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Abstract
The mains current in an ac/dc converter contains periodic current pulses due
to the action of the rectifier and output filters. The high current pulse cause harmonic distortion of the supply current and decrease its power factor. These result in a poor power quality, voltage distortion and poor power factor at input ac mains, slowly varying rippled dc output at load end and low efficiency. Many input wave shaping methods have been proposed to solve the problem of the poor power factor, which can be classified as active and passive power factor correction (PFC) methods. Most of PFC applications are based on average current mode control and implemented by analog circuits. With the development of digital technique, digital control for AC-DC converters with PFC is considered. Most of the existing digital PFC control methods are based on conventional analog control laws in a digital format. The disadvantages of conventional digital PFC control are: the high calculation requirements, high cost to implement and limited switching frequency. In this thesis, a predictive digital PFC control method is proposed to solve these problems. The proposed control method generates all the duty cycles in advance, based on the reference current and sensed inductor current, input voltage and output voltage. It requires only one multiplication and three addition operations for digital implementation, so that the proposed PFC control method can be implemented by using a low cost DSP or microprocessor to achieve high switching frequency. In order to evaluate the proposed control, a power factor correction of a rectifier with static load, is controlled using three methods: i) Average current mode control ii) Hysteresis control iii) Proposed predictive control. The proposed PFC control method can achieve a unity power factor under the steady state and transient operations and has a good dynamic performance during the step change of input voltage and load change.
The use of variable speed drive (VSD) systems in the industry is growing
due to emerging high volume of fractional horsepower VSD applications. All VSDs have rectifiers and storage capacitors in their front-end to get dc voltage from an ac power source. This input circuitry lowers the power factor (PF) of the VSD systems and pollutes ac power systems. The emerging requirements in the drive applications for high efficiency and low cost coupled a sinusoidal current operation from the utility are necessary. The study of power factor correction (PFC) integrated with inverter-fed induction motor drives becomes very important. The effects of the PFC circuit on the magnitude input current, harmonic content and input power factor of the drive system are studied.
Simulation results for the system under consideration (static load) show that
the predictive control has low total harmonic distortion THD, high PF, high
switching frequency, lower cost and better performance. Furthermore, a sinusoidal input current with low THD can be achieved under a distorted input voltage. Also, the simulation results for ac drive system with and without PFC circuit are studied in both transient and steady-state operations. The prototypes of boost PFC for both static load and ac drive system, controlled by a DSP 1104 evaluation board, were built and tested in the laboratory. The experimental results show that the proposed PFC control method can achieve sinusoidal input current waveform for both transient and steady-state operations with step change of input voltage and load torque. The simulation results are proved the good agreement when compared with the relevant experimental results.