Electronique de puissance DC‐AC Converter Inverters Introduction • Conversion of a DC source into an AC output with a variable frequency. • The frequency of the output AC can be controlled by the conduction period of power po er switches s itches of the inverter. in erter • p voltage g of inverter has a no sinusoidal p periodic waveform but can Output be made to closely approximate this desired waveform. • Inverters are used in industrial applications, including speed control off induction and synchronous motor, aircraft power supplies, uninterruptible power supplies (UPS), and high‐voltage DC transmision. Half-Bridge Inverter Square‐wave output Step wave output Step‐wave Half‐Bridge VSI (Voltage Source Inverter) Half‐Bridge VSI (Voltage Source Inverter) Full‐Bridge g VSI State S1 S2 S3 S4 Output Voltage 1 On Off Off On +E 2 Off On On Off ‐E 3 On Off Off On +E 4 Off On On Off ‐E With Inductive RL load Square-Wave output Step-Wave output Inverter Voltage Control Techniques 1 Control of DC input voltage supplied to the inverter 12- Control of AC output voltage of the inverter 3- Control of the voltage within the inverter Pulse-Width Modulation (A) Single Pulse‐Width Modulation (B) Multiple Pulse‐Width Modulation Variable duty cycle with m= 5 (a) m= 2 (b) m= 3 (C) SINUSOIDAL PULSE‐WIDTH MODULATION PWM Inverters Single-phase Full-Bridge Inverters Three-Phase Three Phase Inverter • Three inverter legs; g ; capacitor p mid-point p is fictitious Three-Phase PWM Waveforms Three-Phase Inverter Harmonics Three-Phase Inverter Output p • Linear and over-modulation over modulation ranges Three-Phase Inverter: Square-Wave Mode • Harmonics are of the fundamental frequency Square-Wave Square Wave and PWM Operation • PWM results in much smaller ripple current DC-Side DC Side Current in a Three-Phase Three Phase Inverter • The current consists of a dc component and the switching-frequency related harmonics Square-Wave Operation • devices conducting are indicated PWM Operation • devices conducting are indicated Tolerance-Band Current Control • Results in a variable frequency operation Fixed-Frequency Fixed Frequency Operation • Better control is possible using dq analysis Summary of DC-AC Inverters • Functional representation in a block-diagram form
