An off the wall transformer question/problem * * * SOLVED WELL NOT REALLY * * * - Page 1
Forum section: electronics / general

author: 99tito99
author_rank: Regular Contributor
timestamp: October 15, 2013, 05:50:58 am
content: Hi Folk:On my last post on this:https://www.eevblog.com/forum/chat/an-off-the-wall-transformer-questionproblem/, I was sure I had this figured out. After replacing the GFI and fixing the breaker, for a couple of days the hum appeared to have stopped.But it has returned. What I hadn't noticed, probably because the transformer was not on consistently, is that the hum is VERY systematic.What I have observed is when it starts the hum occurs 7 times (no more no less), each hum lasts 4 seconds, and each hum is separated by 35 seconds. I have observed this pattern four times over the last two evenings.'G7PSK' in the previous post suggested:"The timing could well be a clue as to what is going on. As stated by Wytnucles there could be a resonance problem, the frequency of the mains alters during the day dropping under load so the utilities run the frequency up at night in order to keep synchronous clocks correct . . ."Well maybe something like that. But first I must state that I do not wear aluminum foil hats.But, doing some web searching there is a bunch of stuff on power line communication (PLC) in which the power companies transmit down the power lines low frequency signal (24 - 500 KHz and low bit rates) to send commands and receive data from sub-stations and other monitored nodes.Although the hum is 60Hz, could the addition of the PLC signal stimulate the transformer to resonated?My next step is to see if I can see this signal with my oscilloscope, which might be a bit of a challenge with my Tek 2213A.If anyone has ideas about this they will be most appreciated. Cheers, Mark

author: G7PSK
author_rank: Super Contributor
timestamp: October 15, 2013, 07:43:52 am
content: There is a line trap also known as a wave trap to stop the HF signal before it reaches the transformer at the substation otherwise the signal is attenuated by the transformer to a level that would compromise the data transmission, so it is highly unlikely that any control signal is getting to your transformer as it would be lost in all the power company's transformers apart from being stopped intentionally by the line traps.Some more information in the PDF.

author: JackOfVA
author_rank: Unknown
timestamp: October 15, 2013, 08:58:17 pm
content: Maybe line voltage induced? Many (most all?) electrical utility companies use automatic transfomer tap changers in their networks to compensate for voltage changes caused by changing loads over the day and week.Tap changes are motor driven switches that select transformer taps to increase or decrease the distribution voltage so as to maintain the voltage at the outlet within tolerance. If the line voltage is too low or too high, a tap changer will activate and step the voltage up or down to bring it back into an acceptable range. However, to avoid unnecessary operation there's a time factor used - the voltage has to be out of tolerance for a minimum time before the tap changer activates.It's possible that the transformer hum is generated when the line voltage is exceptionally high or low and that the 5 second hum period represents the delay and operate time of the tap changer. That you see 7 cycles of hum with a dead period may be tied to the tap changer hunting - as the voltage is increased or decreased the load current and hence the system voltage drop changes so it may take multiple tap changes before normal voltage conditions are restored. And a certain settling period is incorporated in the tap changer algorithm.This would be a good application for a recording voltmeter -- log the line voltage and see if changes high or low correlate with hum.To be line frequency related is not impossible, but strikes me as unlikely. Power system frequency changes are on the order of some tens of milliHz and in order to cause an acoustical resonance that distinctly changes with a few tens of milliHz change in the mains would require the mechanical structure under resonance to have an exceptionally high Q. While possible, it seems to be a long shot.