Three topics on inverter application technology

introduction

With the large-scale engineering application of frequency converters, a large number of engineers of various technical levels and other related technical levels need to master the application technology of frequency converters. For example, ordinary electrical workers; for example, junior electrical engineers; for example, mechanical engineers. At the same time, frequency converters are increasingly used in various complex engineering environments. Various conventional technologies in books are often difficult to solve special problems in frequency converter applications. This article is based on Delta brand KG series frequency converters and provides principle analysis and design technology for three frequency converter application problems. The technical principles are actually also applicable to other frequency converter engineering application conditions.

1. Inverter anti-interference technology

The interference problem of frequency converter has been bothering many customers. Here are some common interference and elimination methods to introduce to you:

1.1 Common interference pathways

(1) Aerial radiation: Propagation in the air in the form of electromagnetic waves.

(2) Line transmission mode: mainly transmitted through the power supply network.

(3) Line-to-line induction method. The electromagnetic induction generated by inductance or the electrostatic induction generated by capacitance is transmitted through line-to-line induction.

1.2 Elimination of interference sources

(1) High-frequency and high-power DC welding machines should be kept away from inverters. The welding machine itself should be well grounded.

(2) An RC surge absorber should be installed at the on/off contacts of the electromagnet.

(3) For contactors installed in the same electrical cabinet as the inverter, inferior products should be eliminated. Products with low switching noise and good arc extinguishing effect should be selected. If necessary, RC surge absorbers should also be installed.

(4) The impedance of the power supply should be low to avoid the start and stop of hundreds of kilowatts of electrical appliances nearby, which may cause the inverter input voltage to produce an instantaneous sudden change that is too high.

(5) The phase voltage of the power supply must be constant to avoid causing the inverter with 220V single-phase input to operate in an undervoltage or overvoltage state.

(6) For the user plant's self-generating system, the output power supply voltage should not fluctuate. Sudden changes should be avoided and the system should be stable.

1.3 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 grounded. The grounding resistance should be less than 100Ω to absorb surge interference.

(2) Install an inductive magnetic ring filter at the input or output end of the inverter. Taking Delta KG series inverter as an example (many inverter brands have specifications in their manuals), winding 3-4 turns in parallel will help suppress high-order harmonics (this method is simple and inexpensive). If you need to further enhance the anti-interference effect, you can choose a filter device that meets EMC standards for Delta inverters (Delta inverter manuals have specifications).

[page]

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

(4) In the electric control cabinet equipped with the frequency converter, the power line and the signal line should be routed separately through pipes, and the metal hose should be well grounded.

(5) Use shielded cables for analog signal lines, and connect one end to the analog ground at the inverter.

(6) Interference can also be improved by adjusting the carrier frequency of the inverter. The lower the frequency, the smaller the interference, but the greater the electromagnetic noise.

(7) The RS485 communication port must be connected to the host computer using a photoelectric isolation transmission method 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 power device, and try to avoid sharing an internal transformer.

(9) The instruments and equipment affected by interference should also be shielded independently. The temperature controller, PID regulator, PLC, sensor or transmitter on the market should be equipped with a metal shielding shell and connected to the security ground. If necessary, the above-mentioned inductive magnetic ring filter can be installed at the power supply line end of such instruments.

2 Technology to prevent inverter leakage circuit breaker from malfunctioning

In daily use, we have encountered a leakage protector configured in the inverter input circuit, but the leakage circuit breaker often trips after power is supplied, and the reason cannot be found. Many people think that there is a problem with the quality of the inverter. In fact, there is a reason for this. Let's analyze this issue.

2.1 Rated current design of leakage circuit breaker

The inverter 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 the inverter, it is recommended to select a leakage circuit breaker with a sensitivity current of more than 200mA and an operating time of more than 0.1 seconds for each inverter. However, there is no guarantee that the leakage circuit breaker will not trip. The following factors must be considered to determine the size of the system leakage current, and select an appropriate leakage circuit breaker and necessary measures to improve the phenomenon of leakage circuit breaker tripping after power is supplied.

Leakage current path analysis diagram

The general formula for selecting the rated current of a leakage circuit breaker is as follows (see the figure above):

I△n≧10*〔Ig1+Ign+3*(Ig2+Igm)〕

Ig1, Ig2: Leakage current of cables during commercial operation.

Ign: Leakage current of the noise filter on the inverter input side.

Igm: Leakage current of the motor during commercial operation.

From the relevant variable parameters of the above formula, we know that the factors that affect the leakage current are:

(1) Leakage current of the cable (has two parts)

•Leakage current of the leakage circuit breaker filter cable length.

• Leakage current of the inverter motor cable length.

(2) Leakage current of the filter (including the inverter).

(3) Leakage current of the motor.

[page]

2.2 Leakage current value of each part (unit: mA)

(1) Leakage current of cable = A*(actual cable length/1000m); cable manufacturers provide leakage current value A for each wire diameter per 1000m.

(2) Leakage current of the filter (including the inverter) - provided by the inverter supplier. For example: Delta VFD055B43B uses a filter 26TDT1W4B4, and its maximum leakage current is 70mA.

(3) Motor leakage current - provided by the motor supplier.

2.3 Design Example

When using a frequency converter for circular knitting machines, the front end uses a leakage protection, but it often trips. The analysis is as follows: the frequency converter power is 5.5KW, and the leakage current of the leakage circuit breaker is 75mA. When evaluating based on past experience, under normal circumstances, the leakage current of the cable length and the motor body has little effect. The main influencing factors are the leakage current of the filter (including the frequency converter) 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 leakage circuit breaker must be installed on the power supply side, it is recommended to select a leakage circuit breaker with a sensitivity current of more than 200mA and an operating time of more than 0.1 seconds. However, there is no guarantee that the leakage circuit breaker will not trip. It must be effective when 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 risk of the existing residual current circuit breaker (75mA) tripping due to the leakage current of the filter (including the inverter).

(3) Disconnect the existing leakage circuit breaker (75mA) from the power supply system, input the power directly into the filter and then transfer it to the inverter.

3. Frequency Converter Input and Output Protection Technology

The inverter has a powerful protection function, which generally refers to output protection. From a design perspective, the protection of the inverter input is still a difficult problem. The main reason is that there is no device that can quickly cut off high voltage and high current and has a low cost. Therefore, how to prevent the impact of high voltage and high current on the inverter input is an important issue in the application.

3.1 Power supply voltage conditions of the inverter

Take Delta KG series inverter as an example:

230V series single phase power supply 200/208/220/23050/60Hz

460V series three-phase power supply 380/400/415/440/46050/60Hz

Voltage: ±10% Frequency: ±5%

If the input voltage of Delta KG series inverter 220V series and 440V series is too low, the inverter will have undervoltage protection and will not damage the inverter. If the input voltage of Delta inverter 220V series is higher than 265V or the input voltage of 440V series is higher than 500V, the DC bus voltage of the inverter will exceed the limit and may seriously damage the inverter. Therefore, when using the inverter in the occasion of unstable power supply voltage or self-generated power supply, special attention should be paid to whether the rated voltage of the inverter meets the power supply requirements.

3.2 Input contactor

The input contactor in the Delta inverter manual is a switch that provides input power to the inverter. It must not be used as a start or stop switch for the inverter. Otherwise, it may cause damage to the inverter.

3.3 One inverter output controls multiple motors

(1) Multiple motors start and stop synchronously, and increase or decrease speed at the same frequency. In this application, attention should be paid to power matching. The power of the inverter cannot be simply selected to be equal to the sum of the power of multiple motors. The power level of the inverter should be enlarged. Note! The inverter output should be directly connected to the motor, and no relay should be used in between.

(2) Multiple motors are not allowed to start and stop asynchronously. Because of this control method, the inverter output must be connected to a relay. So in principle it is not allowed! When starting asynchronously, there will be no problem with the first motor starting. But when the second motor starts, the voltage on the inverter output side is very high. At this time, the second motor is equivalent to full voltage starting, and its starting current is about 7-8 times its own rated current, which far exceeds the rated current of the inverter.

When the first motor stops asynchronously, the inverter output voltage must be very high. At this time, when the relay switches the motor, the inductive load will generate a very high instantaneous reverse voltage, which is far higher than the rated voltage of the inverter's internal components. The inverter will either alarm with overvoltage or be damaged by overvoltage.

When multiple motors are switched asynchronously, the next inverter can only be started after the previous inverter has stopped.

3.4 Delta inverter E ground wire

(1) Neutral line. The neutral line is the center line of the generator output. Regardless of whether it is at zero potential at the client end, the neutral line cannot be connected to the E terminal of the inverter as a ground line!

(2) The N terminal of the inverter. The N terminal of the inverter is the negative terminal of the DC bus in the inverter and should be connected to the brake module. It should not be used as a ground terminal, nor should it be mistakenly connected to the neutral line of the power supply.

(3) Safety ground. The E ground wire of the Delta inverter should be connected to the safety ground, which is the motor casing. Avoid high voltage surges and noise interference.

4 Conclusion

This article focuses on three special technical problems in the application of inverter engineering, taking Delta KG series inverter as an example, and provides the principle design method to solve practical problems. The technical principle is actually applicable to general inverter engineering application conditions.