How does the frequency converter work?

It is understood that the inverter is to change the power supply voltage by changing the frequency of the power supply. Specifically, what is the principle of inverter operation?

The main circuit is to provide the power conversion part of the voltage regulating and frequency modulation power supply for the asynchronous motor. The main circuit of the frequency converter can be roughly divided into two categories [1]: The voltage type is the inverter that converts the DC of the voltage source into AC, and the DC loop Filtering is a capacitor. The current type is a frequency converter that converts the current from a direct current source to an alternating current, and the direct current loop filter is an inductor. It consists of three parts: a "rectifier" that converts the power-frequency power source into a DC power source, a "fluid-wave circuit" that absorbs the voltage ripple generated by the inverter and the inverter, and an inverter that converts DC power into AC power. Device."

(1) Rectifier: Recently, a large number of diode inverters are used, which converts power frequency power to DC power. It is also possible to use two sets of transistor converters to construct a reversible converter, which can be regenerated due to its reversible power direction.

(2) Smooth wave circuit: In the DC voltage after the rectifier is rectified, it contains the pulsating voltage of 6 times the power supply frequency, and the pulsating current generated by the inverter also changes the DC voltage. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb the ripple voltage (current). When the capacity of the device is small, if the power supply and the main circuit configuration device have a margin, a simple flat wave circuit can be omitted from the inductor.

(3) Inverter: In contrast to a rectifier, an inverter converts DC power to AC power of a desired frequency, and turns the six switching devices on and off in a determined time to obtain a 3-phase AC output. Take the voltage type pwm inverter as an example to illustrate the switching time and voltage waveforms.

The control circuit is the circuit that provides the control signal to the main circuit (voltage, frequency adjustable) of the asynchronous motor. It has the frequency and voltage "operation circuit", the main circuit "voltage and current detection circuit", the motor's "speed detection. "Circuit", a "driver circuit" that amplifies the control signal of the arithmetic circuit, and a "protection circuit" of the inverter and the motor.

(1) Inverter operation circuit: It compares the external speed and torque 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: It is isolated from the main circuit potential to detect voltage and current. Inverter operating principle (3) Inverter drive circuit: The circuit that drives the main circuit device. It is isolated from the control circuit so that the main circuit device turns on and off.

(4) Inverter speed detection circuit: The signal of the speed detector (tg, plg, etc.) installed on the asynchronous motor shaft machine is the speed signal, which is sent to the calculation circuit. According to the instruction and operation, the motor can be operated at the command speed.

(5) Inverter protection circuit: Detects the voltage, current, etc. of the main circuit. When an abnormality such as overload or overvoltage occurs, in order to prevent the damage of the inverter and the asynchronous motor, the inverter stops working or suppresses the voltage and current values.