Abstract : This paper takes the practical problems of Delta KG series frequency converter engineering applications as examples to introduce the anti-interference technology, leakage circuit breaker maloperation prevention technology, and input/output protection technology of frequency converters. The technical principles and handling principles are applicable to general frequency converter engineering applications. Keywords : Frequency converter, anti-interference, leakage circuit breaker maloperation, frequency converter, input/output protection 1 Introduction With the large-scale engineering application of frequency converters, a large number of engineering personnel at various technical levels and other related technical levels need to master frequency converter application technology. For example, ordinary electrical workers; junior electrical engineers; mechanical engineers. At the same time, frequency converters are increasingly used in various complex engineering environments, and the various conventional technologies in books are often insufficient to solve the special problems in frequency converter applications. Based on Delta brand KG series frequency converters, this paper provides a principle-based analysis and design technology for three frequency converter application problems. The technical principles are also applicable to other frequency converter engineering application conditions. 2. Frequency Converter Anti-interference Technology Interference problems of frequency converters have always troubled many customers. Here are some common interferences and elimination methods: 2.1 Common Interference Pathways (1) Air Radiation. Propagated in the air as electromagnetic waves. (2) Line Propagation. Mainly propagated through the power network. (3) Line Induction. Electromagnetic induction generated by inductance or electrostatic induction generated by capacitance propagates through line induction. 2.2 Elimination of Interference Sources (1) High-frequency, high-power DC welding machines should be kept away from frequency converters. The grounding of the welding machine itself should be good. (2) RC surge absorbers should be installed on the on/off contacts of electromagnets. (3) Inferior products should be eliminated from contactors installed in the same electrical cabinet as the frequency converter. Products with low switching noise and good arc extinguishing effect should be selected. RC surge absorbers should also be installed if necessary. (4) The power supply impedance should be low to avoid the start-up and shutdown of electrical appliances with hundreds of kilowatts nearby, which may cause excessively high instantaneous changes in the input voltage of the frequency converter. (5) The phase voltage of the power supply should be constant to avoid the inverter with 220V single-phase input operating under undervoltage or overvoltage conditions. (6) For the user's self-generated power system, the output power supply voltage should not fluctuate. Sudden changes should be avoided and the voltage should be stable. 2.3 Common measures for inverter anti-interference (1) The E terminal of the inverter should be connected to the control cabinet and the motor casing, and should be connected to a safety ground. The grounding resistance should be less than 100 Ω to absorb surge interference. (2) Install an inductive magnetic ring filter at the input or output terminal of the inverter. Taking Delta KG series inverters as an example (many other inverter brands provide specifications in their user manuals), the filter should be flat and wound 3-4 turns to help suppress high-order harmonics (this method is simple, easy to implement, and inexpensive). If further anti-interference effect is required, a filter device that meets the EMC standard for Delta inverters can be selected (specifications are provided in the Delta inverter user manual). (3) The above-mentioned magnetic ring filter can also be wound on the input line of the inverter control signal terminal or analog signal setpoint terminal according to the site conditions. (4) In the electrical control cabinet with inverter, the power line and signal line should be run separately in conduits, and the metal flexible conduit should be well grounded. (5) The analog signal line should be shielded, and one end should be connected to the analog ground at the inverter. (6) The interference can also be improved by adjusting the carrier frequency of the inverter. The lower the frequency, the less interference, but the greater the electromagnetic noise. (7) The RS485 communication port must be connected to the host computer using opto-isolated transmission 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 inverter's power unit, and the sharing of an internal transformer should be avoided as much as possible. (9) The instruments and equipment subject to interference should also be independently shielded. Instruments such as temperature controllers, PID controllers, PLCs, sensors or transmitters on the market should be equipped with metal shielding shells and connected to the safety ground. If necessary, the above-mentioned inductive magnetic ring filter can be installed at the power input terminal of such instruments. 3. Techniques to Prevent Malfunctions of Inverter Leakage Circuit Breakers In daily use, we encounter situations where a leakage current protection device (RCD) is configured in the input circuit of an inverter. However, the RCD frequently trips after power is supplied, and the cause cannot be found. Many people believe that there is a problem with the inverter's quality. In fact, there are reasons for this. We will analyze this problem. 3.1 Rated Current Design of Leakage Circuit Breakers Inverter output is controlled by PWM (Pulse Width Modulation, similar to high-speed switching), which results in high-frequency leakage current. If a general leakage circuit breaker is installed on the primary side of the inverter, it is recommended to select a leakage circuit breaker with a sensitivity current of 200mA or higher and an operating time of 0.1 seconds or higher for each inverter. However, it is not guaranteed that the leakage circuit breaker will not trip. The following factors must be considered to determine the magnitude of the system leakage current and to select an appropriate leakage circuit breaker and necessary measures to improve the phenomenon of leakage circuit breaker tripping after power is supplied. The formula for selecting the rated current of a general residual current circuit breaker is as follows (see Figure 1): I△n ≧ 10＊〔Ig1+Ign+3＊（Ig2+Igm）〕 Ig1, Ig2: Leakage current of the cable during commercial operation. Ign: Leakage current of the noise filter on the input side of the frequency converter. Igm: Leakage current of the motor during commercial operation. [align=center] Figure 1 Leakage current path analysis[/align] From the relevant variable parameters of the above formula, we know that the factors that affect the magnitude of the leakage current are: (1) Leakage current of the cable (two parts) • Leakage current of the cable length of the residual current circuit breaker filter. • Leakage current of the cable length of the frequency converter motor. (2) Leakage current of the filter (including the frequency converter). (3) Leakage current of the motor. 3.2 Leakage current values ​​of each part (unit: mA) (1) Leakage current of cable = A * (actual cable length / 1000m); the cable manufacturer provides the leakage current value A per 1000m for each wire diameter. (2) Leakage current of filter (including inverter) - provided by the inverter supplier. For example: Delta VFD055B43B uses a 26TDT1W4B4 filter, and its maximum leakage current is 70mA. (3) Leakage current of motor - provided by the motor supplier. 3.2 Design example The inverter is used in a circular knitting machine. The front end uses leakage protection, but it often trips. The analysis is as follows: the inverter power is 5.5KW, and the leakage current of the leakage circuit breaker is 75mA. Based on past experience, under normal circumstances, the impact of cable length and motor leakage current is not significant. The main influencing factors are the leakage current of the filter (including the inverter) and whether the load side is constructed according to the third type of grounding (below 10Ω). Therefore, the following suggestions are made: (1) If a residual current circuit breaker must be installed on the power supply side, it is recommended to select a residual current circuit breaker with a sensitivity current of more than 200mA and an operating time of more than 0.1 seconds. However, it is not guaranteed that the residual current circuit breaker will not trip. It must be effective if the leakage current of other objects (cable length and motor) is within the normal range and the load side is constructed according to the third type of grounding (below 10Ω). (2) If an existing residual current circuit breaker (75mA) must be installed on the power supply side, it is recommended that the input power be directly input to the inverter without passing through the existing filter to reduce the tripping of the existing residual current circuit breaker (75mA) due to the leakage current of the filter (including the inverter). (3) Disconnect the existing residual current circuit breaker (75mA) from the power supply system, and directly input the power to the filter before connecting it to the frequency converter. 4. Frequency Converter Input/Output Protection Technology Frequency converters have powerful protection functions, generally referring to output protection. From a design perspective, protecting the input terminal of the frequency converter remains a challenge. The main issue is the lack of a device that can quickly cut off high voltage and high current while also being cost-effective. Therefore, preventing the impact of high voltage and high current on the frequency converter input terminal is a crucial problem in application. 4.1 Power Supply Voltage Requirements for Inverters (Taking Delta KG series inverters as an example): 230V series single-phase power supply: 200/208/220/230V 50/60Hz; 460V series three-phase power supply: 380/400/415/440/460V 50/60Hz. Voltage: ±10%; Frequency: ±5%. If the input voltage of the Delta KG series 220V and 440V inverters is too low, the inverter will activate undervoltage protection and will not be damaged. If the input voltage of the Delta KG series 220V inverter exceeds 265V or the input voltage of the 440V series exceeds 500V, the DC bus voltage of the inverter will exceed the limit, which may seriously damage the inverter. Therefore, when using inverters in situations with unstable power supply voltage or self-generated power, special attention should be paid to whether the rated voltage of the inverter meets the power supply requirements. 4.2 Input Contactor The input contactor in the Delta inverter manual is a switch that provides input power to the inverter. It must never be used as the start or stop switch of the inverter. Otherwise, it may cause damage to the inverter. 4.3 One inverter output controls multiple motors (1) Multiple motors start and stop synchronously, and accelerate and decelerate at the same frequency. In this application, attention should be paid to power matching. The power of the inverter should not be simply selected to be equal to the sum of the power of multiple motors. The power range of the inverter should be increased. Note! The inverter output should be directly connected to the motor. Relays should not be used in between. (2) Asynchronous start and asynchronous stop of multiple motors are not allowed. Because in this control method, the inverter output must be connected to a relay. So it is not allowed in principle! When starting asynchronously, the first motor will start without any problem. But when the second motor starts, the voltage on the output side of the inverter is very high. At this time, the second motor is equivalent to full voltage start. Its starting current is about 7-8 times its own rated current, which is far beyond the rated current of the inverter. When the first motor stops asynchronously, the inverter output voltage will be very high. At this time, when the relay switches the motor, the inductive load will generate a very high instantaneous reverse voltage, which will far exceed the rated voltage of the internal components of the inverter. The inverter will either alarm due to overvoltage or be damaged due to overvoltage. Asynchronous switching of multiple motors must be done after the previous inverter has stopped before switching to the next inverter can start. 4.4 Delta inverter E grounding wire (1) Neutral wire. The neutral wire is the center line of the generator output. Regardless of whether it is at zero potential at the customer end, the neutral wire should not be used as a ground wire and connected to the E terminal of the inverter! (2) N terminal of the inverter. The N terminal of the inverter is the negative terminal of the DC bus inside the inverter and should be connected to the brake module. It should not be used as a grounding wire terminal, and it should not be connected to the power supply neutral wire by mistake. (3) Safety ground. The E grounding wire of the Delta inverter should be connected to the safety ground, which is the motor casing. Avoid high voltage surge impact and noise interference. 5. Conclusion This paper addresses three unique technical issues in inverter engineering applications, using Delta KG series inverters as examples, and presents a principle-based design method for solving these practical problems. The technical principles are actually applicable to general inverter engineering application conditions.