What is sine wave inverter?

 

The basic structure of a sine wave inverter

A sine wave inverter is a device that converts DC power to AC power by controlling the on/off of semiconductor power switching devices such as SCRs, GTOs, GTRs, IGBTs, and power MOSFETs. The circuit that controls the on and off of the power switch is the control circuit of the inverter. The control circuit outputs certain voltage pulses to turn on and off the power switch in the power conversion circuit according to certain rules, and then the output of the main power circuit is a specific combination of harmonics, and finally, the desired voltage waveform is obtained through the filter circuit.

 

Input Circuit

The input to the inverter is usually DC power (or DC power obtained from the mains through rectification and filtering), which includes DC power obtained from the DC grid, batteries, PV cells, and other means. Usually, this electrical energy cannot be directly used as the voltage on the input side of the inverter but has to pass through certain filtering circuits and EMC circuits before it can be used as the input to the inverter.

 

Inverter main circuit

The inverter main circuit is a power conversion circuit consisting of power switching devices. There are many different structural forms of the main circuit for different input and output conditions. Each power conversion circuit has its advantages and disadvantages, and the most suitable circuit topology should be considered as the main circuit structure in the actual design.

 

Control Circuit

The control circuit generates one or more sets of pulses through certain control techniques according to the output requirements of the inverter and acts on the power switching tubes through the drive circuit to turn them on or off in a prescribed sequence, and finally obtains the required voltage waveform at the output of the main circuit. The role of the control circuit is crucial to the inverter system, and the performance of the control circuit directly determines the quality of the inverter output voltage waveform.

 

Output Circuit

The output circuit generally includes the output filter circuit and EMC circuit, and if the output is DC, a rectifier circuit should be added after it. For inverters with isolated output, the output circuit should also have an isolation transformer in the front stage. Depending on whether the output needs a voltage regulator circuit, the output circuit can be divided into open-loop and closed-loop control. The output of an open-loop system is determined by the control circuit only, while the output in a closed-loop system is also influenced by a feedback loop to make the output more stable.

 

Auxiliary Power

Some parts or chips of the control circuit and input and output circuits have specific input voltage requirements, and an auxiliary power supply can meet the specific voltage requirements in the circuit. Typically, the auxiliary power supply consists of one or several DC-DC converters. For AC input, the auxiliary power supply is completed by a combination of rectified voltage and DC-DC converters.

 

Protection Circuitry

Protection circuitry typically includes input overvoltage and under-voltage protection, output overvoltage and under-voltage protection, overload protection, overcurrent, and short circuit protection. Additional protections are available for inverters operating in specific application environments, such as temperature protection at extremely low or high temperatures, barometric protection at certain barometric pressure changes, humidity protection in humid environments, etc.

 

Classification of sine wave inverters

There are many different types of inverters, and they can be divided into three categories based on the nature of the output waveform: sine wave inverters, square wave inverters, and trapezoidal wave inverters. Sine wave inverters output sine wave AC, while square wave inverters output square wave AC of poorer quality.

Inverters are usually divided into voltage input type and current input type. Unlike DC choppers, the width of the modulating pulse is related to the sine wave, so the output current or voltage is close to the sine wave. Current-input inverters are rarely used for electric vehicle drives because analog current sources require a large number of inductive components. The voltage input inverter circuit is simple and can convert energy in both directions, so it is almost always used in electric vehicles.

Depending on the need, its output waveform can be a square wave or pulse-width modulated waveform. Pulse width modulation schemes can be classified as sine wave PWM, current lag PWM, voltage space SVPWM, etc. Inverters can use PWM technology to output pulse-width modulated waveforms to induction motors and permanent magnet synchronous motors. A suitable scheme can effectively suppress harmonics, make better use of DC voltage and reduce DC voltage fluctuations.

 

Sine wave inverter principle

The function of the inverter is to convert DC power to AC power, it consists of an inverter bridge, and an SPWM wave module! driver module and filter circuit, of which the SPWM inverter circuit is the key to generating a pure sine wave. the generation of the SPWM wave module has been a hot topic of research, SPWM is pulse width modulation technology, a pulse waveform with variable duty cycle, and the PWM control technology is based on this conclusion as the theoretical basis. the PWM control technology is based on this conclusion. In the case of pulse width modulation, the on and off of semiconductor switching devices are controlled so that the output gets a series of pulses of equal amplitude and variable width! to equivalently obtain the desired waveform. If the duty cycle of the pulse series is arranged according to a sinusoidal law, the output voltage can be filtered to obtain a sinusoidal waveform! At the same time, the harmonic component of the load current is greatly reduced, which is called sinusoidal pulse width modulation.

 

The history of sine wave inverters

The development of sine wave inverter technology has always been closely integrated with the development of power devices and their control technology and has gone through five stages since the beginning of its development.

 

The first stage

From the 1950s to the 1960s, the birth of thyristor (SCR) created the conditions for the development of sinusoidal inverters.

 

>The second stage

In the 1970s, the emergence of silicon-controlled transistors (GTO) and bipolar transistors enabled the development and application of inverter technology.

 

The third stage

1980s, the power field effect tube, insulated gate type field effect tube, and MOS control intergranular tube appeared, laying the foundation for the high-power development direction of the inverter.

 

The fourth stage

the 1990s, the development of microelectronics technology to make vector control technology, multi-level control technology, fuzzy control technology and repeat control technology, and other new control technology in the field of inverters to better application, greatly promote the development of inverter technology.

 

The fifth stage

At the beginning of the 21st century, with the continuous progress and improvement of power electronics technology, modern control theory, and microelectronics technology, inverter technology develops in the direction of high efficiency, high frequency, high reliability, high power density, and intelligence.

 

Development trend of sine wave inverter/h2>

With the rapid development of power electronics technology and the improvement of various industries' requirements for inverter control performance, the sine wave inverter has also been developed rapidly. At present, the development direction of inverter mainly includes the following aspects

High frequency

High frequency refers to increasing the operating frequency of power switching devices, which can not only reduce the volume of the whole system but also have a good suppression effect on audio noise and improve the dynamic response capability of the inverter output voltage. The high-frequency operation of power switching devices corresponds to the high-frequency isolation transformer, and the application of the high-frequency isolation transformer has further reduced the volume of the whole system.

 

High performance

The effective value is the main parameter of the inverter output voltage. A high-performance inverter has a stable RMS output voltage, as well as high waveform quality and high adaptability to non-linear loads. Since many times the load carried by the inverter can change suddenly, a high-performance inverter requires a high transient response performance of the output voltage. Another important parameter of the AC output voltage is the frequency. A good inverter needs not only a stable output voltage RMS value, but also a stable frequency. An inverter with the above characteristics can only be called a high-performance inverter.

 

Parallel technology

Current inverter technology can produce high-power products, but once this inverter system fails in a high-power application, the system is crippled. In a system consisting of low-power inverters connected in parallel, the normal operation of each unit does not affect the operation of the others, which greatly increases the reliability of the entire system.

 

Miniaturization

Miniaturization is the result of corresponding high frequency because the main methods of inverter miniaturization are increasing the switching tube operating frequency and using high-frequency transformers. Another method is to improve the control method and optimize the spectrum of SPWM waves, thus reducing the size of the filter.

 

High input power factor

Many inverter systems use a circuit of some topology to convert DC power to high-frequency AC pulses, which are then rectified to obtain the desired DC voltage. Spikes in the output current can reduce the input power factor. Improving the power factor on the input side can be an effective solution to the harmonic pollution of the grid by the inverter.

 

Intelligence and digitization

Digitalization of inverters is not simply applying digital devices such as FPGA and microcontroller to the inverter, but the whole system relies on the computing power of digital devices and discrete control methods. With the development of hardware, the processor speed is getting higher and higher, prompting the inverter to develop in the direction of intelligence and digitization.