Study On The Performance Of Single Phase AC Power Supply
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- Development of Cross Direct Cross Frequency Converter
Variable frequency drive(VFD) converts power in motion control system. Motion control systems currently include multiple disciplines. It usually includes AC driver, high-frequency power converter, digital, intelligent and network control.
Therefore, as an important power conversion controller of the system, VFD provides controllable high performance VVF(variable voltage and variable frequency) AC power supply, which develops rapidly.
The intermediate DC link of AC-DC-AC VFD uses large sensors as energy storage elements to buffer reactive power. In the power generation state, the polarity of the output voltage of the grid side control rectifier can be changed, and the regenerative power fed back to the DC side can be easily fed back to the AC power grid. The speed regulation system with 4-quadrant operation capability can be used for single computer applications requiring dynamic performance such as frequent acceleration and deceleration, and for energy-saving speed regulation of large capacity blowers and pumps.
In recent years, with the rapid development of power electronics, computer technology and automatic control technology, AC drive and control technology has become one of the fastest growing technologies. The electric drive technology is facing a historic revolution. AC speed regulation replaces DC speed regulation, and computer digital control replaces analog control technology has become a development trend. AC variable frequency speed regulation technology is one of the main means to save power, 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 lies in the advantages of superior speed regulation, starting and braking performance, high efficiency, high power factor, power saving effect, wide application range, etc. It is of great significance to deeply understand the development trend of AC drive and control technology.
- Principle and waveform
As shown in Figure 1 below, the whole design scheme consists of circuit, filter circuit, inverter circuit, etc. The power is input into the DC through the DC circuit, and the DC circuit smoothes the filter through the filter circuit, and then the inverter circuit is input into the AC with adjustable frequency and voltage.
The single-phase AC-DC-AC conversion circuit consists of two parts. The transition from AC to DC is a current stop process. Select uncontrolled DC diode circuit, select capacitor and inductor, and filter from DC power supply side. Therefore, a relatively straight DC power supply voltage can be obtained. The relatively simple and reliable structure can meet the design requirements. This is a converter that converts direct current to alternating current. Using single-phase bridge converter circuit and PWM control, the output voltage and frequency can be adjusted to PWM control. The intermediate DC link is a capacitor filter, so a voltage converter is selected.
2.1 Main circuit
The input alternating current is converted into direct current through the transformer and bridge stabilizer circuit. The filter circuit is filtered into wires and capacitors. The inverter part uses four IGBT tubes to form a single bridge inverter circuit. The anode modulation mode is adopted to filter the high-order harmonic output through the LC low-pass filter. AC output, frequency adjustable
The DC power supply voltage Ud in the middle of the main circuit can be obtained from the AC voltage stabilizer, and the converter part adopts PWM converter circuit with IGBT single-phase bridge. The control circuit is composed of a single mechanical integrated function generator ICL 8038 that generates two PWM signals. It is used to control two sets of IGBTs(VT1, VT4, VT2 and VT3). ICL 8038 only works with small external components and can be used to generate sine wave, triangle wave and square wave in the range of 0.001~500KHz.
AC-DC-AC converter circuit consists of two parts: AC-DC converter(UI UD) is used as stabilizer part. Uncontrolled diode DC circuit is adopted. The DC side can obtain a flat intermediate DC power supply voltage through capacitors and sensor filters. The structure is simple and reliable, and the performance meets the experimental requirements. DC-AC(Ud Uo) is the commutation part, which adopts single-phase bridge commutation circuit and PWM control. The output voltage and frequency can be adjusted by PWM control. The intermediate DC link is a capacitor filter, so the inverter circuit in the figure is a voltage circuit.
2.2 Control circuit
The working process of the control circuit is signal generation(including signal wave and carrier wave), signal modulation, and IGBT drive signal generation
In this study, the control circuit is centered on two integrated function signal generators ICL 8038, one of which is used to generate sinusoidal modulation Ur and the other is used to generate triangular carrier Uc. After the two signals are asynchronously modulated in the comparison circuit LM311, a series of equal amplitude and unequal amplitude rectangular waves Um, namely SPWM waves, are generated.
After Um passes through the converter, two ± PWM wave channels with a phase difference of 180 degrees will be generated. After triggering the CD4528 delay, two SPWM1 and SPWM2 wave channels are obtained, with a phase difference of 180 degrees and a certain dead time range. They are used as control signals for two pairs of IGBT switches in the main circuit. The control circuit also has over-current protection interface terminal STOP. STOP is at a lower level when there is an overcurrent signal. The lower level is output through the gate, blocking the two SPWM signals, and closing IGBT for protection.
PWM technology, namely pulse width modulation technology, obtains the required waveform by modulating the pulse width. PWM is widely used in converter circuits. At present, almost all medium and small power converter circuits use PWM technology, which has a profound impact on converter circuits
ICL 8038 is a precision waveform generator. The waveform frequency can be from 0.1001 hz to 300 hz.
2.3 Voltage regulator circuit
As a single terminal module commonly used to convert AC to DC, most DC circuits are composed of transformers, main circuits and filters. The main circuit is mainly composed of silicon DC diode and transistor. The filter is connected between the main circuit and the load to filter the AC component in the pulsating DC power supply voltage. The setting of the transformer depends on the specific situation. The function of transformer is to realize the matching of AC input voltage and DC output voltage and the electrical isolation between AC power grid and DC circuit. The structure of this part is simple and reliable, the performance meets the experimental requirements, and the bridge DC circuit is adopted. Function of converting AC energy of fixed frequency and voltage into DC energy
Within the positive half cycle of the secondary side voltage of the transformer, its polarity is from up to positive and down to negative. In this case, diodes D1 and D4 will shift in both directions and D2 and D3 will shift in reverse. The current flows from the top of the secondary side of the transformer. Move to RL only through diode D1, and then return the transformer through diode D4. Then the positive and negative half wave voltage is obtained from the load resistor RL. Therefore, it can be considered that the half wave voltage is equal to the positive half wave voltage on the secondary side of the transformer. During the record cycle, the offset of diodes D2 and D3 is positive, and the offset of D1 and D4 is negative. The current flows from the lower end of the second side of the transformer, flows to RL only through diode D2, and then returns to the transformer only through diode D3. Similarly, when half wave voltage is obtained under load, the polarity is still positive or negative, as shown above.
2.4 Filter circuit
The filter circuit is usually used to filter the texture from the output voltage of the voltage regulator, and is usually composed of impedance elements. These elements include capacitor C, sensor L in series with the load, and various complex filter circuits composed of capacitors and sensors, which connect the two ends of the load resistance in series.
When the AC power supply is switched to the DC power supply, the voltage of the circuit will fluctuate. To suppress voltage fluctuations, a simple capacitor filter is used. When the current through the sensor changes, the induced electromotive force generated by the sensor prevents the current from changing. When the current passing through the sensor increases, the self induced electromotive force generated by the sensor is opposite to the current direction. It prevents the current from increasing, converts part of the electric energy into magnetic energy, and stores it in the electric sense. When the current through the sensor decreases, the self sensing transmission direction is the same as the current. In this way, the stored energy can be released without reducing the current to compensate for the current loss.
Therefore, through the sensing filter, the load current and voltage ripple will be reduced, not only the waveform is smooth, but also the conduction angle of the DC diode will be increased. Under certain conditions of the sensor, the smaller the load resistance is, the smaller the AC component of the output voltage is.
Only RL&Omegal L can achieve better filtering effect. The larger the L, the better the filtering effect. In addition, due to the EMF of the filter sensor, the conduction angle of the diode is close to π, so the impulse current of the diode decreases, and the current makes the diode smooth, thus extending the life of the diode.
2.5 bit conversion circuit
The converter circuit corresponds to the voltage regulator circuit. The AC circuit converts the DC supply voltage to the AC supply voltage of the desired frequency. Converter circuit is one of the core components of universal converter, which plays a very important role. The basic function is to convert the DC power supply from the intermediate DC circuit to the AC power supply. The frequency and voltage can be adjusted freely in the control circuit.
2.6 Drive circuit
As an intermediate step between the control circuit and the main circuit, the task of the drive circuit is to convert the connection signal of the controller in the control circuit into the drive signal of the device.
You can:
(1) The driving signals of the four pipes in the whole bridge circuit are not common grounding, so it is necessary to isolate the control signals. In addition, the voltage flattening 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) Convert the signal generated by the control circuit to the appropriate drive signal required to control the switch.
(3) Turn off the saturation protection function to prevent overcurrent and short circuit damage.
2.7 Circuit configuration
SPWM(sine pulse width modulation) is used to change the modulation frequency and complete the AC-DC-AC frequency conversion.
The design circuit consists of three parts: main circuit, driving circuit and control circuit. AC-DC conversion is an uncontrollable DC circuit. DC-AC conversion is a single-phase bridge converter circuit composed of four IGBT transistors, with anode modulation function. High frequency sine(fundamental wave) output is obtained through LC low-pass filter.
- Simulation
3.1 Single phase full bridge inverter circuit
Set the input voltage UD to 300V. The amplitude is 1, the period is 0.2s, the frequency is 50Hz, and the occupancy is 50%. Two of them have a 0 second delay and add the output to the grids of IGBT1 and IGBT4. The remaining two delays are 0.1 seconds, and the output is added to the grids of IGBT2 and IGBT3. The load resistance is set to 1 Omega and the resistance is set to 0.01H. The cycle and frequency of the drive signal are set to 50 Hz. The simulation time can be set to 2s, and the simulation results can be obtained after running
From top to bottom, it is the input voltage Ud, the voltage Uo at both ends of the load, and the current io passing through the load. The waveforms of AC and DC currents are determined by the characteristics of resistive loads. When the DC current is negative, the current flows to the power supply through the semi parallel diode, and the magnetic field energy storage of the load sensor is sent through the DC bus. If the DC current is negative, the current will flow to the load through IGBT.
For a pure resistive load, the DC current does not change. The analog waveform shows that the AC fundamental wave voltage output by the converter is 220V, which is consistent with the theoretical value in the above mathematical analysis.
Therefore, the amplitude of the fundamental output voltage of the single-phase square wave voltage source converter circuit is greater than that of the DC power supply voltage, and the voltage utilization is high. However, due to the high utilization rate and large harmonic content, it is difficult to meet the requirements of most loads.
3.2 Single phase half bridge inverter circuit
The input voltage Ud is set to 360V(180V per power supply), and the amplitude is 1. The cycle is 0.2s, the frequency is 50Hz, and the blank rate is 50%. The grid of IGBT has a 0 second delay, and the output is added to the grid. The other delay is 0.1 seconds, increasing the output for the grid of IGBT1. The load resistance is set to 1 º and the resistance is 0.04h. The cycle and frequency of the drive signal are set to 50 Hz
It is basically consistent with the theoretical value.
- Experts
Simple – fewer devices
- Const
AC power supply voltage width: Ud/2
Two series capacitors are required on the DC side.
This requires controlling the voltage balance between the two.
Used for small power converters less than several kilowatts.
- Results and analysis
Through the experimental data and waveform, it can be concluded that with the increase of the input voltage frequency, the amplitude of the output voltage remains unchanged and the frequency decreases under the pure resistance load. In the case of resistance and inductive load, with the decrease of input voltage frequency, the output voltage amplitude remains unchanged, and the frequency also decreases. With frequency conversion effect
The amplitude modulation method is completed on the stabilizer link by adjusting the control angle α of the stabilizer. The frequency adjustment method in the converter link is completed by adjusting the frequency of the trigger pulse of the converter(VT1 to VT6). The frequency of the trigger pulse increases, so does the frequency of the output voltage.
When it is a pure resistive load, the current and voltage are in the same phase. The phase of current and voltage changes when resistance and inductive load are applied. Therefore, it can be concluded that different loads affect the phase of current and voltage.