INTRODUCTION TO POWER ELECTRONICS

POWER ELECTRONICS

Power Electronics is an area of study and specialization that aims at applying the speed and other desirable properties of Electronic devices in Electrical Power Systems Engineering.

Over the years, it has been found out that the Thyristors are reliable Power Electronic devices which are fast and are able to withstand high voltages and currents.

In this study, the Thyristor shall be investigated with a view of determining its characteristics and how it can be applied in practice to realize desirable Power Electronics Components, Circuits and Devices.

Areas Of Interect

Types of Converters
Converters are Power Semiconductor Systems used for rectifying, inverting, or otherwise modulating the Power Output of an AC or DC energy source.

Types of Converters

AC Voltage Controller:
Converts Fixed Voltage AC  to variable Voltage AC
Rectifiers (Uncontrolled)
Converts Fixed Voltage AC to Fixed Voltage DC
Rectifiers (Controlled)
Converts Fixed Voltage AC to Variable Voltage DC
DC to DC Converter (Chopper)
Converts Fixed Voltage DC to Variable Voltage DC
Inverters (Uncontrolled)
Converts Fixed Voltage DC to Fixed Voltage AC
Inverters (Controlled)
Converts Fixed Voltage DC to variable Voltage AC

Cycloconverters
ConvertsFixed frequency AC toVariable frequency, variable Voltage AC


Thyristors and their Applications

Switches
Rectification
DC to DC Convertion (Chopper)
Inverters

Thyristors can be used instead of :

Mecury - arc rectifiers
relays and Contactors
Rheostats and Motor Starters
Autotransformers
Mechanical Speed Changers
etc,

Application

Electric drives for rapid-transit systems
Rolling-mill drives
Multiple-drive Systems for textile and paper mills
Machine tool control
Aircraft Power Supply
UPS
DC Power Transmission etc.

Diode and Thyristors

Current passes from anode to cathode of a diode but not in the opposite direction

In the case of a thyristor, when a small current is passed from the gate terminal to the cathode, the thyristor will conduct, provided that the anode is at a higher potential than the cathode.

Once conduction starts, the gate circuit looses control. The device then continues to conduct until the potential of the anode becomes equal to or less than that of the cathode for a brief interval. At the end of which the gate circuit is once again in control.

Where possible, seperate the Cathode-gate circuit from Cathode external circuit.

Anode - Cathode Resistance
The anode - Cathode Resistance of a diode or of a thyristor in the conducting state is very low but not zero. Hence large anode current produce heat. Hence heat sink is used to remove the heat and sefeguard the device.

Switch

A switch presents either infinit or zero resistance to current. It is desirable to have a switch which is capable of instantenous transition from one state to another. The thyristor or the diode can be used as a switch. Eg. an ideal diode.

The diode conducts if source voltage v is positive. The anode - cathode voltage v(AK) is then zero.
The diode does not conduct if the source voltage is negative, then v(AK) is also negative

The operating point of the diode may lie on the +ve axis of i(A) in the range 0>=v(AK)>=-2V