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Transformers 21 2.7. INTRODUCTION TO TRANSFORMER Definition: A transformer is an AC static device which transfers electric power from one circuit to another without changing its frequency. Hence the main function of a transformer is to raise or lower the voltage ina circuit witha corresponding decrease or increase in current at the same frequency. Need : Electrical energy has to be transmitted over long distances. Electrical energy is produced (generated) at power-houses situated at one place and utilized in houses or factories situated at another places, The main advantage of ac voltage over dc voltage is the ease of transformation of voltage potential. The voltage generated at power-houses (generating stations) is usually 6-6 KV or 11 KV but for transmission we require higher voltages. This is because for same amount of power transferred, if the voltage is high, less current will flow which in turn will result in less ?R losses. Hence a transformer is required for lowering or raising the voltage in a system. The transformer is one of the simplest electrical machines. This is due to the fact that there are no rotating parts and hence the theory associated with it is simple. For a user, it requires little attention and its maintenance is easy and cheap compared to other electrical machines. Also there are no friction or windage losses and hence the efficiency of a transformer is very high (95% — 99%). Classification according to input : A transformer when receives the energy at a lower voltage and delivers it at a higher voltage, it is called a step-up transformer. When the energy is received ata higher voltage and delivered at a lower voltage, it is called a step-down transformer. 2:8 BASIC PRINCIPLE AND OPERATION As shown in Fig. 2-10 any transformer has two coils—electrically insulated from one another and wound ona common core, i.e., magnetically coupled. The energy from one coil is transferred to other coil by means of magnetic coupling. The coil which receives energy from an AC source is called primary (P) and the coil which delivers the energy to the load is called secondary (S). In a transformer, either winding may be the primary, the other being the secondary, depending upon which winding receives /— Magnetic linkage t 4 and which delivers energy. Primary A P core s 7} Secondary q If the primary coil has smaller number winding tH [tp winding ‘ead of turns of a thick wire while the secondary ! i a coil has a Jarger number of turns of thin wire, then it is a step-up transformer. Basic-transformer Similarly in a step-down transformer the Fig. 210 primary consists of larger number of turns of thin wire while the secondary has a smaller number of turns of thick wire. In other words we can say that in a step-down transformer the primary is the high-voltage winding whereas the secondary is the low-voltage winding. The converse is true for a step-up transformer. In Fig. 2-10 we have seen that both the primary and secondary windings have been wound ona common core, The effects of a common core are : 1. Agreat increase in the total flux by virtue of improved permeance of the magnetic circuit. Scanned by CamScanner