Main task of Power Electronics

The main task of power electronics is to control and convert electrical power one form to another form. For the control of electric power or power conditioning the conversion of electric power from one form to another form   is necessary and the switching characteristics of the power devices permit these conversions. The static power converters perform these function of power conversions. A converters may be considered as a switching matrix. The power electronics circuits can be classified into six types:

1] Diode Rectifiers

2] AC to dc Converters

3] AC to dc Converters

4] DC to dc Converters

5] DC to ac Converters

6] Static Switches

1] Diode Rectifiers:

          A diode rectifier circuit converts ac voltage into a fixed dc voltage and is shown in fig(1). The input voltage to the rectifier Vi could be either single phase or three phase.

Fig(1)

2] AC to dc Converter:

          A single phase converter with two natural commutated thyristors is shown in fig(2). The average value of the output voltage Vo can be controlled by varying the conduction time of thyristors or firing delay angle. The input could be single or three phase source. These converters are also known as controlled rectifiers.

Fig(2)

3] AC to ac Converters:

          These converters are used to obtain a variable ac output voltage Vo from a fixed voltage source and a single phase converter with a TRIAC is shown in fig(3). The output voltage is controlled by varying the conduction time of a TRIAC or firing delay angle. These types of converters are also known as ac voltage controllers.

Fig(4)

4] DC to dc Converters:

          A DC to dc converter is also known as a chopper or switching regulator and a transistor chopper is shown in fig(4). The average output voltage Vo is controlled by varying the conduction time t, of transistor Q1. If T is chopping period , then t1 = dT. d is called the duty cycle of the chopper.

Fig(4.1)

Fig(4.2)

5] DC to ac Converter:

          A DC to ac converter is also known as an inverter. A single phase transistor inverter is shown in fig(5). If transistors M1 & M2 conduct for one half of a period and M3 & M4 conduct for other half, the output voltage is of the alternating form. The output voltage can be controlled by varying the conduction tme of transiators.

Fig(5)

6] Static Switches:

          Because the power devices can be operated as static switches or contactors, the supply to these switches could be either ac or dc and the switches are called as ac static switches or dc switches. A number of conversion stages are often cascade to produce the desired output as shown in fig(6).

Fig(6)

Power Converters:

•  Rectifier converting an AC voltage  to a DC voltage.
• Inverter converting a DC voltage to AC voltage.
• Choppers or a switch mode power supply that converts a dc voltage to another dc voltage.
• Cycloconverter converting an ac voltage to another ac voltage.
• In addition, SCRs & other power conductor devices are used as static switches.