With the development of electronic technology, frequency converters have undergone significant changes, but their basic principles remain unchanged. A frequency converter has four main components: a rectifier, intermediate circuits, an inverter, and a control circuit.
(A frequency converter has four main components)
Definition of frequency converter
A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of its power supply. A VFD mainly consists of a rectifier (AC/DC), filter, inverter (DC/AC), braking unit, drive unit, detection unit, and microprocessor unit. The VFD relies 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 to achieve energy-saving speed regulation. In addition, the VFD also has multiple protection functions such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, VFDs have been widely used.
The four components of a frequency converter
1) Rectifier
The rectifier circuit of a general-purpose frequency converter consists of a three-phase bridge rectifier. Its function is to rectify the mains frequency power supply, providing the necessary DC power for the inverter and control circuits. The three-phase AC power supply typically needs to be introduced into the input of the rectifier bridge through a network of absorption capacitors and varistors. The network's role is to absorb high-frequency harmonic signals and surge overvoltages from the AC mains, preventing damage to the frequency converter. When the power supply voltage is three-phase 380V, the maximum reverse voltage of the rectifier devices is generally 1200-1600V, and the maximum rectified current is twice the rated current of the frequency converter.
2) Intermediate circuit
There are three types of intermediate circuits: those that convert rectified voltage to DC current; those that stabilize or smooth pulsating DC voltage for use by the inverter; and those that convert a fixed rectified DC voltage to a variable DC voltage. The intermediate circuit can be viewed as an energy storage device; the motor can obtain energy from the intermediate circuit through the inverter. When using a power inverter, the intermediate circuit consists of a large inductor coil and can only be used with a controlled rectifier. The inductor coil converts the variable current voltage output by the rectifier into a variable DC current. The magnitude of the motor voltage depends on the load. To reduce fluctuations in DC voltage and current, the intermediate circuit's filter can filter the output of the rectifier circuit. The filter's capacitor and inductor keep the output voltage constant at a given frequency, thus obtaining the required withstand voltage and capacity. The intermediate circuit can also provide the following additional functions depending on the circuit design.
3) Inverter circuit
The function of an inverter circuit is to convert the DC power output from a DC circuit into an adjustable AC power supply under the control of a 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.
The most common inverter circuit structure is a three-phase bridge inverter circuit using six power switching devices (GTR, IGBT, GTO, etc.). By periodically controlling the on and off of the power switching devices in the inverter, a three-phase AC output of any frequency can be obtained.
Generally, small and medium-capacity frequency inverters typically use integrated or intelligent modules in their main circuitry. Intelligent modules highly integrate rectifier modules, inverter modules, various sensors, protection circuits, and drive circuits. Examples include Mitsubishi's IPMPM50RSA, Fuji's 7120MBP50RA, and Siemens' 060BSM50GD120, which integrate rectifier modules, power factor correction circuits, IGBT inverter modules, and various detection and protection functions. Typical module switching frequencies are 20kHz, and protection functions include outputting fault indicator lights for undervoltage, overvoltage, and overheating faults.
The inverter circuit includes a freewheeling circuit. The function of the freewheeling circuit is to provide a path for the regenerative energy of the asynchronous motor to be fed back to the DC circuit when the frequency decreases, as the synchronous speed of the asynchronous motor also decreases. During the inverter process, parasitic inductance provides a path for energy release. Furthermore, when two switches on the same bridge arm are opened simultaneously, a short circuit will occur, burning out the commutator components. Therefore, in practical general-purpose frequency converters, corresponding auxiliary circuits such as buffer circuits are also included to ensure normal circuit operation and to protect the commutator components in case of accidents.
4) Control circuit
The control circuit transmits signals to the rectifier, intermediate circuits, and inverter, and receives signals from these components. The control section depends on the design of each inverter.
Inverter principle
A variable-frequency drive (VFD) is a power control device that uses frequency conversion and microelectronics technologies to control an AC motor by changing the frequency of its power supply. The power supply used is either AC or DC. DC power is generally obtained by transforming AC power through a transformer, followed by rectification and filtering. AC power accounts for approximately 95% of the total power consumption.
The above information explains the working principle of frequency converters. There are many frequency converter brands on the market now, and their quality varies. It is essential to understand them before purchasing to avoid making a blind purchase.
