Chapter 5 CONCEPT OF REAL AND REACTIVE POWER 5.1 Introduction In this chapter the meaning of ‘real’ and ’reactive’ power are explored for sinusoidal systems. Initially, single phase (so-called two wire) circuits are discussed as to gain an understanding of the energy flow within a circuit config- uration that is representative for electrical machines. We will then extend this analysis to three-phase (three wire) circuits. In the final part of this chapter a set of tutorials is introduced to reinforce the concepts discussed. 5.2 Power in single phase systems The concept of power is introduced with the aid of figure 5.1, which shows a load in the form of an inductance L, resistance R and voltage source u e in series connection. The voltage source u e is generally known as the induced voltage in electrical machines. The circuit configuration as described above is representative for electrical machines hence its use here. A current source i (t) is connected to this network. The reason for using a sinusoidal supply current source instead of a supply voltage source is to simplify the mathematical analysis. Application of Kirchhoff’s voltage laws to this circuit shows that the Figure 5.1. R-L-ue load connected to current source