Concept of Power Electronics
Power electronics belongs partly to power engineers and partly to electronics engineers. engineering is mainly concerned with generation, transmission, distortion and utilization of electric energy at high efficiency. Electronics engineering, on other hand is guided by distortion -less production, transmission and reception of data and signals of high efficiency. In addition, apparatus associated with power engineering is biased mainly on electromagnetic principles whereas that in electronics engineering is based upon physical phenomena in vaccum, gasses/vapours and semiconductors Power electronic converters (or circuits ) can be classified into six types as under
1. Diode rectifiers: A diode rectifier circuit converts ac input voltage into a fixed dc voltage. The input voltage may be singlephase or three phase. Diode rectifiers fine wide use in electric traction, battery charging, electroplating, electrochemical process-ing, power supplies, welding and uninterruptible power supply (UPS) systems.
2. Ac to dc converts (phase controlled rectifiers): These convert constant voltage to line voltage for their commutation, as such these converters. Phase -controlled converters may be fed from 1-phae or 3-phase source. These are used in dc drives, metallurgical and chemical industries, excitation system for synchronous machines etc.c
3. DC to dc converters (dc Choppers): A dc chopper converts fixed dc input voltage to a controllable dc output voltage. The chopper circuits required force, or load commutation to turn-off the thyristors. For lower power circuits, thyristors are replaced by power transistors. Classification of chopper circuits depend upon the type of commutation and also on the direction of power flow. choppers find wide applications in dc drives, subway cars, trolley trucks, battery-driven vehicles etc.
4. Dc to a.c converters (inverters): An inverter converts fixed dc voltage to a variable ac voltage. The output may be a variable voltage and variable frequency. These converters used line, load or forced commutation for turning -off the thyristors. Inverters find wide use in induction-motor and synchronous-motor drives, induction heating, UPS, HVDC transmission etc. At present, conventional thyristors are also being replace by GTOs in high-power applications and by power transistors in low-power applications.
5. Ac to ac converters: These convert fixed ac input voltage into variable ac output voltage. These are of two types as under
(a) Ac voltage controllers (ac voltage regulators): These converter circuit convert fixed ac voltage directly to a variable ac voltage at the same frequency.
Ac voltage controller employ two thyristors in anti parallel or a triac. Turn-off of both the devices is obtained by line commutation. Output voltage is controlled by varying the firing angle delay. Ac voltage controllers are widely used for lighting control, speed control of fans, pumps etc.
(b) Cycloconverters: These circuits convert input power at one frequency to output power at a different frequency through one-stage conversion, Line commutation is more common in these converters, though forced and load commutated cycloconverters are also employed.
These are primarily used for slow-speed large ac drives like rotary kiln etc.
6. Static switches: The power semiconductor devices can operate as static switches or contractors. Static switches posses may advantage over mechanical and electromechanical circuit breakers. Depending upon the input supply, the static switches are called ac static switches or dc static switches.