# Chapter 4 DC to AC Converters ( Inverters )

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Chapter 4 DC to AC Converters ( Inverters )
Power Electronics Chapter DC to AC Converters ( Inverters )

Applications of Inverters
Conversion of electric power from DC type energy sources to AC type load Battery Photovoltaic cell (Solar cell) Fuel cell As a part of composite converter AC-DC-AC frequency converter (for AC motor drive) AC-DC-AC constant-voltage constant-frequency converter (for uninterruptable power supplies) AC-DC-AC Converters for induction heating AC-DC-AC-DC switching power supplies

Outline 4.1 Commutation 4.2 Voltage source inverters
4.3 Current source inverters 4.4 Multiple-inverter connections and multi-level inverters

4.1 Commutation types Basic operation principle of inverters
Load S 1 2 3 4 i o u U d A classification of inverters Square-wave inverters (are discussed in this chapter) PWM inverters ( will be discussed in Chapter 6) The concept of commutation

4 types of commutation Device commutation:
Fully-controlled devices: GTO, IGBT, MOSFET Line commutation Phase-controlled rectifier Phase-controlled AC controller Thyristor cycloconverter Load commutation Forced commutation

Forced commutation (capacitance commutation)
Direct-Coupled With Coupling-Inductor

Another classification of commutations
4 types of Commutations For fully-controlled devices Device commutation Self-commutation Forced commutation Line commutation For thyristors External commutation Load commutation

2 classes of inverters Voltage Source Inverter (VSI)
Current Source Inverter (CSI)

4.2 Voltage source inverter (VSI)
+ - C R L U d V 1 2 3 4 VD u o i Features DC side is constant voltage, low impedance (voltage source, or bulk cap) AC side voltage is square wave or quasi-square wave. AC side current is determined by the load. Anti-parallel diodes are necessary to provide energy feedback path. (freewheeling diodes , feedback diodes)

Single-phase half bridge VSI
U d i o u V 1 2 VD u o U m i t 1 2 3 4 5 6 V VD G1 G2 The current conducting path is determined by the polarity of load voltage and load current. (This is true for analysis of many power electronics circuits.) The magnitude of output square-wave voltage is Ud/2.

Single-phase full bridge VSI
o U m i t 1 2 3 4 5 6 V VD G1,4 G2,3 Operation principle + V 3 VD 3 VD 1 C V 1 R i L U o d u V 2 o VD VD 4 2 V 4 - The magnitude of output square-wave voltage is Ud. The effective value of output voltage (or fundamental output voltage) can be changed by changing Ud.

Single-phase full bridge VSI
Quantitative analysis Fourier series extension of output voltage Magnitude of output voltage fundamental component Effective value of output voltage fundamental component (4-1) (4-2) (4-3)

Single-phase full bridge VSI
Output voltage control by phase-shift + - C R L U d V 1 2 3 4 VD u o i

Inverter with center-tapped transformer —push-pull inverter

Dead time (blanking time) to avoid “shoot through”
Three-phase VSI 180o conduction Dead time (blanking time) to avoid “shoot through”

Basic equations to obtain voltage waveforms
Three-phase VSI Basic equations to obtain voltage waveforms For line voltage For phase voltage of the load

Three-phase VSI Quantitative analysis
Fourier series extension of output line-to-line voltage Magnitude of output voltage (line-to-line) fundamental component Effective value of output voltage (line-to-line) fundamental component (4-8) (4-10) (4-11)

4.3 Current source inverter (CSI)
Features DC side is constant current, high impedance (current source, or large inductor) AC side current is quasi square wave. AC side voltage is determined by the load. No anti-parallel diodes are needed. sometimes series diodes are needed to block reverse voltage for other power semiconductor devices.

Single-phase bridge CSI
Parallel Resonant Inverter A C R L d I VT 1 2 3 4 T T3 T4 u o i Switching frequency is a little higher than the resonant frequency so that the load becomes capacitive and load current is leading voltage to realize load commutation.

Three-phase self-commutated CSI
120o conduction

Three-phase force-commutated CSI

4.4 Multiple-inverter connections and multi-level inverters
Series connection of 2 single-phase VSIs

Series connection of 2 3-phase VSIs

Multi-level Inverters
Ways to deal with higher voltage and achieve better waveform Series connection of multiple converters Series connection of multiple switch devices Major type of multi-level inverters Neutral point clamped multi-level inverter Flying-capacitor multi-level inverter Cascade H-bridge( series connected H-bridges) In broad sense, previously discussed series connection of multiple inverters is also called multi-level inverter. In narrow sense, only NPC and FC structures are called multi-level inverters.

Neutral-Point-Clamped 3-level inverter

Neutral-Point-Clamped 5-level inverter

Flying-Capacitor 3-level inverter

Series connection of 3 H-bridges