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An important property of the synchronous machine is its ability to supply variable 'leading' or 'lagging' reactive power when working in conjunction with an active network. This phasemodifying, or VAr-correcting property is sometimes loosely termed 'synchronous condenser operation', and this seems to give rise to a common misapprehension about whether the power factor is leading or lagging. The following notes are an attempt to show how this confusion has arisen and the conclusion is drawn that it is far better not to use the term 'synchronous condenser' at all.The power factor of any circuit is defined as the ratio of the power absorbed or generated to the product of the voltage and the current.' 1, 2 > It could be left at that, but it is often useful to define a circuit as having a 'leading' or a 'lagging' power factor to indicate whether the voltage or the current has its peak first.This important concept is particularly significant in synchronous machine design, because a current in the stator windings which lags the machine e.m.f. will tend to oppose the magnetization of the iron circuit and so reduce the flux, whereas a current which leads the machine e.m.f. can assist the magnetization. Therefore, if it is required to produce a constant voltage output when a lagging current flows in the stator windings, more magnetizing current must be sent through the exciting winding on the rotor in order to overcome the demagnetizing effect. This extra current will increase the Joule heating in the rotor winding which must therefore be designed to dissipate this extra heat. It is therefore particularly unfortunate that confusion can arise between leading and lagging power factors in connection with synchronous machines. The Basic Circuit (i) With ResistanceConsider a circuit containing only resistance and a source of voltage which we will assume to have a sinusoidal waveform. Fig. 1

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