Matrix Converter An Introduction

Variable-speed motor drive that uses an AC Drive has enjoyed widespread use because of its great energy-saving effect. What is yet unsolved are the suppression of a power harmonic current and the effective use of regenerative energy during deceleration. In order to fully solve these technical issues, we employ the Matrix converter technology, which directly converts from AC power source to AC output. Matrix Converter Basic Principle The matrix converter directly converts AC to AC rather than AC to DC to AC as in conventional voltage source PWM AC Drives. Matrix converters have capability to regenerate power and suppress input current harmonics and noticed as optimum drives for applications ranging from cranes, elevators and centrifuges where regeneration occurs, to air-conditioning fans and feed-water pumps where high harmonic countermeasures are required. Figure 1 shows main circuit configurations of the matrix converter and the conventional voltage source PWM AC Drive. The main circuit of the matrix converter consists of small input filters, which consist of reactors and capacitors, and 9 bi-directional switches. The bi-directional switches consist of the combination of IGBTs shown in Fig.1. On the other hand, the voltage source PWM AC Drive consists of a power AC Drive circuit with the combination of a rectifying circuit on the input side, a smoothing circuit with capacitors on the intermediate part, and IGBTs on the output side. Matrix Converter Voltage source PWM AC Drive

Fig.1 Main circuit configurations of matrix converter and voltage source PWM AC Drive PWM method The matrix converter directly PWM switches 3-phase AC power supply voltage and outputs any given voltage and frequency. Figure 2 shows an example of switching waveform of the matrix converter. Zone 1 of Fig.1 shows that the switching is made from phase-T voltage which is the standard, through phase S which is the intermediate voltage phase, to phase R which is the highest voltage phase. This allows smaller voltage fluctuation than as in conventional AC Drives and therefore suppresses surge voltage and leakage current. PWM waveform example

Fig.2 PWM waveform example Input current control The matrix converter uses electric potential at all the three input phases to PWM switch. Therefore, output voltage and input current can be controlled at the same time. Matrix converters hold two advantages; its energy-saving regeneration and high harmonics suppression effect by controlling a current. Figure 3 shows I/O waveforms of a conventional AC Drive and the matrix converter.

Fig.3 I/O waveforms of conventional AC Drive and matrix converter Figure 4 shows the comparison of input current harmonics. The input current of the matrix converter is almost sine wave and problems associated with harmonics can be easily solved by the matrix converter alone.

Fig.4 Comparison of input current harmonics Effects of matrix converters Matrix converter circuits offer the following effects comparing to conventional AC Drive circuits. Suppression of power harmonics Realizes less than 7% THD of input current and more than 98% input power factor without any specific measures taken. Longer operating life The main circuit does not have endurable parts such as an electrolytic capacitor. This makes the operating life of the main circuit longer and the maintenance interval longer. Elimination of derating With the elimination of current constriction on any specific device, the reduced operation during low-frequency operation is unnecessary. Power regeneration Unique bi-directional switches for directly connecting the power supply and loads enable continuous regeneration. High-efficiency Only the bi-directional switches are used to connect the power supply and loads, allowing higher-efficient operation than as in conventional AC Drives.