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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