A frequency converter is a static variable frequency power supply device composed of electronic components that converts a fixed-frequency alternating current into an alternating current with an adjustable frequency within a certain range. Its output frequency range is generally between 0 and several hundred hertz. It can achieve high-performance stepless speed regulation and save energy in some applications. It is now widely used in speed control systems for AC asynchronous motors. Its drawback is that the output voltage waveform is not sinusoidal, meaning it has a higher harmonic content. A higher number of high-order harmonics can negatively impact motor efficiency, temperature rise, noise, and other performance characteristics.
Commonly used low-voltage frequency converters are classified as follows:
(1) Voltage type and current type
①From the perspective of the main circuit method, voltage-type converters convert DC power from a voltage source into AC power, which is more widely used; current-type converters convert DC power from a current source into AC power.
② Based on the components of the rectifier and smoother, the voltage type is more commonly used, which uses thyristors or diodes to form the rectifier and capacitors as the smoother; the current type uses thyristors to form the rectifier and reactors as the smoother.
③ In terms of controller type, voltage-type control signals are voltage, while current-type control signals are current. The former is general-purpose, while the latter is suitable for applications requiring fast response.
(2) "Intersection-Direction-Intersection" type and "Intersection-Intersection" type
Based on the voltage change process, there are two commonly used types: "AC-DC-AC" and "AC-AC".
(3) Ordinary type and vector control intelligent type
There are general-purpose constant voltage-frequency ratio types, special-purpose types to meet certain special requirements, and more advanced intelligent types. Intelligent types include slip frequency control types and vector control types.
Vector control intelligent inverters have a "self-learning" function. That is, after the motor is first turned on, the inverter tests and reads certain performance parameters of the controlled motor (such as stator and rotor reactance, values that often require complex experiments to obtain), stores them, and then uses them to control the motor's operation. This is currently the most advanced type of inverter.
(4) "High-Low-High" and "High-High" types
These two types are used for high-voltage frequency converters . The "high-low-high" type first reduces the high voltage to low voltage, then uses a low-voltage frequency converter to complete the frequency conversion, and finally converts it back to high voltage for use by high-voltage equipment; while the "high-high" type directly uses a high-voltage frequency converter.
(5) Open-loop control and closed-loop control
The main difference between the two lies in whether there is a feedback signal involved in the input and output control of the frequency converter. If there is, it is closed-loop control; if not, it is open-loop control.
(6) Pulse amplitude modulation (PAM), pulse width modulation (PWM) and high carrier frequency PWM control.
The commonly used types of low-voltage frequency converters are summarized above for reference and reading.
