What is a frequency converter?
A frequency converter is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the power supply. The frequency converter adjusts the voltage and frequency of the output power supply by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, frequency converters have many protection functions, such as overcurrent, overvoltage, and overload protection.
Variable frequency drive components
A frequency converter mainly consists of a rectification unit (AC to DC), a filter, an inverter (DC to AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit. The frequency converter adjusts the output voltage and frequency by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, the frequency converter has many protection functions, such as overcurrent, overvoltage, and overload protection.
1. Main circuit
The main circuit is the power conversion section that provides voltage and frequency adjustable power to the asynchronous motor. The main circuit of a frequency converter can be broadly divided into two categories: voltage-source converters, which convert DC power from a voltage source to AC power, and current-source converters, which convert DC power from a current source to AC power, and inductors, which filter the DC circuit. It consists of three parts: a rectifier that converts the mains frequency power to DC power; a smoothing circuit that absorbs voltage ripples generated in the converter and inverter; and an inverter that converts DC power to AC power.
2. Rectifier
Diode-based converters are widely used to convert mains frequency power into DC power. Two sets of transistor converters can also be used to construct a reversible converter, which can operate in regenerative mode because its power direction is reversible.
3. Smoothing circuit
The DC voltage after rectification contains a pulsating voltage at six times the frequency of the power supply. Furthermore, the pulsating current generated by the inverter also causes DC voltage fluctuations. To suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). For small device capacities, if the power supply and main circuit components have sufficient capacity, the inductor can be omitted, and a simple smoothing circuit can be used.
4. Inverter
Unlike rectifiers, inverters convert DC power into AC power at the required frequency. A three-phase AC output is obtained by switching six switching devices on and off at predetermined times. The switching time and voltage waveforms are shown as an example using a voltage-source PWM inverter.
The control circuit is a loop that provides control signals to the main circuit that supplies power (voltage and frequency are adjustable) to the asynchronous motor. It consists of a frequency and voltage "operation circuit", a main circuit "voltage and current detection circuit", a motor "speed detection circuit", a "drive circuit" that amplifies the control signals from the operation circuit, and an inverter and motor "protection circuit".
With the continuous improvement of industrial automation, frequency converters have been widely used.
