IEC/EN 61000-3-2 sets limits for mains harmonic current emissions up to the 40th harmonic. It has been a 'standard result' for a long time that for single-phase rectifiers with capacitive filters, the harmonic spectrum has nulls or quasi-nulls, depending on the 'conduction angle' of the rectifier diode(s). (The conduction angle is the fraction of the half-cycle during which the diode(s) conduct(s), expressed as an angle.)
For example, a simplified theoretical analysis of the spectrum for a full-wave (bi-phase) rectifier with 60 degrees conduction angle shows zero amplitude for the 9th, 15th, 21st, 27th... every triplen harmonic of order 9 or greater. For 55 degrees conduction angle, the amplitudes of the 17th, 23rd and 29th harmonics are small but not zero, and the 15th and higher triplen harmonics are far from zero amplitude. The conduction angle is dependent on the supply voltage, the load current, the filter capacitor value, the values (real and imaginary) of any impedance in series with the rectifier, including the source impedance of the mains supply, and the temperatures of some components, notably the diodes, of course. The combined effect of all these may me such that the results of two measurements of the same equipment, even with the same test equipment at different times, may differ in detail significantly. For people beginning to become accustomed to 'uncertainty' estimates of a few dB, this effect is disconcerting, and needs to be explained. The subject was brought to the attention of the IEC by the German regulatory authorities, through the German National Committee of the IEC. After some investigation, the principle was established (I hope: the dust hasn't quite settled yet!) that what matters is whether any of the results show that a limit is violated: for the cases where 'variability' is known to occur, differences between test results that do not involve any violation of a limit should not be considered a cause for concern. If this thread runs a bit, I will put some relevant Word documents on an unlinked page on my web site. -- Regards, John Woodgate, OOO - Own Opinions Only. Deadlines are 90% of deadliness. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
