The composition of general purpose VFD

Frequency conversion speed regulation is to change the synchronous speed of the rotating magnetic field by changing the power supply frequency of the motor stator. It is an efficient speed regulation method without additional slip loss. The outstanding advantages are high speed regulation efficiency, low start-up energy consumption, wide speed regulation range, stepless speed regulation, fast dynamic response, high speed regulation precision, convenient operation, and easy realization of production process control automation. General purpose VFD generally include: rectifier circuit, smoothing circuit, control circuit and inverter circuit. The following briefly introduces the composition and function of each circuit.

Rectifier circuit
The function of the rectifier circuit is to convert AC power into DC power. The rectifier circuit is generally a separate rectifier module, and the damage of the rectifier module is a common fault of the inverter. In the static state, the multimeter is used to block the forward and reverse measurements to judge whether the rectifier module is damaged. Of course, we can also use the withstand voltage meter to test.

Smoothing circuit
The DC voltage rectified by the rectifier contains a pulsating voltage of 6 times the frequency of the power supply. In addition, the pulsating current generated by the inverter also makes the DC voltage unstable. In order to suppress the voltage fluctuation, inductors and capacitors are used to absorb the pulsating voltage (current). The DC part of the power supply has a margin for the components of the main circuit, and the inductance is omitted and a simple capacitor filter circuit is used. To test the capacity and withstand voltage of the filter capacitor, we can also observe whether the safety valve on the capacitor bursts open, whether there is liquid leakage, etc. to judge whether it is good or bad.

Control circuit
Modern frequency conversion speed regulation basically uses 16-bit, 32-bit single-chip microcomputer or DSP as the control core, so as to realize full digital control. The frequency converter is a speed regulating device with adjustable output voltage and frequency. The loop that provides the control signal is called the main control circuit, and the control circuit is composed of the following circuits: frequency and voltage computing circuits, voltage and current detection circuits of the main circuit, motor speed detection circuits, and drive circuits for further amplification of the control signals of the computing circuits And the protection circuit of inverter and motor.

Inverter circuit

The inverter circuit is the opposite of the rectifier circuit. The inverter circuit converts the DC voltage into the AC voltage of the desired frequency, and turns on and off the 5 upper bridge and 6 lower bridge power switching devices at the determined time. Thus, a three-phase AC voltage with a phase difference of 2/3 can be obtained on the output terminals U, V, and W. The inverter circuit usually refers to the IGBT inverter module (the inverter produced in the early stage is a power module such as GTR). The damage of the IGBT module is also a common fault of the inverter. For IGBT module damage, most of the cases will damage the drive components. The most easily damaged components are Zener tubes and optocouplers. Conversely, if there are problems with the components of the drive circuit, such as capacitor leakage, breakdown, and aging of the optocoupler, it will also cause the IGBT module to burn out or the frequency conversion output voltage to be unbalanced. Check whether there is a problem with the drive circuit. You can compare whether the resistance of each trigger terminal is consistent when the power is not turned on. Turn on the power to measure the voltage waveform of the trigger terminal. However, some inverters cannot be turned on without a module installed. At this time, a dummy load is connected in series at the P terminal of the module to prevent the module from burning out due to accidental contact with the trigger terminal or other lines during inspection.

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