Three problems of application technology of the ho

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Three problems of inverter application technology

Abstract: This paper takes the practical problems of delta kg series inverter engineering application as an example to introduce the anti-interference technology of inverter; Technology to prevent accidental tripping of leakage circuit breaker; Input and output protection technology. The technical principles and treatment principles are applicable to the engineering application of general frequency converters

key words: frequency converter anti-interference leakage circuit breaker misoperation frequency converter input and output protection

1 introduction

with the large-scale engineering application of frequency converter, a large number of engineering personnel at all technical levels and other related technical levels need to master the application technology of frequency converter. For example, ordinary electrical workers; For example, Junior Electrical Engineer; For example, mechanical engineer. At the same time, frequency converters are increasingly applied to various complex engineering environments. Various conventional technologies in books are often difficult to solve special problems in the application of frequency converters. Based on the kg series inverter of delta brand, this paper gives a theoretical analysis and design technology for three inverter application problems. In fact, the technical principle is also applicable to other inverter engineering application conditions

2 anti interference contact of frequency converter: Mr. Lin Lin's interference technology

the interference problem of frequency converter has been puzzling many customers. Here, I will introduce some common interference and troubleshooting methods:

2.1 common interference ways

(1) air radiation mode. It spreads in the air in the form of electromagnetic waves

(2) line propagation mode. Mainly through the power network

(3) line to line induction mode. Electromagnetic induction generated by inductance or electrostatic induction generated by capacitance propagates through line to line induction

2.2 elimination of interference sources

(1) high frequency and high-power DC welding machine should be far away from the frequency converter. The electric welding machine itself should be well grounded

(2) RC surge absorber shall be installed on the on-off contact of electromagnet

(3) the contactors installed in the same electrical cabinet with the frequency converter should be removed from inferior products. Choose products with low switching noise and good arc extinguishing effect. RC surge absorber shall also be installed when necessary

(4) the impedance of the power supply should be low, so as to avoid the start and stop of hundreds of kilowatts of electrical appliances nearby, resulting in the instantaneous mutation of too high input voltage of the frequency converter

(5) the phase voltage of the power supply should be constant, so as not to cause the 220V single-phase input frequency converter to work under voltage or overvoltage

(6) for the self generating system of the user's plant, the output power supply voltage is required not to be high or low. We should avoid mutation and be stable

2.3 common anti-interference measures of frequency converter

(1) the e end of the frequency converter should be connected with the control cabinet and the housing of the motor, and it should be grounded. The grounding resistance should be less than 100 Ω, which can absorb surge interference

(2) inductive magnetic loop filter is installed at the input or output end of the frequency converter. Take delta kg series inverter as an example (there are also specifications in the user manual of many inverter brands). Flatness and winding are helpful to suppress high-order harmonics (this method is simple and cheap). If the anti-interference effect needs to be further strengthened, a filter device that meets the EMC standard dedicated to delta inverter can be selected (specifications are provided in the manual of delta inverter)

(3) the above magnetic loop filter can also be wound on the incoming line of the control signal end of the frequency converter or the given end of the analog signal according to the field situation

(4) in the electric control cabinet equipped with frequency converter, the power line and signal line should be threaded separately, and the metal hose should be well grounded

(5) the shielded wire shall be selected for the analog signal wire, and the single end shall be grounded at the frequency converter

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

(7) the transmission mode of photoelectric isolation must be adopted to connect the RS485 communication port with the upper computer, so as 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 power unit of the frequency converter, and try to avoid sharing an internal transformer

(9) independent shielding should also be carried out on the disturbed instrument equipment. Temperature controllers, PID regulators, PLCs, sensors or transmitters and other instruments in the market should be equipped with metal shielding shells and connected to the security ground. If necessary, the above inductive magnetic loop filter can be installed at the power incoming end of such instruments

3 technology to prevent the misoperation of the leakage circuit breaker of the frequency converter

we encounter that there is a leakage protector configured in the input circuit of the frequency converter in our daily use, but the leakage circuit breaker often trips after power transmission, and the reason can not be found. Many people think that there is a problem with the quality of the frequency converter. In fact, there is a reason. Let's make an analysis on this problem

3.1 rated current design of leakage circuit breaker

the output of the frequency converter is controlled by PWM (pulse width modulation, similar to high-speed switch), so high-frequency leakage current will occur. If you want to install a general leakage circuit breaker on the primary side of the frequency converter, it is recommended to use a leakage circuit breaker with a sensitivity current of more than 200mA and an action time of more than 0.1s for each frequency converter, but it is not guaranteed that the leakage circuit breaker will not trip when it is turned off, The following factors must be considered to determine the magnitude of the leakage current of the system, and appropriate leakage circuit breaker and necessary measures must be selected to improve the tripping of the leakage circuit breaker after power transmission

the calculation formula for selecting the rated current of general leakage circuit breaker is as follows (see Figure 1):

i △ n ≥ 10* [ig1+ign+3* (ig2+igm)]

ig1, ig2: the leakage current of cable during commercial operation

ign: leakage current of the noise filter at the input side of the frequency converter

igm: leakage current of motor during commercial operation

Figure 1 leakage current path analysis

according to the relevant change parameters of the above formula, the factors that will affect the magnitude of leakage current are:

(1) leakage current of cable (with two parts)

· leakage current of cable length of leakage circuit breaker filter

· leakage current of the cable length of the inverter motor

(2) leakage current of filter (including frequency converter)

(3) leakage current of motor

3.2 leakage current value of each part (unit: MA)

(1) leakage current of cable =a* (actual cable length/1000m); The cable manufacturer shall provide the leakage current value a per 1000m of each wire diameter

(2) leakage current of filter (including frequency converter) - provided by frequency converter supplier. For example, the filter used by delta vfd055b43b is 26tdt1w4b4, and the maximum leakage current is 70ma

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

3.2 design example

the frequency converter is used in the application of disc knitting machine. The leakage protection is used in the front end, but it often trips. The analysis is as follows: the power of the frequency converter is 5.5kW. When it rises to a height of 1 meter, the lifting frame and the falling weight stop running, and the leakage current of the leakage circuit breaker is 75ma. Based on past experience, under normal conditions, the length of the cable and the leakage current of the motor body have little influence, and 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 grounding (below 10 Ω), so the suggestions are as follows:

(1) if the power side must be equipped with a leakage circuit breaker, It is recommended to select an inductive current of more than 200mA. According to the strength limit of low carbon steel and the cross-sectional area flow of the test piece, the leakage circuit breaker with an action time of more than 0.1s should be turned off, but it is not guaranteed that the leakage circuit breaker will not trip when it is turned off. It must be valid only when other objects (cable length and motor) are within the normal leakage current range and the load side is grounded according to the third type (below 10 Ω)

(2) if the existing leakage circuit breaker (75ma) must be installed on the power side, it is recommended that the input power supply be directly input to the frequency converter without the existing filter, so as to reduce the tripping of the existing leakage circuit breaker (75ma) caused by the leakage current of the filter (including the frequency converter)

(3) disconnect the existing leakage circuit breaker (75ma) from the power system, input the power directly into the filter, and then transfer it to the frequency converter

4 input and output protection technology of frequency converter

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

4.1 power supply voltage condition of frequency converter

take delta kg series frequency converter 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 220V series and 440V series of delta kg series frequency converter is too low, the frequency converter will have undervoltage protection and will not be damaged. 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, which may seriously damage the inverter. Therefore, when using the frequency converter in situations where the power supply voltage is unstable or in spontaneous power supply, special attention should be paid to whether the rated voltage of the frequency converter meets the requirements of the power supply

4.2 input contactor

the input contactor on the delta inverter manual is a switch that provides input power to the inverter. Never use it as the start or stop switch of the frequency converter. Otherwise, the frequency converter may be damaged

4.3 one inverter output controls multiple motors

(1) multiple motors start and stop synchronously, and speed up and down at the same frequency. Pay attention to power matching in this application mode. The power of the frequency converter cannot be simply selected equal to the sum of the power of multiple motors, but the power of the frequency converter should be amplified. Attention! The output of the frequency converter should be directly connected with the motor, and the relay should not be used in the middle

(2) asynchronous start and asynchronous stop of multiple motors are not allowed. Because of this control mode, the output of the frequency converter should be connected to the relay. So in principle, it is not allowed! When starting asynchronously, there will be no problem in starting the first motor. However, when the second motor is started, the voltage at the output side of the frequency converter 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, far exceeding the rated current of the frequency converter

when the first motor stops asynchronously, the output voltage of the frequency converter must be very high. At this time, when the relay switches the motor, the inductive load will produce a very high instantaneous reverse voltage, which is far more than the rated voltage of the internal devices of the frequency converter. The frequency converter is either overvoltage alarm or overvoltage damage

asynchronous switching of multiple motors can only be switched to the start of the next frequency converter after the previous frequency converter stops

4.4 e grounding wire of delta inverter

(1) zero line. The zero line is the center line of the generator output. No matter whether it is zero potential at the client or not, the zero line cannot be connected to the e end of the frequency converter as a ground line

(2) N terminal of frequency converter. The N line end of the frequency converter is the negative end of the DC bus in the frequency converter and should be connected to the brake module. It can not be regarded as the ground wire terminal, nor can it be wrongly connected to the zero line of the power supply

(3) security place. The e grounding wire of delta frequency converter should be connected to the security ground, that is, the housing of the motor. Avoid high-voltage shock wave and noise interference

5 conclusion

the root of this article

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