Three problems in VFD (Variable-frequency Drive) application technology

This article takes Delta KG series VFD (Variable-frequency Drive) engineering application as an example to introduce the anti-interference technology of VFD (Variable-frequency Drive); the technology of preventing false tripping of leakage circuit breakers; and the technology of input and output protection. The technical principles and processing principles are applicable to the engineering application of general VFD (Variable-frequency Drive).

1 Introduction

???With the large-scale engineering application of VFD (Variable-frequency Drive), a large number of engineers of various technical levels and other related technical levels need to master VFD (Variable-frequency Drive) application technology. Such as general electrical workers; such as junior electrical engineers; such as mechanical engineers. At the same time, VFD (Variable-frequency Drive) is more and more applied to various complex engineering environments, and various conventional technologies in books are often difficult to solve special problems in VFD (Variable-frequency Drive) applications. Based on the Delta brand KG series VFD (Variable-frequency Drive), this paper provides a theoretical analysis and design technology for three VFD (Variable-frequency Drive) application problems. The technical principle is also applicable to other VFD (Variable-frequency Drive) engineering application conditions.

2 VFD (Variable-frequency Drive) anti-jamming technology

???The interference problem of VFD (Variable-frequency Drive) has been troubled by many customers. Here, I will introduce some common interferences and their elimination methods:

???2.1 Common Interference Paths

???(1) Air radiation mode. It propagates through the air in the form of electromagnetic waves.

???(2) Line transmission mode. It is mainly spread through the power network.

???(3) Line-to-line induction. Electromagnetic induction generated by inductance or electrostatic induction generated by capacitance propagates by way of induction between lines.

???2.2 Elimination of interference sources

???(1) High-frequency and high-power DC welding machines should be kept away from VFD (Variable-frequency Drive). The grounding of the welding machine itself should be good.

???(2) The on-off contact of the electromagnet should be equipped with an RC surge absorber.

???(3) The contactors installed in the same electric cabinet as the VFD (Variable-frequency Drive) should eliminate inferior products. It is necessary to choose products with low switching noise and good arc extinguishing effect. If necessary, an RC surge absorber should also be installed.

???(4) The impedance of the power supply should be low, so as to avoid the start-up and stop of hundreds of kilowatts of electrical appliances nearby, causing the VFD (Variable-frequency Drive) input voltage to produce too high a sudden sudden change.

???(5) The phase voltage of the power supply should be constant, so as not to cause the VFD (Variable-frequency Drive) with 220V single-phase input to work under the condition of undervoltage or overvoltage.

???(6) For the self-generating system of the user's factory, it is required that the output power supply voltage should not fluctuate. To avoid mutation, to be stable.

???2.3 Common measures for VFD (Variable-frequency Drive) anti-interference

???(1) The E terminal of the VFD (Variable-frequency Drive) should be connected with the control cabinet and the casing of the motor, and should be connected to the safety ground, and the grounding resistance should be less than 100

Ω, can absorb surge interference.

???(2) Install an inductive magnetic ring filter at the input or output of VFD (Variable-frequency Drive). Take the Delta KG series VFD (Variable-frequency Drive) as an example (there are many VFD (Variable-frequency Drive) brand manuals with specifications), flatness and 3-4 turns, help to suppress high-order harmonics (This method is simple and easy, and the price is low). If it is necessary to further enhance the anti-interference effect, an EMC-compliant filter device dedicated to Delta’s VFD (Variable-frequency Drive) can be selected (specifications are provided in the Delta VFD (Variable-frequency Drive) user manual).

???(3) The above-mentioned magnetic ring filter can also be wound on the VFD (Variable-frequency Drive) control signal terminal or the incoming line of the analog signal given terminal according to the site conditions.

???(4) In the electric control cabinet equipped with VFD (Variable-frequency Drive), the power line and signal line should be routed separately, and the metal hose should be well grounded.

???(5) Shielded wires should be used for the analog signal wires, and the single end should be connected to the simulated ground at the VFD (Variable-frequency Drive).

???(6) Interference can also be improved by adjusting the carrier frequency of VFD (Variable-frequency Drive). The lower the frequency, the less interference, but the greater the electromagnetic noise.

???(7) The connection between the RS485 communication port and the upper computer must adopt the transmission mode of photoelectric isolation to improve the anti-interference performance of the communication system.

???(8) The power supply of the external computer or instrument should be separated from the power supply of the VFD (Variable-frequency Drive) power unit, and try to avoid sharing an internal transformer.

???(9) Independent shielding should also be carried out on the disturbed instrumentation equipment. Instruments such as thermostats, PID regulators, PLCs, sensors or transmitters on the market must be equipped with metal shielding shells and connected to security grounds. If necessary, the above-mentioned inductive magnetic ring filter can be installed at the power inlet end of this type of instrument.

3 Technology to prevent misoperation of VFD (Variable-frequency Drive) leakage circuit breaker

???In our daily use, we have seen that a leakage protector is installed in the VFD (Variable-frequency Drive) input circuit, but the leakage circuit breaker often trips after power transmission, and the reason cannot be found. Many people think it is VFD (Variable-frequency Drive) Variable-frequency Drive) quality problem, in fact, there is a reason for this, make an analysis on this problem.

???3.1 Rated current design of leakage circuit breaker

???VFD (Variable-frequency Drive) output is controlled by PWM (Pulse Width Modulation, similar to high-speed switching), so high-frequency leakage current will occur. If you want to install a general leakage circuit breaker on the primary side of VFD (Variable-frequency Drive) For each VFD (Variable-frequency Drive), it is recommended to use a leakage circuit breaker with a sensitivity current of more than 200mA and an action time of 0.1 seconds or more, but there is no guarantee that the leakage circuit breaker will not trip, and it must be considered The following factors can determine the size of the leakage current of the system, and choose