The DC voltage after rectification by the rectifier contains a pulsating voltage with a frequency six times that of the power supply. In addition, the pulsating current generated by the inverter also causes the DC voltage to fluctuate.
The main circuit is the power conversion part that provides voltage and frequency adjustable power to the asynchronous motor. The main circuit of the frequency converter can be broadly divided into two categories: voltage type and current type.
A current-source inverter is a power conversion section that provides regulated voltage and frequency power to an asynchronous motor. A voltage-source inverter converts DC power from a voltage source to AC power; its DC circuit filtering uses capacitors. A current-source inverter converts DC power from a current source to AC power; its DC circuit filtering uses inductors. It consists of three parts: a rectifier that converts mains frequency power to DC power, a smoothing circuit that absorbs voltage ripples generated in the inverter and frequency converter, and an inverter that converts DC power to AC power.
(1) Rectifier: Recently, diode-based frequency converters have been widely used, which convert power frequency power into DC power. Two sets of transistor converters can also be used to form a reversible converter, which can perform regenerative operation 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 aluminum electrolytic 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 adjustable) to the asynchronous motor. It consists of frequency and voltage calculation circuits, voltage and current detection circuits of the main circuit, motor speed detection circuit, drive circuit that amplifies the control signals from the calculation circuit, and protection circuits for the inverter and motor.
(1) Inverter operation circuit: compares and calculates 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) Inverter voltage and current detection circuit: isolated from the main circuit potential to detect voltage, current, etc.
(3) Inverter 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) Frequency converter speed detection circuit: The speed signal is sent to the operation circuit by the speed detector (tg, plg, etc.) installed on the shaft of the asynchronous motor. According to the instructions and operations, the motor can run at the command speed.
(5) Inverter 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.
In summary, based on the above six points, the lifespan of aluminum electrolytic capacitors is also very important in frequency converters.
