1) Rectifier
The rectifier circuit of the universal inverter consists of a three-phase bridge rectifier bridge. Its function is to rectify the power frequency power supply and supply DC power to the inverter circuit and control circuit. Three-phase AC power supplies generally require the input of the rectifier bridge through a network of absorbing capacitors and varistors. The function of the network is to absorb the high-frequency harmonic signals and surge overvoltages of the communication power grid to prevent damage to the VFD. When the power supply voltage is three-phase 380, the maximum reverse voltage of the V-type rectifier equipment is generally 1200-1600V, and the maximum rectifier current is twice the rated current of VFD.
2) Center circuit
There are three types of central circuits: converting the rectifier voltage into DC current; Make the pulsating DC voltage of the inverter stable or smooth; The rectified fixed DC voltage is converted into a variable DC voltage. The central circuit can be regarded as an energy storage device, and the motor can obtain energy from the central circuit through the inverter. When using a power inverter, the central circuit consists of a large inductance coil and can only be used in conjunction with a controllable rectifier. An inductor converts a variable current voltage from the rectifier output into a variable DC current. The voltage of the motor depends on the load. In order to reduce the fluctuation of DC voltage and current, the filter of the central circuit can filter the output of the rectifier circuit. The capacitance and inductance of the filter keep the output voltage constant at a given frequency, and then achieve the required withstand voltage and capacitance. According to the circuit plan, the central circuit can also provide the following additional functions.
3) Inverter circuit
The function of the inverter circuit is to convert the DC power output by the DC circuit into communication energy that can be adjusted arbitrarily under the action of the control circuit. The output of the inverter circuit is the output of the VFD, so the inverter circuit is one of the central circuits of the VFD and plays a very important role.
The most common inverter circuit structure is a three-phase bridge inverter circuit with six power switching devices (GTR, IGBT, GTO, etc.). ), regularly control the on/off of the power switching equipment in the inverter to obtain the three-phase communication output of arbitrary frequency.
Generally, small and medium-capacity VFD main circuit equipment generally adopts integrated modules or intelligent modules. The intelligent module is highly integrated with a rectifier module, an inverter module, various sensors, maintenance circuits and drive circuits. It integrates rectifier module, power factor calibration circuit, IGBT inverter module and various detection and maintenance functions. The typical switching frequency of the module is 20KHz, and its maintenance function is to output fault signal light when undervoltage, overvoltage, and overtemperature are faulty.
The freewheeling circuit is set in the inverter circuit. The function of the freewheeling circuit is that when the frequency drops, the synchronous speed of the asynchronous motor also decreases. It provides a channel for the regenerative energy of the asynchronous motor to be fed back to the DC circuit. During the inverter, the parasitic inductance provides a channel for the release of energy. In addition, when two switches on the same arm are turned on together, a short circuit will be generated and the converter device will be burned. Therefore, in the actual general-purpose VFD, auxiliary circuits such as corresponding buffer circuits are also set up to ensure the normal operation of circuits and converter equipment in the event of an accident.
4) Control the circuit
The requirement takes into account that part of the control function is based on the planning effect of the VFD itself, because the control circuit is able to send signals to and receive signals to the rectifier, central circuit, and inverter.
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