Nowadays, electrical power is generated for industrial and utility purposes by thermal power stations and large hydroelectric plants in the three-phase form at a frequency of 50 Hz (in India). The generated frequency in some other countries is 60 Hz. The generated voltage at the generating station is 6.6 kV, 11 kV or higher. For transmission purposes, it is required to step it up to a voltage of 132 kV or higher. Again, in urban and rural areas it is required to step it down to 3.3 kV and 6.6 kV, respectively, and 11 kV at the substation. For domestic purposes, it is required to step it down to 400 V or 230 V Three-phase transformers are used to step up the generated voltage before transmission of electrical power and also to step down the high voltage before distribution, that is, at the substation. Before the study of three-phase transformers, knowledge of single-phase transformers is essential. The aim of this chapter is to discuss single-phase transformers only.
A transformer is a static or stationary electromagnetic device, consisting of two coils, by means of which electrical power in one circuit is transformed into electrical power of the same frequency in another circuit.
1.2 BASIC PRINCIPLE
Figure 1.1 shows a basic single-phase transformer having two windings wound on a common magnetic core. From the principle of mutual induction, when two coils are inductively coupled and the current in one coil is changed uniformly, an emf (electromagnetic force) is induced in the other coil. If a closed path is provided at the secondary circuit, this induced emf at the secondary drives a current. As shown in Figure 1.1, the transformer has two coils, which are electrically separated and magnetically linked through a common magnetic path. The basic principle of the transformer is the same as the principle of mutual induction. The coils of the transformer have high mutual inductance.
In brief, we can say the following:
- The transformer is a static device.
- It transfers electrical power from one circuit to another.
- During transfer of power, there is no change of frequency.
- It uses electromagnetic induction to transfer electrical power.
- The two electrical circuits are in mutual inductive influence of each other.