What Is a Resonant Inverter?

Resonant pole soft inverter refers to the electronic components whose DC link voltage is not affected by resonance, and the auxiliary circuit is connected to the output of the inverter. ^[1]

Brushless DC motors have developed rapidly due to their superior performance. However, brushless DC motors are usually driven by hard-switching inverters. Hard-switching inverters have low system efficiency, high voltage and current stresses on switching devices, and heat dissipation. The volume and weight of the device are large, and the excessively high du / dt and di / dt generated by the hard on / off have caused very serious electromagnetic interference to the system, affecting the insulation life of the motor, and the voltage at the moment of the power switch device turning on and off And current spikes may cause the operating trajectory of the power device to exceed the safe operating area, resulting in damage to the switching device and affecting system reliability. Therefore, hard-switching inverters limit the further improvement of power density and performance of high-power brushless DC motor drive systems. Controller efficiency, electromagnetic interference, and weight and weight issues have become increasingly prominent and become the main factors restricting their rapid development.

In order to solve the many problems of hard-switching inverters, people have put more effort into soft-switching technology. Since the concept of a resonant switch, a soft switch, was introduced in the early 1980s, soft switching technology has attracted a large number of researchers and has become a research hotspot in the field of power electronics and motion control. Soft-switching voltage source inverters for AC motor drives mainly include two types: resonant DC link inverters and resonant pole inverters. Research on resonant DC link soft-switching inverters has produced many topologies and control methods. The common feature of this type of inverter is that a power device is connected in series on the DC bus, and its conduction loss will seriously affect the improvement of inverter efficiency. , And the DC bus groove affects the utilization of the DC voltage.

The auxiliary circuit of the resonant pole inverter is connected to the three output terminals of the inverter, and the DC link voltage is not affected by resonance. Typical circuits of resonant pole inverters include quasi-resonant current mode inverters, auxiliary resonant commutating pole inverters, zero-voltage conversion inverters, star buffers and delta buffers. However, the research of these resonant pole inverters is mainly focused on asynchronous motor drives. Asynchronous motors control the sinusoidal current in the three-phase windings, and the brushless DC motors work in a star-phase three-phase six-phase 120 ° commutation. For square wave control, only two phases are energized and the square wave, the control method and the action of the switching device are greatly different. Therefore, the traditional resonant soft-switching inverter technology is not completely suitable for brushless DC motors. It is necessary to study Resonant pole soft switching inverter suitable for brushless DC motor. A resonant soft-switching inverter dedicated to a brushless DC motor drive system is proposed, as shown in Figure 1. The poles of the three-phase bridge arms are connected to a resonant network. Each phase of the resonant network contains a bidirectional switch and a small buffer resonant capacitor. The three phases share a resonant inductor and two capacitors that form the midpoint voltage of the DC bus. The buffer resonance capacitor provides the ZVS turn-off condition for the PWM modulation of the main switching device of this phase. The bidirectional switch controls the resonance process to realize the potential change of the buffer resonance capacitor, and provides the ZVS turn-on condition for the PWM modulation of the main switching device. The bidirectional switch is switched on and off under ZCS conditions, and its voltage stress is half that of the DC power supply.

Direct torque control theory, with its direct control methods, simple structure, and excellent performance, largely solves the complex control in vector control, its characteristics are easily affected by changes in motor parameters, and its actual performance is difficult to reach the theoretical analysis results. Many major issues. However, direct torque control has the disadvantage of torque ripple at low speeds. At this time, if the switching frequency can be increased, the low frequency pulsation of the rotation speed can be improved. In the high-speed range, it is usually necessary to relax the torque tolerance to avoid excessive switching frequency. Setting the torque tolerance narrower is beneficial to reduce the torque ripple, but it increases the switching frequency of the inverter. , Making it a disadvantageous factor to further improve the speed regulation accuracy of the direct torque control system. In the conventional hard-switching inverter, it is difficult to further increase the switching frequency due to the switching consumption. Soft switching technology is the best way to increase switching frequency and reduce switching losses. ^[3]