Noise And Leakage Currents; Leakage Currents And Countermeasures - Mitsubishi Electric FR-A720-0.4K Instruction Manual

Noise and leakage currents

3.1 Noise 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 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 (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 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 (7.5K
or less), the external thermal relay is likely to operate unnecessarily because the ratio of the leakage current to the rated
motor current increases.
Line-to-line leakage current data example (200V class)
Motor
Rated Motor
Capacity
Current(A)
(kW)
0.4
0.75
1.5
2.2
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 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 (ground) leakage breaker, use the Mitsubishi earth (ground) leakage breaker designed for
harmonics and surge suppression.
54
Leakage Currents(mA)
Wiring length 50m
1.8 310
3.2 340
5.8 370
8.1 400
440
490
535
*The leakage currents of the 400V class are about twice as large.
MCCB MC
Wiring length 100m
500
530
560
590
630
680
725
Thermal relay
Inverter
Line-to-line static
capacitances
Line-to-line leakage currents path
⋅ Dedicated motor SF-JR 4P
⋅ Carrier frequency: 14.5kHz
2
⋅ Used wire: 2mm
, 4cores
Cabtyre cable
Motor
IM