chapter  21
Transformers
Pages 25

At the end of this chapter you should be able to:

• understand the principle of operation of a transformer • understand the term ‘rating’ of a transformer • use V1/V2=N1/N2 = I2/I1 in calculations on transformers • construct a transformer no-load phasor diagram and calculate magnetizing and core loss components of

the no-load current • state the e.m.f. equation for a transformer E=4.44 f mN and use it in calculations • construct a transformer on-load phasor diagram for an inductive circuit assuming the volt drop in the

windings is negligible • describe transformer construction • derive the equivalent resistance, reactance and impedance referred to the primary of a transformer

• understand voltage regulation • describe losses in transformers and calculate efficiency • appreciate the concept of resistance matching and how it may be achieved • perform calculations using R1=(N1/N2)2RL • describe an auto transformer, its advantages/disadvantages and uses • describe an isolating transformer, stating uses • describe a three-phase transformer • describe current and voltage transformers

A transformer is a device which uses the phenomenon ofmutual induction (see Chapter 9) to change the values of alternating voltages and currents. In fact, one of the main advantages of a.c. transmission and distribution is the ease with which an alternating voltage can be increased or decreased by transformers. Losses in transformers are generally low and thus efficiency is high. Being static they have a long life and are very stable. Transformers range in size from theminiature units used in electronic applications to the large power transformers used in power stations; the principle of operation is the same for each. A transformer is represented in Fig. 21.1(a) as consisting of two electrical circuits linked by a common ferromagnetic core. One coil is termed the primary winding, which is connected to the supply of electricity, and the other the secondary winding, which may be connected to a load. A circuit diagram symbol for a transformer is shown in Fig. 21.1(b).