Switch-Mode DC-AC Inverter: Basic concepts

1. Inverters with single- and three-phase ac outputs will be focused
   
2. Input is dc voltage source
   
3. Such inverters are called voltage-source inverters (VSI)
   
4. The other type of inverter is a current source inverter (CSI) where the input is a dc current source
   
5. Discussion will be limited to VSI
   
6. Four-quadrant operation
   
7. Rectifier mode: quadrants 2 and 4
   
8. Inverter mode: quadrants 1 and 3
   
9. v_o can be assumed to be sinusoidal
   
10. i_o will lag v_o since the inverter will drive an inductive load such as a motor
11. In interval 1, both v_o and i_o are positive and interval 3, both v_o and i_o are negative
12. Therefore during intervals, 1 and 3, p_o=v_oi_o will be positive, and power will flow from the dc to ac side which is the inverter mode of operation
13. During the intervals 2 and 4, v_o and i_o will be opposite signs, and power will flow from the ac side to the dc side which is the rectifier mode of operation
14. Thus, switch-mode inverters are capable of operating in all four quadrants

Figure 3: Single-phase switch-mode inverter.

Freewheeling Diodes in Inverters

1. The inverter circuits normally use IGBT or MOSFET switches shunted by an anti-parallel connected diode.
   
2. The diode allows current flow in the opposite direction when the switch is open.
   
3. These freewheeling diodes prevent inductive current interruption
   
4. This provides protection against transient over voltage, which may cause reverse breakdown of the IGBT and MOSFET switches.
   

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DC-AC CONVERTERS (INVERTERS)

INTRODUCTION

1. An inverter converts the dc voltage and current to an ac voltage and current.
2. The frequency of the ac voltage can be regulated by the inverter operating frequency.
3. The main devices used for inverters are power MOSFETs, IGBTs, IGCTs etc. Currently only the inverters with a high power rating, such as 500 kW or higher, are likely to be built with GTOs.

Figure 1: Switch-mode in ac motor drive.

Switch-Mode DC-AC Inverter: Bi-directional power flow

Figure 2: Switch-mode converters for motoring and regenerative breaking in ac motor drive.

Applications

1. Emergency lighting systems;
2. Uninterrupted power supplies;
3. Frequency converters;
4. AC motor drives;
5. Solar power conversion to 50 Hz;
6. Wind power conversion;
7. Fuel cells; and
8. High-voltage dc transmission.

Types based on Source

1. Inverters can broadly be classified into Voltage Source (VSI),
   Current Source Inverters (CSI).
   
2. Voltage source inverters are supplied from ideal (or low impedance) DC voltage sources.
   
3. The DC source can be fixed or variable but very low source impedance is assumed. With such a source, the output AC voltage is determined completely by the DC source voltage and the states of the switching devices in the inverter. This of course assumes that the load current does not become discontinuous.
   
4. The DC source may be in the form of batteries, generators, solar cells or for large power applications, controlled or uncontrolled ac-dc rectifier circuits terminated with a C- or LC-filter which offers low impedance path to the lagging components of the load current which must sink into the DC source.
   
5. Current source inverters have a stiff dc current supply, usually through a large inductor or a closed-loop controlled current source. The DC source impedance of such inverters is very high. 
   

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