Working principle of DC inverter

Frequency converter is a commonly used component in electronic products. Do you know how it works?

The variable-frequency drive (VFD) principle is to apply the principle of frequency conversion technology and microelectronics technology to control the power control equipment of AC motor by changing the working power frequency of the motor. The power used is divided into AC power and DC power. Most DC power is obtained by transforming AC power through a transformer, rectifying and filtering. AC power accounts for about 95% of the total power used by people.

The inverter consists of a main circuit (including a rectifier, an intermediate DC link, and an inverter) and a control circuit. The functions of each part are as follows:

1. Rectifier Its function is to rectify three-phase (or single-phase) AC power into DC. In SPWM inverters, full-wave rectifier circuits are mostly used. In most medium and small capacity inverters, the rectifier device uses uncontrollable rectifier diodes or diode modules.

2. The function of the inverter is opposite to that of the rectifier. It converts DC power into AC power with variable voltage and frequency to achieve variable frequency speed regulation of AC motors. The inverter circuit is composed of switching devices, most of which use bridge circuits, often called inverter bridges. In the SPWM inverter, the switching device is controlled by the SPWM modulation signal in the control circuit to convert DC power into three-phase AC power.

3. Control circuit This part of the circuit is composed of operation circuit, detection circuit, drive circuit, protection circuit, etc., and generally adopts large-scale integrated circuits.

Working principle of DC inverter

The so-called DC inverter can only drive a brushless DC motor (it is different from the structure of AC voltage type and AC current type inverter, which is connected to AC induction motor or AC variable frequency motor). It uses semiconductor technology to first rectify the AC power and convert it into DC power, and then send it to the IGBT field effect tube or electronic module, which is controlled by the microprocessor chip command to switch. It is controlled by the Hall element installed inside the DC motor. The two complement each other and are indispensable.

The brushless DC motor is shown in the figure below.

Brushless motors are divided into; brushless DC motors (BLDC) and permanent magnet synchronous motors (PMSM).

Common control methods include: 1. Three-phase six-step control, commonly known as square wave control; 2. Sine wave control, also called pulse modulation (PWM);

The brushless DC motor is a new type of DC motor that uses transistor commutation technology to replace the traditional commutator commutator. Its structure is shown in the figure above.

There are two dead zones in the structure of the above brushless DC motor, that is, when the rotor rotates to the center point between the N and S poles, the Hall at this position cannot sense the magnetic field and must rotate by inertia. In order to overcome the above problem, the modulation width must be used to overcome it. The working principle of the brushless motor is as follows;

The stator winding of the motor must switch the direction of the current according to the magnetic pole position of the rotor in order to make the rotor rotate continuously. Therefore, a rotor magnetic pole position sensor must be installed in the brushless DC motor. This sensor usually uses a Hall element.

The Hall element is a magnetic induction sensor that can detect the polarity of the magnetic field, convert the polarity of the magnetic field into an electrical signal, and send it to the control electrode of the corresponding transistor. The excitation current in the stator winding is switched according to the signal of the Hall element, so that a rotating magnetic field can be formed to drive the rotor to rotate.

The Hall element is connected to the DC power supply through the current limiting resistor, and the bias current flows through the transistor to turn off or on in the corresponding direction. (As shown in the figure above), in the stator W1 coil and the stator W2 and W3 stator coils, it is changed by the signal detected by the Hall element change, forming a rotational motion. Generally, the Hall element is installed near the rotor pole of the brushless DC motor.

There are two dead zones in the above brushless DC motor structure, that is, when the rotor rotates to the position between the N and S poles as the neutral point, the Hall element cannot sense the magnetic field at this position, so there is no output, and the stator winding will also have no current, and the motor can only rotate by inertia. If the motor happens to stop at this position, it will not start. In order to overcome the above problems, people have also developed a variety of methods in the real line.

Schematic diagram of the internal structure of a brushless DC motor. It has three Hall elements distributed at 120 degrees in the bubble machine, the rotor is a single-pole (N, S) permanent magnet, and the stator winding is 3 groups. It is driven by 6 transistors V1~V6 to drive their respective windings, and the rotor position is detected by two Hall elements. The direction marked (red 1) in the figure. During the rotation of the rotor magnetic pole, when the N pole approaches the Hall element HG1, it will sense the magnetic field signal and convert it into an electrical signal of the corresponding polarity, and so on. There is current in winding L1 and no current in L2. The magnetic field S pole generated by L1 will attract the N pole and repel the S pole, causing the rotor to rotate counterclockwise.