Study on the Performance of Single Phase AC-DC-AC Inverter Circuit

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1. Development of AC-DC-AC VFD

Variable Frequency Drive(VFD) converts power in the motion control system. The motion control system now contains a variety of disciplines. It generally includes AC drive, high frequency power converter, digital, intelligent and networked control. 
Therefore, the VFD, as an important power conversion controller of the system, provides controllable high performance VVVF(variable voltage and variable frequency) AC power supply and has been rapidly developed. 

The middle DC link of AC-DC-AC VFD uses large inductor as energy storage element, buffering reactive power. In the state of power generation, the regenerative power fed back to the DC side can be easily fed back to the AC power grid by changing the polarity of the output voltage of the grid side controlled rectifier. The speed regulation system with four quadrant operation ability can be used in single machine applications required frequent acceleration and deceleration and other dynamic performance, as well as in the energy-saving speed regulation of large capacity fans and pumps.

In recent years, as the rapid development of power electronic technology, computer technology and automatic control technology, AC drive and control technology has become one of the most rapidly developed technologies. Electric drive technology is facing a historical revolution. It has become the development trend that AC speed regulation replacing DC speed regulation, as well as computer digital control replacing analog control technology. AC variable frequency speed regulation technology is one of the main methods to save electricity, improve process flow, improve product quality and environment, and promote technological progress.

Variable frequency speed regulation is recognized as the most promising speed regulation mode at home and abroad. The reason is its excellent speed regulation, starting and braking performance, high efficiency, high power factor and power saving effect, as well as wide application range and many other advantages. It is of great significance to deeply understand the trend of AC drive and control technology.

2. Principle and Waveform

As shown in the following figure 1, the overall design scheme consists of rectifier circuit, filter circuit, inverter circuit, etc. The electricity is converted into DC through rectifier circuit, and the DC is filtered smoothly through filter circuit, and then input inverter circuit to AC with adjustable frequency and voltage.

The single-phase AC-DC-AC inverter circuit is composed of two parts. The conversion of AC to DC is the rectification process. The uncontrolled rectifier diode circuit is selected, and the capacitor and inductance are selected for filtering on the DC power side. So a relatively straight DC supply voltage can be obtained. The structure is relatively simple, reliable and the performance meets the design requirements. It is an inverter that converts DC to AC. Single-phase bridge inverter circuit and PWM control is used, and the output voltage and frequency can be adjusted by PWM control. The intermediate DC link is capacitance filter, so voltage inverter is selected.

Figure 1: Principle of AC-DC-AC inverter circuit

2.1 Main Circuit

The figure 2 depicted the AC-DC-AC inverter circuit that is uncontrollable rectifier circuit. The input AC is converted to DC through transformer and bridge rectifier circuit. The filter circuit is filtered by inductance and capacitance. The inverter part adopts four IGBT tubes to form a single bridge inverter circuit. By adopting bipolar modulation mode, the output is filtered by LC low-pass filter to filter out the high-order harmonic wave. There is AC output with adjustable frequency.

The DC supply voltage Ud in the middle of the main circuit is obtained by alternating current rectifier, while the inverter part is single-phase bridge PWM inverter circuit, where IGBT is used in the power device. The control circuit is composed of single chip integrated function generator ICL8038, which generates two PWM signals. These are used to control two groups of IGBT (VT1, VT4, VT2 and VT3). ICL8038 can work normally only with small external elements, which can be used to generate sine wave, triangle wave, and square wave, with the frequency range from 0.001 to 500KHz.

Figure 2: Single-phase AC-DC-AC inverter circuit diagram

The AC-DC-AC inverter circuit consists of two parts. The AC-DC converter (ui—Ud) is used as the rectifier part. The uncontrollable diode rectifier circuit is adopted. The DC side is filtered by capacitors and inductors, and the flat intermediate DC supply voltage can be obtained. The structure is simple and reliable, and its performance meets the needs of experiments. DC to AC (Ud—uo) is the inverter part, which adopts single-phase bridge inverter circuit and PWM control. The output voltage and frequency can be adjusted by PWM control. Because the middle DC link is capacitor filter, the inverter circuit in the figure is voltage circuit.

2.2 Control Circuit

The work process of the control circuit is: signal generation (including generating signal wave and carrier), signal modulation, generating IGBT driving signal.

In this study, the control circuit uses two integrated function signal generators ICL8038 as the core, one of which generates sinusoidal modulation wave Ur, the other is used to generate triangular carrier Uc. After the two signals are asynchronously modulated by the comparison circuit LM311, a series of equal amplitude and unequal width rectangular wave Um, namely SPWM wave, are generated.

After the Um passes through the inverter, two channels of ± PWM waves with phase difference of 180 degrees are generated. After delay by the trigger CD4528, two channels of SPWM1 and SPWM2 waves with phase difference of 180 degrees and a certain dead time range are obtained. They are used as the control signals of two pairs of switch IGBT in the main circuit. The control circuit is also equipped with an over-current protection interface terminal STOP. When there is an over-current signal, the STOP is in a low level and outputs a low level through the and gate, which blocks two SPWM signals and turns off the IGBT, thus playing a protective role.

Figure 3: Control circuit of of the AC-DC-AC VFD.

PWM technology, namely pulse width modulation technology, is to obtain the required waveform by modulating the pulse width. PWM is widely used in inverter circuit. At present, almost all medium and small power inverter circuits are used with PWM technology, which has a profound influence on inverter circuit.

ICL8038 is a precise waveform generator. The frequency of the waveform can be from 0.1001hz to 300Hz. Its internal structure is shown in the figure 4.

Figure 4: Internal principle diagram of ICL8038

Figure 5: Waveform of unipolar PWM

Figure 6: Waveform of bipolar PWM

2.3 Rectifier Circuit

Rectifier circuit, generally a single rectifier module, is used to convert AC into DC. Most rectifier circuits are composed of transformer, main circuit and filter. The main circuit is mainly composed of silicon rectifier diodes and thyristors. The filter is connected between the main circuit and the load to filter the AC component present in the pulsating DC supply voltage. The setting of transformer depends on the specific situation. The function of transformer is to realize the matching between AC input voltage and DC output voltage, as well as the electrical isolation between AC power grid and rectifier circuit. The structure of this part is simple and reliable, and its performance meets the needs of the experiment, so the bridge rectifier circuit is used. Its function is to convert AC energy with fixed frequency and voltage into DC energy.

During the positive half cycle of the secondary side voltage of the transformer, its polarity is positive at the top and negative at the bottom. At this time, diodes D1 and D4 will be forward biased, and D2 and D3 will be reverse biased. The current flows out from the upper end of the secondary side of the transformer. It only flows to RL through diode D1, and then flows back to the transformer through diode D4. Then a half wave voltage, whose polarity is positive up and negative down, is obtained on the load resistor RL. Therefore, it can be considered that the half wave voltage is the same as the positive half wave voltage of the secondary side of the transformer. During the negative half cycle, diodes D2 and D3 will be forward biased, and D1 and D4 will be reverse biased. The current flows out from the lower end of the secondary side of the transformer, and can only flow to RL through diode D2, and then flows back to the transformer through diode D3. Similarly, a half wave voltage is obtained on the load, and the polarity is still positive up and negative down, which is the same as the previous one.

Figure 7: Rectifier circuit diagram

2.4 Filter Circuit

Filter circuit is often used to filter out the ripple in rectifier output voltage, which is generally composed of reactance elements. These elements include capacitor C in parallel at both ends of load resistance, inductor L in series with load, and various complex filter circuits composed of capacitor and inductor.

After the conversion of AC power to DC power, there will be voltage fluctuation in the circuit. In order to suppress the voltage fluctuation, a simple capacitor filter is used. When the current flowing through the inductor changes, the induced electromotive force generated in the inductors will prevent the current from changing. When the current through the inductors increases, the self induced electromotive force generated by the inductors is opposite to the current direction. This prevents the current from increasing and converts part of the electric energy into magnetic energy to store in the inductance. When the current through the inductors decreases, the self induced electromotive force direction is the same as the current. This prevents the current from decreasing, and releases the stored energy to compensate the current loss.

Therefore, after inductance filtering, not only the pulsation of load current and voltage is reduced whose waveform becomes smooth, but also the conduction angle of rectifier diode is increased. Under the constant condition of inductors, the smaller the load resistance is, the smaller the AC component in output voltage is.

Only when RL>>ωL can better filtering effect be obtained. The larger L is, the better filtering effect is. In addition, due to the EMF of the filter inductor, the conduction angle of the diode is close to π, which make the impulse current of the diode reduced and the current flowing smooth through the diode, thus prolonging the service life of the rectifier diode.

2.5 Inverter Circuit

The inverter circuit is opposite to the rectifier circuit. The inverter circuit converts the DC supply voltage into the AC supply voltage of the required frequency. It transforms the low voltage into the high voltage and DC into AC. Inverter circuit is one of the core components of general VFD, which plays a very important role. Its basic function is to convert the DC power supply from the intermediate DC circuit into the AC power supply whose frequency and voltage can be adjusted arbitrarily under the control circuit.

Figure 8: Single phase full bridge inverter circuit diagram

Figure 9: Working waveform of single phase full bridge inverter circuit

2.6 Drive Circuit

As the middle link between the control circuit and the main circuit, the task of the drive circuit is to convert the on-off signal of the controller generated by the control circuit into the driving signal of the device.

Figure 10: Drive circuit diagram

It can complete the following work:

(1) The drive signals of the four tubes of the whole bridge circuit are not all common ground, so the control signals need to be isolated. In addition, the voltage level of the control circuit is low, and the voltage level of the main circuit is high. In order to avoid interference, electrical isolation must be carried out.

(2) Transform the signal generated by the control circuit into the appropriate drive signal needed for the control on-off.

(3) The function of withdraw saturation protection prevents the damage of over current and short circuit.

2.7 Circuit Composition

SPWM (sine wave pulse width modulation) is used to complete AC-DC-AC frequency conversion by changing the modulation frequency.

The design circuit consists of three parts: main circuit, drive circuit and control circuit. The AC-DC conversion is an uncontrollable rectifying circuit, and the DC-AC conversion is a single-phase bridge inverter circuit composed of four IGBT transistors with bipolar modulation. The output is filtered by LC low-pass filter to get high frequency sine wave (fundamental) output.

3. Simulation Experiment

3.1 Single-Phase Full Bridge Inverter Circuit

Set the input voltage UD to 300V. The amplitude is 1, the period is 0.2 s, the frequency is 50 Hz, and the duty cycle is 50%. Two of them have a delay of 0 s and their outputs are added to the gates of IGBT1 and IGBT4. The other two have a delay of 0.1 s and their outputs are added to the gates of IGBT2 and IGBT3. The load resistance is set to 1 Ω and inductance 0.01 H. The period and frequency of drive signal are both set to 50 Hz. The simulation time is set as 2 s, and the simulation results can be obtained after running.

The figure 11 depicted waveform obtained in the oscilloscope. From top to bottom, there are input voltage Ud, voltage Uo at both ends of the load and Current io flowing through the load in order. The waveforms of AC current and DC current are determined by the characteristics of resistive load. When the DC current is negative, the current flows to the power supply through the anti parallel diode, and the magnetic field energy storage of the load inductor feeds to the DC bus. When the DC current is negative, the current flows to the load through the IGBT.

If it is a pure resistance load, there will not be fluctuation in the DC current. According to the simulation waveform, the AC fundamental voltage output by the inverter is 220 V, which is consistent with the theoretical value in the above mathematical analysis.

So the fundamental amplitude of output voltage of single-phase square wave type voltage source inverter circuit is greater than that of DC supply voltage, in which the voltage utilization rate is higher. But the harmonic utilization rate is also higher and the harmonic content is larger, which is difficult to meet the requirements of most loads.

Figure 11: Simulation waveform of single-phase full bridge inverter circuit

3.2 Single-Phase Half Bridge Inverter Circuit

Figure 12: Single-phase half bridge inverter circuit diagram

Set the input voltage Ud to 360V (180V for each power supply), and the amplitude is 1. The period is 0.2 s, the frequency is 50 Hz, and the duty cycle is 50%. One has a delay of 0 s and its output is added to the gate of IGBT. The other has a delay of 0.1 s and its output is added to the gate of IGBT1. The load resistance is set to 1 Ω and inductance 0.04h. The period and frequency of drive signal are set to 50 Hz. The simulation time is set to 2 s and the waveform obtained in the oscilloscope is shown in the figure 13.

It is basically consistent with the theoretical value.

* PROS

- Simple
- Less devices

* CONS

- AC supply voltage amplitude is Ud / 2.
- DC side need two capacitors in series.
- It requires to control the voltage balance between the two.
- It is used for small power inverter below several kW.

Figure 13: Simulation waveform of single-phase full bridge inverter circuit

4. Results and Analysis

Figure 14: Waveform of single phase AC-DC-AC inverter circuit

From the experimental data and waveforms, it can be concluded that when it is pure resistance load, with the increase of the input voltage frequency, the output voltage amplitude remains unchanged and the frequency decreases. When it is resistance and inductance load, with the decrease of the input voltage frequency, the output voltage amplitude also remains unchanged and the frequency decreases. Both of them have the effect of frequency conversion.

The method of amplitude adjustment is completed in the rectifier link by adjusting the control angle α of the rectifier. The method of frequency adjustment is completed in the inverter link by adjusting the frequency of the trigger pulse of the inverter(VT1 ~ VT6). When the frequency of trigger pulse increases, the frequency of output voltage will increase too.

When it is pure resistance load, the phase of current and voltage is the same. When it is resistance and inductance load, the phase of current and voltage changes. Therefore, it can be concluded that different load can affect the phase of current and voltage.

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