In the dc voltage rectified by the rectifier, there is a pulsation voltage of 6 times the frequency of the power supply. In addition, the pulsation current generated by the inverter also causes the dc voltage to change.
The main circuit is the power conversion part which provides the induction motor with the voltage and frequency modulation power supply.
The current mode is the power conversion part of the main circuit that provides the induction motor with voltage and frequency modulation power supply.
The voltage type is the converter that converts the dc of the voltage source into ac, and the filter of the dc circuit is the capacitor.
The current type is a converter that converts the dc of the current source into ac, and the dc circuit filter is the inductance.
It consists of three parts: a rectifier that converts power supply to dc power, a flat-wave circuit that absorbs voltage pulsation generated in the converter and inverter, and an inverter that converts dc power to ac power.
(1) rectifier: recently, a lot of use is diode converter, which converts power supply into dc power supply.Two sets of transistor converters can also be used to constitute a reversible converter.
(2) flat-wave circuit: in the dc voltage rectified by the rectifier, there is a pulsation voltage of 6 times the frequency of the power supply. In addition, the pulsation current generated by the inverter also causes the dc voltage to change.To suppress voltage fluctuations, an inductor and an aluminum electrolytic capacitor are used to absorb the pulsating voltage (current).
Device capacity is small, if the power supply and main circuit components have a margin, can be eliminated inductance using a simple flat wave circuit.
(3) inverter: in contrast to the rectifier, the inverter converts the dc power into the ac power of the required frequency, so that the 3-phase ac output can be obtained by switching the 6 switching devices on and off at the specified time.The switching time and voltage waveform are demonstrated by taking the voltage PWM inverter as an example.
Control circuit is for asynchronous motor power supply (voltage, frequency adjustable) of main circuit provides the control signal circuit, it has the operation frequency, voltage circuit, main circuit of voltage, current detection circuit, motor speed detection circuit, the operation control signal amplification of the drive circuit of the circuit, and the composition of inverter and motor protection circuit.
(1) inverter operation circuit: compare the external speed, torque and other commands with the current and voltage signals of the detection circuit, and determine the output voltage and frequency of the inverter.
(2) inverter voltage and current detection circuit: isolated from the main circuit potential to detect voltage and current, etc.
(3) makes the drive circuit, the circuit that drives the main circuit device.It is isolated from the control circuit to make the main circuit device on and off.
(4) speed detection circuit of frequency converter:Take the speed signals of speed detectors (tg, PLG, etc.) installed on the shaft of asynchronous motor as speed signals,And send them into the calculation loop. The motor can run at the command speed according to the instructions and calculations.
(5) makes protection circuit:Detect the voltage and current of the main circuit. In case of overload or overvoltage,Make the makes stop working or suppress the voltage and current value in order to prevent the makes and asynchronous motor from being on.
Conclusion: according to the above findings, the service life of aluminum electrolytic capacitor is also very important in the frequency converter.