Harmonics in most industrial, commercial, and residential systems are caused by the presence of non-linear loads. If the load is linear (e.g. resistance heating, constant speed induction motors, and incandescent lighting), then the current is sinusoidal with a smooth sinewave.

However, if the load is non-linear (e.g. variable-speed drives, office equipment, household appliances, resistance welding, electric arc furnaces, and UPS’s), then the current does not have the same waveform as the supply voltage. It then becomes non-sinusoidal, distorting the sinewave and potentially causing a drop in voltage, an increase in heating in the equipment, and other power quality concerns.