ABSTRACT
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 = 4.44fmN 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
• 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
20.1 Introduction
A transformer is a device which uses the phenomenon of mutual 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 easewithwhich an alternating voltage can be increased or decreased by transformers.
