Today’s commercial, industrial, retail and even domestic premises are increasingly populated by electronic devices such as PCs, monitors, servers and photocopiers which are usually powered by switched mode power supplies (SMPS). If not properly designed, these can present non-linear loads which impose harmonic currents and possibly voltages onto the mains power network.
Harmonics can damage cabling and equipment within this network, as well as other equipment connected to it. Problems include overheating and fire risk, high voltages and circulating currents, equipment malfunctions and component failures, and other possible consequences.
A non-linear load is liable to generate these harmonics if it has a poor power factor. Other loads can present poor power factors without creating harmonics. This post looks at these issues, the circumstances that can lead to damaging harmonic generation, and practical approaches to reducing it.
The two causes of poor power factor
At the simplest level, we could say that an electrical or electronic device’s power factor is the ratio of the power that it draws from the mains supply and the power that it actually consumes. An ‘ideal’ device has a power factor of 1.0 and consumes all the power that it draws. It would present a load that is linear and entirely resistive: that is, one that remains constant irrespective of input voltage, and has no significant inductance or capacitance. Fig. 1. shows the input waveforms that such a device would exhibit. Firstly, the current waveform is in phase with the voltage, and secondly both waveforms are sinusoidal.
In practice, some devices do have unity power factors, but many others do not. A device has a poor power factor for one of two reasons; either it draws current out of phase with the supply voltage, or it draws current in a non-sinusoidal waveform. The out of phase case, known as ‘displacement’ power factor, is typically associated with electric motors inside industrial equipment, while the non-sinusoidal case, known as ‘distortion’ power factor, is typically seen with electronic devices such as PCs, copiers and battery chargers driven by switched-mode power supplies (SMPSs). We shall look briefly at the displacement power factor before moving on to the distortion case, which is of more immediate concern to electronic power system designers. However it is important to be aware of both cases. For example, some engineering courses discuss the power factor issue only in terms of motors, which causes confusion when their students later encounter poor power factor as exhibited by an SMPS.
Electric motors and displacement power factor problems
Electric motors create powerful magnetic fields which produce a voltage, or back emf, in opposition to the applied voltage. This causes the supply current to lag the applied voltage. The resulting out of phase current component cannot deliver usable power, yet it adds to the facility’s required supply capacity and electricity costs. Fitting capacitors across motors reduces the phase lag and improves their power factor.