Power electronics base_________________________________________________________________________________67

Introduction

Now everywhere electric power practically is produced and transferred as a power of three-phase alternating current. However near 25 % of the whole electric power is consumed as a power of direct current. It is electrochemistry, electrometallurgy, power engineering, electric drive of high quality, electric transport etc. It is augmented necessity of electric power with other number of phases and frequency than standard. It is the electric drive of alternating current, frequency converters of different employment etc. In home appliances also there is a necessity of converting and smooth regulating of the electric power (drive of electrical household appliances, illumination, conditioning etc.).

Thus, it is increased necessity by devices for converting electrical energy of standard frequency without essential losses. These devices should have necessary high-speed and should ensure regulation of needed specified parameters (current, voltage, power, frequency).

The device is intended for converting of electrical energy with certain parameters to electrical energy with other parameters, adjustable or non- adjustable, names as the converter.

Earlier for this purpose were applied:

1). Motor-generator sets.

Faults: large losses and inertia because of intermediate transformation of energy from electric form into mechanical form and vice versa.

2). Traditional gas-filled rectifiers of classical power electronics engineering (mercury rectifiers, thyratrons, ignitrons).

Faults: great dimension, considerable voltage drop across a valve (up to 24 V), working capacity over a restricted temperature range (from 20 up to 55 Celsius degrees), difficulty of realization of complex circuits, fragility, prolonged time of restoring of locking properties.

3). Magnetic amplifiers.

Faults: great dimension and mass, inertia, losses of no-load operation.

Creation of semiconductor triodes (transistors) in the end of 40-s - beginning 50-s and power semiconductor rectifiers (thyristors) in 50-s was marked revolution at power electronics engineering.

The volumes of converter’s sets were reduced from 3 to 7 times; losses of power have decreased as the voltage drop across a semiconductor rectifier does not exceed 1.2V. The efficiency can reach to values greater than 98 %. There are ensured high-speed and a longevity.

There is possibility of a choice of power semiconductor devices for different currents and voltages ratings, for combination with different coolers, for parallel and series connection of components allowing building up families of converters over a wide range of power.

The semiconductor rectifiers, dinistors and thyristors, have allowed creation the varieties of schemes, impracticable with the help of other devices.

The modern state of power electronics ensures transformation of an energy with low losses at any combinations of frequency and voltage in a primary network U1, f1 and in an active or passive secondary network U2, f2.

The basic types of converters are shown in a fig. 1.

Figure 1.