变频调速英文版课件Chapter 6.ppt

Chapter 6 Current-Fed Converters The application of current-fed converters may include the following: 6.2 General Operation of a Six-Step Thyristor Inverter 6.2.1 Inverter Operation Modes Fig. 6.5 Summary of modes for ac machine operation 6.4 Force-commutated Inverters6.4.1 Auto-Sequential Current-Fed Inverter (ASCI) Idealized voltage and current waveforms for an induction motor fed by a six-step current source inverter 6.7 Inverters with Self-Commutated Devices6.7.1 Six-Step Inverter 6.7.1.1 Load Harmonic Resonance Problem Fig. 6.22 Frequency variation of inverter to excite machine resonance at harmonics 6.7.2 PWM Inverters6.7.2.1 Trapezoidal PWM Fig. 6.24 Harmonics in PWM current wave with different modulation index and M=21 (pulses per half-cycle) Fig. 6.26 Relation between inverter frequency and GTO switching frequency 6.7.2.2 Selected Harmonic Elimination PWM (SHE-PWM) 6.8 Current-Fed vs. Voltage-Fed Converters * 6.1 Introduction ? Current-fed converter, as the name indicates, likes to see a stiff dc current source (ideally with infinite Thevenin impedance) at the input. ? A variable voltage source can be converted to a variable current source by connecting a large inductance in series and controlling the voltage within a feedback current control loop. ? The power semiconductor devices in a current-fed inverter must withstand reverse voltage, and therefore, standard asymmetric voltage blocking devices, such as power MOSFETs, BJTs, IGBTs, MCTs, IGCTs, and GTOs cannot be used. Symmetric voltage blocking GTOs and thyristor devices should be used. Of course, forward-blocking devices can be used with series diodes. (b) (c) (d) idealized ac line current waveforms; (e) Thyristor gating sequence (6.1) approximate induction motor equivalent circuit; (b)(c)(d) voltage and current waveforms (6.28) ?The general relation between the number of harmonics removed (K) and M is given by K=(M-1)/2