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.

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Power Electronics

Power electronics is a subject or a field which combines the knowledge of Power (Electric Power), Electronics and Control System to process and control the flow of electric power by supplying voltages and currents in the form optimally suited for user load. Power engineering deals with the static and rotating power equipment for the generation, transmission and distribution of electric power. Electronics deals with the study of solid state semiconductor power devices and circuits for power conversion to meet the desired control objectives i.e. to control the output voltage and output power. So power electronics may be defined as the subject of applications of solid state power semiconductor devices for the control and conversion of electric power. The basic block diagram of power electronics is shown in fig 1 below.

Fig 1: Basic block diagram of Power Electronics

Power electronics refers to control & conversion of electrical power by power semiconductor devices where these devices operate as switch. Power electronics uses new switches circuit topologies to make smaller lower weight & higher efficiency power supplier from 1W to MW levels. Power electronics is defined as a subject that deals with the apparatus and equipment working on the principle of electronics but rated at power level rather than signal level.

Example: Semiconductor power switches such as thyristors, GTOs, etc.

The main goal of power electronics is to convert the electrical energy from one form to another form and also to control the flow of energy from an electrical source to an electrical load with highest efficiency, high availability and high reliability with the lowest cost, smallest size and light weight. Power electronics deals with the study and design of Thyristorised power controllers for variety of applications like Heat Control, Light Illumination Control and Motor control – AC/DC motor drives used in industries. Today power electronics systems are widely spread for adapting energy to the load in many different cases in industries, transportation and household, e.g.

• ·        Power supply and electrical transmission e.g. high voltage DC lines (HVDC)
• ·         Electric drives in electric cars, trams, locomotives and many other cases
• ·         Electric drives in conveying devices, rolling mills, machine tools and others
• ·         Power supply for welding, electroplating and electrolysis
• ·         Power supply for consumer goods, e.g. switching mode power supply for computers, TV and many more other products, furthermore light dimming, speed control of fans, vacuum cleaners and many others electric appliances.

The main task of power electronics is to convert and control the electric power, i.e. for the control of electric power or power conditioning, the conversion of electrical power from one form to another form is necessary and the switching characteristics of power devices permits these conditions. The static power converters perform these functions of power conversions. A converter may be considered as a switching matrix. The four main forms of conversions are:

Ø  AC to DC Conversion (Rectification)

Ø  AC to AC Conversion (Phase Controlled Rectifiers)

Ø  DC to DC Conversions (DC Choppers)

Ø  DC to AC Conversions (Inverters)

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