Others; Leakage Currents And Countermeasures - Mitsubishi Electric F700 Instruction Manual

2.3 Others

2.3.1

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 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 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.
Countermeasures
⋅ 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).
To-earth leakage currents
⋅ Take caution as long wiring will increase the leakage current. Decreasing the carrier frequency of the inverter
reduces the leakage current.
⋅ Increasig the motor capacity increases the leakage current.
(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
Motor Rated Motor
Capacity(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
.
Power
supply
Countermeasures
⋅ 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.
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 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 breaker, use the Mitsubishi earth leakage breaker designed for
harmonics and surge suppression.
Leakage Currents(mA)
Wiring length 50m
620
680
740
800
880
980
1070
MCCB MC
Inverter
⋅ Dedicated motor SF-JR 4P
⋅ Carrier frequency: 14.5kHz
Wiring length 100m
⋅ Used wire: 2mm
1000
Cabtyre cable
1060
1120
1180
1260
1360
1450
Thermal relay
line-to-line static
capacitances
Line-to-line leakage currents path
Others
2
, 4cores
Motor
IM
29
2