Development of a new type of variable frequency power supply for industrial vibrating rods

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Development of a new type of variable frequency power supply for industrial vibrating rods [Copy link]

Abstract: This paper introduces a new type of variable frequency power supply for industrial vibrators, which greatly improves the defects of existing vibrator power supplies in terms of volume and weight. The design of several important parts of the circuit is introduced in detail, and through the analysis and experiments of each part, it is shown that this design has many advantages such as small size, light weight and high performance.

Keywords: active power factor correction; forward converter; vibrating rod; variable frequency speed regulation

　

　

0 Introduction

With the rapid development of power electronics technology, microcomputers and large-scale integrated circuits, the AC motor variable frequency speed regulation system composed of inverters has rapidly developed and matured, and has been used more and more widely.

Existing vibrator products are basically a kind of motor with a generator, and then the generator provides 200Hz AC power to drive the high-speed vibrator motor to run. Its prominent disadvantage is that it is relatively large in size and weight, which causes great inconvenience in use and movement during on-site construction. The focus of this paper is to apply AC variable frequency speed regulation technology to a small construction machine such as a vibrator, and develop a new variable frequency power supply. While realizing the function of the vibrator, the volume and weight of the whole machine are greatly reduced, the power factor at the input end is improved, the voltage and frequency at the output end are stabilized, and the cost of the product can be reduced. The basic performance indicators of the variable frequency power supply are as follows: the power frequency of the built-in asynchronous eccentric vibration motor for the vibrator is 200Hz, single-phase input, three-phase output, the line voltage of the motor is 42V, the single-machine power is 350W, and it is required to be able to run with two machines.

1 Main circuit of voltage source inverter

The variable frequency power supply not only needs to realize the functions of voltage conversion and frequency conversion, but also needs to achieve electrical isolation between the input and output, and meet the harmonic requirements of the power grid. Its basic structure generally includes several important parts such as AC/DC, DC/DC and DC/AC.

The main circuit of this power supply consists of three parts: APFC pre-stage, DC/DC and three-phase inverter. After the input is rectified by full-bridge uncontrolled, the Boost circuit is used as the circuit topology of APFC for voltage pre-regulation. The DC/DC part adopts a single-ended forward converter to achieve the functions of step-down and isolation. The three-phase inverter part adopts SPWM control, and its basic structure is shown in Figure 1. Due to the use of power factor correction technology, the input power factor is high, the grid-side current harmonics are small, and the harmonic pollution to the grid is very small; and when the grid voltage fluctuates or the load changes, the control of the DC/DC link can keep the DC side voltage of the three-phase inverter part stable, so that the output voltage of the system is stable, and there is no need to change the output voltage by adjusting the modulation depth of the three-phase inverter part. Therefore, the requirements for the control chip of the inverter part can be reduced, and a relatively cheap CPU can be used. In addition, since it is a low-voltage inverter, a low-voltage MOS tube can be used as the power switch tube of the inverter circuit.

Figure 1 Basic principle block diagram of inverter power supply

2 Active Power Factor Correction (APFC) Circuit

The use of an average current controlled Boost circuit to implement APFC is currently the most widely used APFC control method in high-frequency switching power supplies. There are many integrated circuit chips available on the market for the control circuit of the power factor corrector using the average current control method. Among them, the UC3854 of Unitrode Company in the United States is a very representative one and has been widely used in practice. In this scheme, the UC3854 chip of Unitrode Company is used to implement it. Its circuit schematic diagram is shown in Figure 2, and the experimental results of the input voltage and current are shown in Figure 3. The corresponding relationship between the actual voltage and the voltage in the figure is 1V:1V, and the corresponding relationship between the actual current and the current in the figure is 4A:1V.

Figure 2 UC3854 power factor correction circuit schematic

Figure 3 Correction circuit experimental waveform

3 Design of Forward Converter

The vibrator is a handheld electric product. For the personal safety of the operator, electrical isolation must be achieved between the input and output. The input and output of the APFC front stage are not isolated, and the isolation function is completed by the DC/DC part. Since a high-frequency DC/DC conversion circuit is used, the volume of the transformer can be made very small. In addition, since the output voltage of the APFC is about 350-400V, considering the voltage stress problem of the switch tube of the inverter circuit behind, the DC/DC part should also have the function of stepping down. Based on this consideration, in this scheme, the DC/DC part adopts a forward conversion circuit (Forward Converter). The biggest advantage of the forward converter is its simple structure, high reliability, and reduced cost and weight. Considering the magnetic reset problem of the transformer, this scheme adopts the circuit shown in Figure 4. When the switch tube is turned on, the transformer transmits energy. When the switch tube is turned off, the output diode D1 is _reverse biased and has no energy discharge circuit. The magnetization energy will cause a large reverse voltage to be added between the drain and source of the MOS tube. The role of using _the N2 coil is to return the stored energy to the power supply through the diode D. As long as the number of turns of N2 _and N1 is the same, the drain-source voltage of the switch tube is 2 V _s . The leakage inductance can be reduced by winding N1 and _{N2 in parallel. In the circuit of Figure 4, the control chip of} the power switch is _UC3844 from Unitrode.

Figure 4 Schematic diagram of the forward circuit with demagnetization reset

4 Design of three-phase inverter control, drive and protection circuits

4.1 Design of inverter control circuit

Since the inverter part of this scheme does not need to change the output voltage by adjusting the modulation depth, it only needs to realize the frequency conversion function. Therefore, the chip used in the control circuit is INTEL's 87C51FX series 8-bit single-chip microcomputer, which is much cheaper than the general Intel196 single-chip microcomputer, and the performance is sufficient. Generally speaking, the typical usage of CPU to generate PWM is to use the timing method, and determine the three-phase output by querying in the timing interrupt. However, this method is only applicable to the case where the output PWM pulse frequency is very low. When the output frequency is greater than 1kHz, the interrupt query time may be longer than the minimum output pulse width, which will cause the output pulse width to increase or decrease, increase the output harmonics, and affect the symmetry between the three phases. Compared with the ordinary 51 series single-chip microcomputer, 87C51FX adds a programmable counter array (PCA), which consists of a 16-bit timer/counter and 5 16-bit comparison/capture modules, as shown in Figure 5. Its function is similar to the EPA of Intel196 single-chip microcomputer. The PCA's 16-bit timer/counter is used as the timing standard for the compare/capture module. Therefore, it is mainly used as a timer. Each compare/capture module has four uses, namely capturing the time when the output level on the external pin CEXn jumps, software timer, high-speed output and pulse width modulation output.

Figure 5 Programmable Counter Array

This scheme uses asymmetric regular sampling method to generate three-phase 6-way control pulses. Compared with symmetric regular sampling method, the step wave formed by asymmetric regular sampling method is closer to sine wave. The calculated three-phase pulse width value is stored in a data table as a timing reference. The 6-way control pulse can be obtained by querying these timing times in the program. The working principle is briefly described as follows: Using the software timer and high-speed output mode of 87C51FX, in the 16-bit comparison mode, the count value of the 16-bit PCA timer and the preset value in the 16-bit comparison register in the module are compared 3 times in each machine cycle. If they are equal, a matching signal is generated to make the module work in high-speed output mode, that is, when the PCA timer count value and the comparison register of the module are equal, a matching signal is generated. This signal causes the output level on the external pin CEXn to jump, and if allowed, a PCA interrupt is also generated. By setting the initial state of the output level on CEXn by software, the pin can be made to jump positive (negative) when the predetermined time is reached. In this way, a 16-bit PWM wave can be generated.

Since the jump of the pin does not need to be completed by the CPU calculation, the pulse width change caused by the minimum pulse width being too narrow is avoided. The program mainly consists of two parts: the main program and the interrupt service program. The main program is mainly for initialization, assigning initial values to the timer and each register. The interrupt program mainly includes the PCA interrupt service program and the protection interrupt program for generating PWM pulses: in the PCA interrupt service program, the main thing is to assign the next timing time to the comparison register of each module; the protection interrupt program is mainly to deal with blocking the PWM output when a protection signal arrives.

4.2 Design of driving circuit

The driver chip used in this solution is IR2130. The biggest advantage of IR2130 is that it can run with a common ground, so only one control power supply is needed. Moreover, 3 of its 6 output signals also have level conversion functions, which can drive both low-voltage and high-voltage power devices. IR2130 also has current amplification and overcurrent protection functions; undervoltage lockout and can indicate undervoltage and overcurrent status functions; input noise suppression function; and can automatically generate the dead time (2μs) required for upper and lower side driving. The driving circuit in actual application is shown in Figure 6.

Figure 6: Driving circuit using IR2130

4.3 Design of protection circuit and main circuit

Since the driving circuit part has the current protection function, the protection circuit part is only designed with voltage protection, including input overvoltage, undervoltage protection and output overvoltage, undervoltage protection. The protection circuit is shown in Figure 7. Among them, the implementation circuits of these protection functions are similar, that is, the output (or input) voltage is sent to the inverting end of the comparator after voltage division, and the inverting end of the comparator is connected to the given voltage. The difference between them lies in the different outputs of the comparator, that is, when the input is overvoltage and the output is overvoltage, the comparator outputs a low level; when the input is undervoltage and the output is undervoltage, the comparator outputs a high level. After the output of the first three protection circuits is calculated by 4011, it becomes an "or" relationship, that is, as long as one fault occurs, the fault signal obtained is a high level and is sent to the external interrupt port of the CPU for corresponding processing. When the output is undervoltage, the comparator outputs a high level, the light-emitting diode lights up, and the buzzer sounds an alarm.

Figure 7 Protection circuit diagram

Figure 8 Main circuit of the inverter

Since the output voltage of the DC/DC part is relatively low, the power switch tube used in the main circuit is a low-voltage MOSFET. At the same time, in order to reduce the burden of the power tube during the switching process, a buffer circuit is used in the main circuit, as shown in Figure 8. The three-phase inverter bridge is composed of 6 MOSFETs, D1 _~ D6 _is the fast recovery diode integrated with the MOSFET, and R , D, C form a buffer circuit (it can also be seen that it is the equivalent circuit of the U, V, W three-phase buffer circuit).

5 Experimental analysis and conclusion

In this design, circuit simulation and experiments were carried out on each part, and a prototype was completed. The experimental waveforms of the motor line voltage and current during normal operation are shown in Figures 9 and 10. This design has the advantages of small size, light weight, stable output voltage, small current harmonics, and high power factor. It is expected that after overcoming the voltage and current stress problem of the power switch of the forward converter caused by the increase in output power, the products produced according to this design will have higher cost performance and be more competitive in the market.

Figure 9 Vibrating rod line voltage waveform at 200Hz

Figure 10 Vibrating rod phase current waveform at 200Hz

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The step wave formed by the asymmetric regular sampling method is closer to a sine wave

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