Digital signal processor controlled PWM phase modulator for two phase voltage source inverter

Abstract

The work is aimed at proposing a phase modulated PWM scheme for two-phase voltage source inverter optimizing the pattern for minimum third harmonic content. The proposed two phase PWM controller is implemented with a Digital Signal Processor (DSP, TMS320C50). The experimental waveforms is compared with the simulated ones for different characteristic parameters like distortion factors. This work results in an improved controller for two phase motor drives and position controllers employing DSP. Both three level and two level PWM schemes are adopted for the generation of PWM signals for the inverter. For implementation of the scheme two different programs are written in C by using ANSI C compiler supplied with the TI's TMS320C50. For compiling and running the program an INTEL Pentium IV microcomputer is used. After running the program the code is uploaded to the TMS320C50 DSP kit. After that the DSP kit supplies the calculated switching points through its VO ports to the external hardware interfacing circuit using EPROMs. The byte patterns for PWM signals generated using MATLAB are stored in two EPROMs (27C256) as data. The DSP kit sends calculated duty cycles to the EPROMs and then according to the calculated duty cycles required patterns are obtained at the output of the EPROMs. For controlling the whole process a 12-bit counter, a high frequency clock circuit, a monostable timer and buffers are used. To test the performance of the scheme these signals are then applied to a two phase halfbridge inverter using MOSFETs. To drive the inverter properly an electronic controller circuit is also developed. The supplied DC voltage to the inverter is 90V. The output of the inverter is fed to a prototype 0.5 hp two phase induction motor. Phase currents of the motor are obtained using two current transformers and resistors. Finally the system performance IS evaluated from the MATLAB simulation results and experimental waveforms. Upon completion of the work it is found that the two level scheme gives the better system performance especially the distortion factors of output current is much lower in this scheme than that in the three level scheme. The experimental result shows good agreement with the simulated one.