For the past three years, this column has covered a different power quality-related topic each month. Now that most of the major areas have been covered, it seems as though it is time to use a different approach.
The forthcoming months will each feature a “PQ Rule” or a single concept that can be used in many applications to solve or interpret the data gathered from a power quality monitor. Ideas, suggestions or other rules that readers have would be greatly appreciated. Please e-mail them to me at firstname.lastname@example.org. Please also reference ELECTRICAL CONTRACTOR magazine, so that I don’t just reply directly to you.
PQ Rule No. 1
The presence of a high percentage of even harmonics usually indicates that there is a significantly large half-wave rectifier on the circuit, or that a full-wave rectifier is damaged and is acting like a half-wave rectifier.
The majority of the harmonic spectrum on most electrical circuits consists of odd harmonics. Due to the way that loads draw current (the h=n*p +/-1 rule), it is unusual to find even harmonics. The exception is where current is being drawn on only half of the cycle, either intentionally or due to something not functioning properly. The Fourier expansion of a half-wave rectified signal is composed entirely of even harmonics, whereas the typical electronic loads, such as PCs, laser printers or adjustable speed drives (ASDs), have predominantly odd harmonic spectrums. Even harmonics are often detectable by the lack of quarter-wave symmetry of the waveform. EC
BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at 732.287.3680.