Analysis of the working principle of PWM transistor frequency converters : Actual frequency converters use semiconductor switches instead of mechanical switches to achieve high-speed switching. Early frequency converters used thyristors or transistors, but current low-noise frequency converters mainly use IGBTs (Insulated Gate Bipolar Transistors).
The diagram below shows the basic circuit of a transistor inverter that uses semiconductor switches.
The following diagram shows the actual main circuit structure of a frequency converter.
The functions of each are described below.
1. Surge voltage absorption circuit
The inverter is protected by suppressing surge voltages that enter the inverter from the power supply.
2. Rectifier section
Alternating current (AC) is converted to direct current (DC) using a diode. The resulting DC voltage exhibits pulsation.
3. Inrush current suppression circuit
When the inverter is powered on, it suppresses the large current flowing from the diode to the smoothing capacitor, thus protecting the smoothing capacitor.
4. Smoothing capacitor
This smooths out the pulsating DC voltage generated by the rectifier, making it a flat DC voltage.
5. Braking circuit
When the frequency converter decelerates, energy returns from the motor to the frequency converter, causing the DC voltage to rise.
The rising voltage flows to the resistor to prevent overvoltage in the frequency converter.
6. Inverter Section
AC with variable frequency and variable voltage is generated by switching DC voltage with transistors.
The buffer circuit has the function of suppressing the surge voltage generated when the transistor is switched, thus protecting the transistor.
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