances exist between the inverter I/O cables, other cables and earth and in the motor, through which a leakage
current flows. Since its value depends on the static 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 circuit breaker according to its rated sensitivity current, independently of the carrier
frequency setting.
(1) 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) leakage circuit breakers and earth leakage relays
unnecessarily.
z
Suppression technique
⋅
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 selection
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).
z
To-earth (ground) 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. The leakage current of the 400V class is larger than
that of the 200V class.
(2) Line-to-line leakage currents
Harmonics of leakage currents flowing in static capacitances 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 (7.5K or less), the external thermal relay is likely to operate unnecessarily because the ratio of the leakage
current to the rated motor current increases.
z
Line-to-line leakage current data example (200V class)
*The leakage currents of the 400V class are about twice as large.
z
Measures
⋅
Use
Pr. 9 Electronic thermal O/L relay
.
⋅
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 selection
makes the sound inoffensive. 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.
z
Installation and selection of moulded case circuit breaker
Install a moulded case circuit breaker (MCCB) on the power receiving side to protect the wiring of the inverter input
side. Select the MCCB according to the inverter input side power factor (which depends on the power supply voltage,
output frequency and load). Especially for a completely electromagnetic MCCB, one of a slightly large capacity must
be selected since its operation characteristic varies with harmonic currents. (Check it in the data of the corresponding
breaker.) As an earth leakage circuit breaker, use the Mitsubishi earth leakage circuit breaker designed for
harmonics and surge suppression.
Motor Capacity
(kW)
Rated Motor
Current(A)
Leakage Currents(mA)
Wiring length 50m
Wiring length 100m
3.7
12.8
440
630
5.5
19.4
490
680
7.5
25.6
535
725
⋅
Motor SF-JR 4P
⋅
Carrier frequency: 14.5kHz
⋅
Used wire: 2mm
2
, 4cores
Cabtyre cable
Power
supply
Thermal relay
Line-to-line static
capacitances
MCCB
MC
Line-to-line leakage currents path
Motor
Inverter
IM
Summary of Contents for FR-A741-11K
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