Detailed explanation of the principle, application, structure and selection of the inverter
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Inverter air conditioners are commonly heard of, but what exactly is an inverter? The principle of the frequency converter lies in modern power electronics technology. It acts as a common conversion device between DC and AC, capable of changing the frequency of AC to control the power equipment of an AC motor. A frequency converter typically consists of several key components: rectification (AC to DC), filtering, inversion (DC to AC), braking unit, drive unit, and a microprocessor-based detection unit.
The inverter adjusts the voltage and frequency of the output power supply by using internal IGBTs to switch, providing the required power supply voltage based on the actual needs of the motor. This allows for energy savings and speed regulation. Additionally, the frequency converter includes various protection functions such as overcurrent, overvoltage, and overload protection.
The figure above illustrates a circuit with variable AC frequency. P and N represent converter circuits that can convert AC to DC and load it onto the load Z. When the pulse signal to P is sinusoidal, the DC becomes sinusoidal, forming the upper half of the sine wave. The period corresponds to the regular period of the pulse signal, which is an integer multiple of the AC period. Through the cooperation of P and N, a periodic sinusoidal current on the load Z is formed, allowing the frequency to be adjusted according to the pulse signal period—this is how the frequency converter operates.
In terms of application, inverter air conditioners use this type of frequency converter to run at different frequencies. They automatically adjust their operating frequency based on the set temperature and the room temperature. When there's a large temperature difference, the frequency increases, enabling faster cooling or heating. Conversely, when the room temperature approaches the set temperature, the frequency decreases, reducing power consumption and avoiding frequent start-ups of the outdoor compressor, thus saving energy.
Beyond air conditioning, frequency converters are widely used in motor control, enabling smooth speed regulation. They are extensively applied in automotive and locomotive fields, offering continuous speed adjustment capabilities.
The main circuit structure of an inverter can be classified into two types: voltage-type and current-type structures. From a working perspective, the main function of the frequency converter is to convert alternating current to alternating current. Therefore, the circuit operates through intersection transformation or similar forms.
The diagram above shows a typical voltage-type inverter circuit. In such a setup, the DC intermediate stage is omitted, but the number of switching tubes remains the same. This structure is often found in ultra-high-power low-speed systems. However, the biggest disadvantage is that the output frequency must be less than one-third or one-half of the grid frequency, otherwise the output voltage waveform becomes distorted, making it suitable for low-speed motors.
Another versatile circuit is the cross-connected main circuit structure, which can be divided into voltage-type and current-type based on its operation mode. The voltage-type structure is more widely used due to its stability and efficiency.
It features a DC bus voltage provided by an electrolytic capacitor, with the front stage using diode uncontrolled rectification. This ensures simplicity and reliability. The inverter uses three-phase PWM modulation, allowing precise control over the inverter's output. With a stable DC bus voltage, controlling the switching sequence and duty ratio of the IGBTs enables excellent performance.
Voltage-type AC-DC converters offer advantages such as a simple structure, lower harmonic content, and adjustable stator and rotor power factors. These features improve the generator’s operating state and output quality. The separation of the grid side and rotor side is achieved via the DC bus capacitor, enabling decoupled control of active and reactive power.
To adapt to different grid conditions, inverters may include additional components like boost circuits or filter stages. These help maintain normal operation under varying voltage levels and high noise environments.
Choosing the right inverter involves considering factors such as the site environment, controlled object, speed range, steady-state accuracy, torque requirements, and wiring conditions. The selection should balance production process and economic efficiency.
The inverter's operating current curve should match the load current curve. This involves ensuring voltage, current, and torque compatibility. A thorough understanding of the system's power consumption and voltage waveform quality is essential. Additionally, knowledge of the load's performance curve determines the inverter's application method.
Typically, the inverter's power rating should slightly exceed the motor's power to allow for a margin. If the motor requires frequent starts and stops, a braking resistor should be installed. In harsh environments, water-cooled inverters can prevent module failure and reduce noise. For aging tests, four-quadrant products can minimize power loss. If a separate DC power source is available, purely inverter modules can save costs.
High-power inverters should include a reactor at the input to improve power quality and power factor. Depending on the motor type, different control methods such as constant voltage or constant current can be used. The inverter's functionality should align with the motor's control purpose.
When driving high-speed motors, the inverter's output current increases due to reduced reactance and higher harmonics. Therefore, the inverter should be slightly larger than for standard motors. If long cables are used, measures must be taken to suppress ground coupling capacitance effects, possibly requiring an output reactor. In special applications like high temperature or altitude, derating may be necessary, so the inverter capacity should be increased accordingly.
In critical precision applications, testing the motor and selecting the appropriate type is essential, sometimes requiring specific encoder specifications. Ultimately, choosing the right inverter depends on local conditions and tailored solutions for each site.