Development Of SPWM Three-Phase Frequency Inverter Experiment Unit

Frequency inverter is a kind of circuit that uses power electronic device to change AC from 50Hz to other frequencies and includes AC/AC and AC/DC/AC types. The AC/DC/AC frequency inverter with sinusoidal pulse width modulation has evident advantage in many aspects and is widely being used.The steady frequency three-phase AC can be changed into the AC which frequency is continuously adjusted in the experiment unit composed of rectifier, inverter, control circuit, and relay safeguard circuit. Firstly, the input three-phase AC is converted into DC after it passes rectifier and filter circuit. Secondly, the DC is changed into three-phase AC which frequency and value can be continuously adjusted. Lastly, the output three-phase AC is supplied to load. The control circuit includes DC regulated power supply, SPWM signal circuit, and driving circuit. The DC regulated power supply provides steady voltage to signal circuit, driving circuit, relay safeguard circuit and so on. By means of modulating the clock signal input of HEF4752, six SPWM signals are produced. The driving circuit uses EXB841 that is very popular to magnify and separate the six SPWM signals, and easily to control the switch on or off of six IGBTs in inverter bridges. To switch the main circuit and other related circuits is achieved by relays and contactors. The function of the safeguard circuit is to compare the signals that come from the voltage and current sensors to predetermined signals. When the signals exceed the rated value, the safeguard turns off the driving signals of IGBTs. At the same time the input AC of the main circuit is cut off to protect electronic devices.The theories and design principles of each circuit of the SPWM three-phase frequency inverter experiment unit are in detail introduced in this thesis. At the same time some experiment results are shown that include the output voltage and current waves when the load is in resistivity, inductivity or asynchronous motor. The comparison of experiment results with theory analysis shows that the experiment unit has stable operation and good technique performance.