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Power conversion deals with the process of converting electric power from one form to another. The power
electronic apparatuses performing the power conversion are called power converters.Because they contain no moving parts, they are often referred to as static power converters. The power conversion is achieved using power semiconductor devices, which are used as switches. The power devices used are SCRs (silicon controlled rectifiers, or thyristors), triacs, power transistors, power MOSFETs, insulated gate bipolar transistors (IGBTs), and MCTs (MOS-controlled thyristors). The power converters are generally classified as:
1. ac-dc converters (phase-controlled converters)
2. direct ac-ac converters (cycloconverters)
3. dc-ac converters (inverters)
4. dc-dc converters (choppers, buck and boost converters)
The basic function of a phase-controlled converter is to convert an alternating voltage of variable amplitude
and frequency to a variable dc voltage. The power devices used for this application are generally SCRs. The
average value of the output voltage is controlled by varying the conduction time of the SCRs. The turn-on of
the SCR is achieved by providing a gate pulse when it is forward-biased. The turn-off is achieved by the commutation
of current from one device to another at the instant the incoming ac voltage has a higher instantaneous potential than that of the outgoing wave. Thus there is a natural tendency for current to be commutated from the outgoing to the incoming SCR, without the aid of any external commutation circuitry. This commutation process is often referred to as natural commutation.
A single-phase half-wave converter is shown in Fig. 30.9. When the SCR is turned on at an angle a , full supply voltage (neglecting the SCR drop) is applied to the load. For a purely resistive load, during the positive half cycle, the output voltage waveform follows the input ac voltage waveform. During the negative half cycle, the SCR is turned off. In the case of inductive load, the energy stored in the inductance causes the current to flow in the load circuit even after the reversal of the s
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