To be able to connect, you must use larger special specifications, the inverter outputs higher harmonics, and the leakage current is larger than that of conventional leakage switches, which simply trip, and requires the use of large-scale or special types of leakage switches. Many VFD manuals have special information on the installation and selection of leakage switches, but leakage maintenance can generally be avoided.
There is a large distributed capacitance between the windings and the housing of a general motor. In the case of grid power supply, only 50Hz power frequency voltage is required on the power line. Because of the low frequency, the leakage current through the distributed capacitor is very small. However, when a VFD is used to drive a motor, the output voltage of the VFD is a high-frequency pulse-width modulation (PWM) voltage waveform of a few kHz, and the output voltage is a pulsating voltage that jumps rapidly between 0V and 530V. For the same distributed capacitance of the same motor, the leakage current will be increased by more than 100 times, which is determined by the operating principle of VFD. The VFD is equipped with a ground leakage monitor at the input that can be serviced by a grid-side fuse or a circuit breaker for high-power equipment.
The VFD is able to select a grid-side fuse for maintenance instead of installing a ground leakage monitor at the input, or a circuit breaker for maintenance of high-power equipment. If the leakage switch is adjusted after the device, the following methods can be used:
Method 1: There is generally an adjuster on the leakage maintainer, which can be adjusted;
Method 2: Replace the leakage maintainer with a VFD special leakage maintainer, there are VFD special leakage maintainers on the market.
Method 3: Add the equipment load, that is, the motor load, and the leakage current will not be very large when the VFD starts.
Method 4: Short-circuit leakage maintainer.
Generally, the VFD will not be connected under the leakage maintenance switch, and the leakage maintenance switch will not be installed on the VFD in practical applications.
The specific reasons can be summarized and analyzed from four aspects: leakage maintenance function, VFD maintenance function, VFD operating principle, VFD and leakage maintenance switch.
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