Page 69 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 59

The voltages and currents in equation (2.34) are phase quantities. In terms of line
p
quantities, for a wye connection we have V L ˆ 3V ph and I L ˆ I ph , whereas for a
p
delta connection we have I L ˆ 3I ph and V LL ˆ V ph . In both cases, therefore,
p 
P ˆ 3V LL I L cos f (2:35)
where f is the angle between the phasors V ph and I ph .

2.10.4 Power measurement

Classical electro-dynamometer wattmeter or `Wattmeter'
The classical wattmeter circuit symbol (Figure 2.30) is derived from the classical
wattmeter (Figure 2.31), which is still widely used. Accuracy is typically 0.5% in
calibrated instruments. The readings of these instruments are usually reliable if the
voltage and current waveforms are sinusoidal with fairly high power factor. They are
generally not suitable with distorted waveforms, but special versions have been
manufactured for use at low power factor.

Fig. 2.30 Wattmeter symbol.

Fig. 2.31 Classical electro-dynamometer wattmeter.