Simply put, it's a process of converting AC to DC and then back to AC. The final conversion to AC is a type of AC with frequency and voltage controlled by pre-set parameters. This allows the AC motor acting as a load to change from a fixed speed to an arbitrary speed with minimal torque variation, which can be programmably controlled. It also includes internal protection devices for overcurrent, overvoltage, undervoltage, and overload, as well as displays for speed, current, and voltage, and can easily display recent fault information.
It can be operated via panel, button, or PLC , allowing for both on-site and remote control.
Inverter working principle
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 inductors, which filter the DC circuit. It consists of three parts: a rectifier that converts the mains frequency power to DC power; a smoothing circuit that absorbs voltage ripples generated in the converter and inverter; and an inverter that converts DC power to AC power.
( 1 ) Rectifier: Recently, diode converters have been widely used to convert power frequency power into DC power. Two sets of transistor converters can also be used to form a reversible converter, which can be regenerated because its power direction is reversible.
( 2 ) Smoothing circuit: The DC voltage after rectification by the rectifier contains a pulsating voltage with a frequency of 6 times that of the power supply. In addition, the pulsating current generated by the inverter also causes the DC voltage to fluctuate. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). When the device capacity is small, if the power supply and main circuit components have a margin, the inductor can be omitted and a simple smoothing circuit can be used.
( 3 ) Inverter: In contrast to rectifier, inverter converts DC power into AC power at the required frequency. By turning on and off 6 switching devices at a predetermined time, a 3- phase AC output can be obtained. The switching time and voltage waveform are shown as an example of a voltage-type PWM inverter.
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".
( 1 ) Operational circuit: compares and calculates the external speed, torque and other commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter.
( 2 ) Voltage and current detection circuit: isolated from the main circuit potential to detect voltage, current, etc.
( 3 ) Drive circuit: The circuit that drives the main circuit devices. It is isolated from the control circuit to turn the main circuit devices on and off.
( 4 ) Speed detection circuit : The speed signal is sent to the operation circuit by the speed detector (tg , plg , etc. ) installed on the asynchronous motor shaft . According to the instructions and operations, the motor can run at the command speed.
( 5 ) Protection circuit : detects the voltage and current of the main circuit. When an overload or overvoltage occurs, in order to prevent damage to the inverter and asynchronous motor, the inverter stops working or suppresses the voltage and current values.
