Instrumentation Application: Working Principle of Inverter
The main circuit is the power conversion part that provides variable voltage and frequency power supply for asynchronous motors. Broadly speaking, the main circuit of an inverter can be divided into two categories [1]: Voltage type - this converts a DC voltage source to AC, with a capacitor used as the filter in the DC loop. Current type - this converts a DC current source to AC, with an inductor used as the filter in the DC loop. It consists of three parts: the "rectifier" which converts industrial frequency power to DC power, the "smoothing circuit" which absorbs voltage ripples generated by the inverter and rectifier, and the "inverter" which converts DC power to AC power.
(1) Rectifier: Recently, diode-based inverters have been widely used; they convert industrial frequency power to DC power. Two groups of thyristor converters can also be used to form a reversible converter. Since its power direction is reversible, it can perform regenerative operation.
(2) Smoothing Circuit: In the DC voltage rectified by the rectifier, there is a pulsating voltage at six times the power supply frequency. Additionally, the pulsating current generated by the inverter causes fluctuations in the DC voltage. To suppress these voltage fluctuations, inductors and capacitors are used to absorb pulsating voltages (currents). When the installation capacity is small, if there is sufficient margin in the power supply and the main circuit components, the inductor can be omitted, using a simple smoothing circuit.
(3) Inverter: The inverter does the opposite of the rectifier, converting DC power to AC power at the required frequency. By turning on and off six switching devices at predetermined times, a three-phase AC output can be obtained. For example, the switching time and voltage waveform of a voltage-type PWM inverter are shown.
The control circuit provides control signals for the main circuit that supplies power (with adjustable voltage and frequency) to the asynchronous motor. It consists of a "calculation circuit" for frequency and voltage, a "voltage and current detection circuit" for the main circuit, a "speed detection circuit" for the motor, a "drive circuit" that amplifies the control signals from the calculation circuit, and a "protection circuit" for the inverter and motor.
(1) Inverter Calculation Circuit: This compares external speed and torque commands with the current and voltage signals detected by the detection circuit to determine the output voltage and frequency of the inverter.
(2) Inverter Voltage and Current Detection Circuit: This detects voltage and current while being electrically isolated from the main circuit potential.
(3) Inverter Drive Circuit: This drives the main circuit components. It ensures the conduction and cutoff of the main circuit components while being isolated from the control circuit.
(4) Inverter Speed Detection Circuit: This uses signals from a speed detector (such as TG or PLG) mounted on the shaft of the asynchronous motor as speed signals, sending them to the calculation circuit. According to instructions and calculations, it enables the motor to operate at the instructed speed.
(5) Inverter Protection Circuit: This detects voltage and current in the main circuit. When overloads or overvoltages occur, it stops the inverter or suppresses the voltage and current values to prevent damage to the inverter and asynchronous motor.