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How Does a Frequency Converter Work?

Views:79     Author:Site Editor     Publish Time: 2019-11-19      Origin:Site


The AC power supply is characterized by stable frequency, stable voltage, internal resistance equal to zero, and a pure sine wave (no distortion). The variable frequency power supply is very close to the ideal AC power supply. Therefore, the advanced developed countries are increasingly using the variable frequency power supply as the standard power supply to provide the best power supply environment for the electrical appliances, and to objectively assess the technical performance of the electrical appliances.


There are two main types of variable frequency power supply: linear amplification type and PWM switching type HY series program-controlled variable frequency power supply. The microprocessor is the core, and the MPWM method is used. The active component IGBT module is designed. The digital frequency division, D/A conversion, instantaneous value feedback, sinusoidal pulse width modulation and other technologies are adopted, so that the single unit capacity can reach 100kVA, isolating the transformer output to increase the stability of the whole machine, with strong load adaptability, good output waveform quality, easy operation, small size, light weight, etc. It has the protection functions of short circuit, over current, overload, overheating to ensure reliable operation of the power supply.


In the early stage of the development of frequency modulation voltage regulation technology, PAM mode is always adopted. Therefore, the AC voltage waveform output by the power inverter can only be a square wave. The square wave RMS value can only be changed by changing the amplitude of the square wave. With the emergence of fully-controlled fast switching devices such as GTR, IGBT, MOSFET, etc., it has gradually developed into a PWM mode. The inverter process can complete the voltage regulation and frequency modulation tasks at the same time. The rectifier does not need to be controlled, the device structure is simpler, and the control is more convenient. The output voltage is improved from a square wave to a PWM wave, reducing the low harmonic content of the output voltage.


SPWM is a control method in which a sine wave is used as a reference wave (modulated wave), and a series of equal-amplitude triangular waves (carriers) are compared with a reference sine wave to generate a PWM wave. When the reference sine wave is higher than the triangular wave, the corresponding switching device is turned on, when the reference sine wave is lower than the triangular wave, the corresponding switching device is turned off. Therefore, the output voltage waveform of the inverter is a pulse train, which is characterized in that: in a half cycle, the pulses are equidistantly equal in width, always wide in the middle, narrow on both sides. The pulse wave is low-pass filtered to obtain a sine wave of the same frequency as the modulated wave. The amplitude and frequency of the sine wave are determined by the amplitude and frequency of the modulated wave. This is the principle of frequency conversion power supply voltage regulation.


The three-phase AC voltage is rectified into a pulsating DC voltage through a three-phase bridge uncontrolled rectifier circuit, and becomes a smooth DC voltage through the the energy storage and filtering of the intermediate filter capacitor. The inverter process consists of four IGBTs to form a full-bridge inverter. The anti-parallel diode completes the freewheeling operation when the IGBT is turned off. R, C, and D form an RCD blocking discharge type absorption buffer circuit. The inverter part adopts SPWM control mode to invert the DC voltage into SPWM pulse wave with adjustable voltage and frequency. The inductor L and the capacitor C3 form a low pass filter LC, which filters out the high frequency carrier component. In order to limit the capacitor charging current, a current limiting resistor R1 is connected between the output end of the rectifier bridge and the storage capacitor. The current limiting resistor R1 is only connected in the initial short period of the power supply. 


Whether the design of the low-pass filter LC output is suitable, it directly affects the distortion of the output voltage waveform of the variable frequency power supply. Therefore, the design principle of the filter is to consider the highest output frequency.



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