The main circuit is the power conversion part of the power supply for the induction motor. The main circuit of the frequency converter can be roughly divided into two types: the voltage type is the converter that converts the dc of the voltage source into AC, and the filter of the DC circuit is capacitance.
The current type is a converter that converts the dc of the current source to ac, and the dc loop filter is the inductor.
It consists of three parts: a "rectifier" that converts power-frequency power into DC power, a "flat-wave circuit" that absorbs the voltage pulsation generated by the converter and inverter, and an "inverter" that converts DC power into AC power.
The most recent use is of diode converters, which convert power to dc power.
Two groups of transistor converters can also be used to form a reversible converter, which can be regenerated due to its reversible power direction.
Flat wave loop
In the rectified DC voltage of the rectifier, there is a pulsating voltage of 6 times the frequency of the power supply. In addition, the pulsating current generated by the inverter also makes the DC voltage change.
To suppress voltage fluctuations, inductors and capacitors are used to absorb fluctuating voltages (currents).
The device capacity is small, if the power supply and the main circuit component has a margin, the inductance can be eliminated and the simple flat wave circuit can be adopted.
In contrast to the rectifier, the inverter converts the DC power to the ac power of the required frequency, so that the 3-phase AC output can be obtained by turning six switching devices on and off in the specified time.
Taking voltage PWM inverter as an example, the switching time and voltage waveform are presented.
Control circuit for asynchronous motor power supply (voltage, frequency adjustable) of main circuit provides the control signal circuit, it has "operation circuit of frequency, voltage, the voltage, current detection circuit of main circuit, motor speed detection circuit, the operation circuit of the control signal amplification" drive circuit ", and "protection circuit of inverter and motor.
(1) Operation circuit: Compare the external speed, torque and other instructions 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: detect voltage and current separately from the main circuit potential.
(3) Drive circuit: The circuit that drives the main circuit device.
It is isolated from the control circuit to make the main circuit components on and off.
(4) Speed detection circuit: The signal of the speed detector (TG, PLG, etc.) installed on the shaft of the asynchronous motor is taken as the speed signal and sent to the operation loop. The motor can operate at the command speed according to the instructions and operations.
(5) Protection circuit: Detect the voltage and current of the main circuit. In case of overload or overvoltage and other abnormalities, in order to prevent the inverter and asynchronous motor from damage, make the inverter stop working or suppress the voltage and current value.