Text 4. THREE-PHASE ELECTRIC MACHINES

Modern electric engineering deals with a great variety of three-phase machines based on the operation of three-phase electrical circuits. Such wide popularity of these systems can be explained by their numerous advantages which must involve the factors of economy, efficiency and certainly reliability.

A three-phase electric generator contains two essential parts ‒ an armature or a rotor and an electromagnet or a stator. As a result of the operation of these parts three electromotive forces are induced in three turns. These electromotive forces are able to generate three electric currents with a phase difference of 120'. Thus a three-phase electric generator is able to convert mechanical energy directly into a three-phase electric current. In order to produce electricity under the most economical conditions, the generators must be as large as possible and they should always be kept fully loaded.

The construction and the operation of a three-phase electric motor have much in common with a three-phase electric generator. The both operate on the same principle of electromagnetic induction discovered by the famous English scientist M. Faraday. But an electric motor serves an opposite designation. A three-phase motor converts electrical energy directly into mechanical one. It should be noted that a three-phase motor has to drive only three-phase loads.

Text 5. TRANSFORMERS

As you know Russian scientists contributed greatly to the development of transformers. An induction coil invented by the famous Russian scientist P. Yablochkov was the forerunner of the modern transistor. In 1882 another Russian electric engineer and inventor Ivan Usagin improved the Yablochkov's transformer. And M. Dolivo-Dobrovolsky designed the first three-phase transformer in 1890.

 It is well known that transformers serve for changing the electric current from one voltage to another. In other words they are used for increasing or decreasing voltage. Transformers found wide application in the long distance energy transmission, in the distribution of this energy among consumers, and also in various devices.

A transformer has two insulated windings or coils, arranged on an iron core. The primary winding is connected to the voltage source; it receives power. The secondary winding is connected to the load; it supplies energy to the load. The value of voltage in secondary depends on the number of turns in it. In case the secondary has more turns than the primary, the output voltage is greater than the inputvoltage. A device of this type steps up the voltage and is called a step-up transformer. In case the secondary winding has fewer turns than the primary, the output voltage is lower than the input. This device decreases or steps down the voltage and is termed a step-down transformer.

It should be noted that electromagnetic induction enables the transformer to transmit energy from the primary to the secondary winding.