Discrete-pulse-modulated current control for voltage-source inverter with induction heating load

Abstract

High performance current control techniques based on discrete-pulse- modulation of the voltage-source inverter (VSI) with transformer-coupled load are investigated for high-frequency power supply of induction heating systems. The uses of induction heating process in industries are getting increased due to its features such as the accurately heated depths and surface area, high efficiency, clean environments, and ease of automation. A new class of control techniques that uses the discrete pulse modulation strategies were used for many advantages such as the low switching loss, low electromagnetic interference, and high power density. However, large ripple and non-zero offset of output current are known to be the major obstacles for a practical uses of the strategy. Although various efforts to cope with these problems have been reported, they cannot be used for the induction heating application since they have not considered the transformer flux constraints. In this dissertation, therefore, improved current control techniques based on discrete pulse modulation are proposed to solve these problems under the consideration of flux constraints. Firstly, the modeling and analysis of discrete-pulse-controlled VSI is carried out. Based on the analysis, two types of discrete pulse modulation strategies are investigated:current regulated delta modulation (CRDM) and pulse density modulation (PDM). CRDM is a method to obtain a robust and fast response, whereas an offset exists due to the regularly sampled nature. The intelligent integrator is employed to compensate the offset problem and the integral wind-up. The boundary of integrator gain is quantititively derived and shown to be inversely proportional to load quality factor and sampling time. The simulation and experimental results verifies that the offset is almost removed but the ripple magnitude increased due to the flux-limiting action. Another control technique using an optimal pulse density modulation is proposed t...