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. Frequency converters rely on internal IGBTs to adjust the voltage and frequency of the output power supply, providing the required power voltage according to the actual needs of the motor, thus achieving energy-saving speed regulation.
In addition, frequency converters also have multiple protection functions such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, frequency converters have been widely used.
A frequency converter mainly consists of a rectifier (AC/DC), a filter, an inverter (DC/AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit.
1. Rectifier
The rectifier circuit of a general-purpose frequency converter consists of a three-phase bridge rectifier. Its function is to rectify the power frequency power supply, providing the required DC power for the inverter and control circuits. Recently, diode converters have been widely used, which convert the power frequency power supply into DC power. Two sets of transistor converters can also be used to construct a reversible converter; because its power direction is reversible, it can perform regenerative operation.
2. 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.
3. Inverter
Unlike rectifiers, inverters convert DC power into AC power at the required frequency. By switching six switching devices on and off at predetermined times, a three-phase AC output can be obtained. The function of the inverter circuit is to convert the DC power output from the DC circuit into an adjustable AC power supply under the control of the control circuit. The output of the inverter circuit is the output of the frequency converter; therefore, the inverter circuit is one of the core circuits of the frequency converter and plays a very important role.
4. Main circuit and control 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 voltage to AC, and current-source converters, which convert DC current to AC, and current-source converters, which use inductors for DC circuit filtering.
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".
