In message <[email protected]>, dated Tue, 20 Dec 2005, [email protected] writes
>You have helped me to think about this differently. To tell if it is >the transformer secondary load that is the culprit one can disconnect >the load. If it still hums then the load is not the problem. True, and the hum will get worse if the cause is core saturation. This is because there is no longer so much voltage drop across the primary resistance; all the applied voltage is going to drive the magnetizing current, which creates induction in the core. For such a big (in electronics terms) transformer, the increase may be 5%, but if, as is likely, the core is very near saturation, that 5% is significant. > >What can the problem be then? It could be the transformer design or it >can be the distorted AC waveform being fed to the transformer. Or a >combination in which a transformer deficiency is revealed by the >distorted AC waveform. The distorted waveform could have only a minor effect. > >The previously mentioned experiment to apply a swept sinewave to the >transformer can reveal a transformer defect. This signal can be applied >to the transformer primary or the secondary. I really wouldn't bother to do that. > >A manufacturing change was made where the core area was reduced along >with a reduction in the number of windings of 5%. This would tend to >make the transformer saturate at a lower voltage - less volt-seconds >will do it. This should have made the hum worse, all other things being >equal, if transformer saturation is the issue. Indeed, it is by far the most likely cause. Ask your client to get a big rotary transformer (Variac) and turn the supply volts down to, say, 230 V. See if that reduces the hum. If it does, saturation is the cause. > >It can be of great academic interest to get to the root cause of a >problem but sometimes in the interest of time I prefer to solve a long >distance problem such as this by fixing more than one thing at once. It's a risky strategy, because the results can be very confusing. > We never now for sure which fix solved the problem but the problem is >gone. That is why I like the idea that was suggested of potting the >noisy critter. That will compromise the temperature rise, which is where you came in! > There was also the suggestion of mounting it on some type of vibration >isolation. Isn't there any? Toroids should always be clamped between rubber pads. That is normally sufficient. The rubber needs to be quite stiff, of course, and for a big toroid I'd suggest 4 to 5 mm thick. -- Due to a virus attack, I've lost all my clever signature tags. Are you sorry? OOO - Own Opinions Only. See http://www.jmwa.demon.co.uk and http://www.isce.org.uk John Woodgate - 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/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: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
